AnonymousFox

//usr/share/doc/pcre-devel-7.8/pcre.txt

-----------------------------------------------------------------------------
This file contains a concatenation of the PCRE man pages, converted to plain
text format for ease of searching with a text editor, or for use on systems
that do not have a man page processor. The small individual files that give
synopses of each function in the library have not been included. There are
separate text files for the pcregrep and pcretest commands.
-----------------------------------------------------------------------------

PCRE(3)                                                                PCRE(3)

NAME
       PCRE - Perl-compatible regular expressions

INTRODUCTION

The  PCRE  library is a set of functions that implement regular expres-
       sion pattern matching using the same syntax and semantics as Perl, with
       just  a  few  differences. Certain features that appeared in Python and
       PCRE before they appeared in Perl are also available using  the  Python
       syntax.  There is also some support for certain .NET and Oniguruma syn-
       tax items, and there is an option for  requesting  some  minor  changes
       that give better JavaScript compatibility.

The  current  implementation of PCRE (release 7.x) corresponds approxi-
       mately with Perl 5.10, including support for UTF-8 encoded strings  and
       Unicode general category properties. However, UTF-8 and Unicode support
       has to be explicitly enabled; it is not the default. The Unicode tables
       correspond to Unicode release 5.0.0.

In  addition to the Perl-compatible matching function, PCRE contains an
       alternative matching function that matches the same  compiled  patterns
       in  a different way. In certain circumstances, the alternative function
       has some advantages. For a discussion of the two  matching  algorithms,
       see the pcrematching page.

PCRE  is  written  in C and released as a C library. A number of people
       have written wrappers and interfaces of various kinds.  In  particular,
       Google  Inc.   have  provided  a comprehensive C++ wrapper. This is now
       included as part of the PCRE distribution. The pcrecpp page has details
       of  this  interface.  Other  people's contributions can be found in the
       Contrib directory at the primary FTP site, which is:

ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre

Details of exactly which Perl regular expression features are  and  are
       not supported by PCRE are given in separate documents. See the pcrepat-
       tern and pcrecompat pages. There is a syntax summary in the  pcresyntax
       page.

Some  features  of  PCRE can be included, excluded, or changed when the
       library is built. The pcre_config() function makes it  possible  for  a
       client  to  discover  which  features are available. The features them-
       selves are described in the pcrebuild page. Documentation about  build-
       ing  PCRE for various operating systems can be found in the README file
       in the source distribution.

The library contains a number of undocumented  internal  functions  and
       data  tables  that  are  used by more than one of the exported external
       functions, but which are not intended  for  use  by  external  callers.
       Their  names  all begin with "_pcre_", which hopefully will not provoke
       any name clashes. In some environments, it is possible to control which
       external  symbols  are  exported when a shared library is built, and in
       these cases the undocumented symbols are not exported.

USER DOCUMENTATION

The user documentation for PCRE comprises a number  of  different  sec-
       tions.  In the "man" format, each of these is a separate "man page". In
       the HTML format, each is a separate page, linked from the  index  page.
       In  the  plain text format, all the sections are concatenated, for ease
       of searching. The sections are as follows:

pcre              this document
         pcre-config       show PCRE installation configuration information
         pcreapi           details of PCRE's native C API
         pcrebuild         options for building PCRE
         pcrecallout       details of the callout feature
         pcrecompat        discussion of Perl compatibility
         pcrecpp           details of the C++ wrapper
         pcregrep          description of the pcregrep command
         pcrematching      discussion of the two matching algorithms
         pcrepartial       details of the partial matching facility
         pcrepattern       syntax and semantics of supported
                             regular expressions
         pcresyntax        quick syntax reference
         pcreperform       discussion of performance issues
         pcreposix         the POSIX-compatible C API
         pcreprecompile    details of saving and re-using precompiled patterns
         pcresample        discussion of the sample program
         pcrestack         discussion of stack usage
         pcretest          description of the pcretest testing command

In  addition,  in the "man" and HTML formats, there is a short page for
       each C library function, listing its arguments and results.

LIMITATIONS

There are some size limitations in PCRE but it is hoped that they  will
       never in practice be relevant.

The  maximum  length of a compiled pattern is 65539 (sic) bytes if PCRE
       is compiled with the default internal linkage size of 2. If you want to
       process  regular  expressions  that are truly enormous, you can compile
       PCRE with an internal linkage size of 3 or 4 (see the  README  file  in
       the  source  distribution and the pcrebuild documentation for details).
       In these cases the limit is substantially larger.  However,  the  speed
       of execution is slower.

All values in repeating quantifiers must be less than 65536.

There is no limit to the number of parenthesized subpatterns, but there
       can be no more than 65535 capturing subpatterns.

The maximum length of name for a named subpattern is 32 characters, and
       the maximum number of named subpatterns is 10000.

The  maximum  length of a subject string is the largest positive number
       that an integer variable can hold. However, when using the  traditional
       matching function, PCRE uses recursion to handle subpatterns and indef-
       inite repetition.  This means that the available stack space may  limit
       the size of a subject string that can be processed by certain patterns.
       For a discussion of stack issues, see the pcrestack documentation.

UTF-8 AND UNICODE PROPERTY SUPPORT

From release 3.3, PCRE has  had  some  support  for  character  strings
       encoded  in the UTF-8 format. For release 4.0 this was greatly extended
       to cover most common requirements, and in release 5.0  additional  sup-
       port for Unicode general category properties was added.

In  order  process  UTF-8 strings, you must build PCRE to include UTF-8
       support in the code, and, in addition,  you  must  call  pcre_compile()
       with  the PCRE_UTF8 option flag. When you do this, both the pattern and
       any subject strings that are matched against it are  treated  as  UTF-8
       strings instead of just strings of bytes.

If  you compile PCRE with UTF-8 support, but do not use it at run time,
       the library will be a bit bigger, but the additional run time  overhead
       is limited to testing the PCRE_UTF8 flag occasionally, so should not be
       very big.

If PCRE is built with Unicode character property support (which implies
       UTF-8  support),  the  escape sequences \p{..}, \P{..}, and \X are sup-
       ported.  The available properties that can be tested are limited to the
       general  category  properties such as Lu for an upper case letter or Nd
       for a decimal number, the Unicode script names such as Arabic  or  Han,
       and  the  derived  properties  Any  and L&. A full list is given in the
       pcrepattern documentation. Only the short names for properties are sup-
       ported.  For example, \p{L} matches a letter. Its Perl synonym, \p{Let-
       ter}, is not supported.  Furthermore,  in  Perl,  many  properties  may
       optionally  be  prefixed by "Is", for compatibility with Perl 5.6. PCRE
       does not support this.

Validity of UTF-8 strings

When you set the PCRE_UTF8 flag, the strings  passed  as  patterns  and
       subjects are (by default) checked for validity on entry to the relevant
       functions. From release 7.3 of PCRE, the check is according  the  rules
       of  RFC  3629, which are themselves derived from the Unicode specifica-
       tion. Earlier releases of PCRE followed the rules of  RFC  2279,  which
       allows  the  full range of 31-bit values (0 to 0x7FFFFFFF). The current
       check allows only values in the range U+0 to U+10FFFF, excluding U+D800
       to U+DFFF.

The  excluded  code  points are the "Low Surrogate Area" of Unicode, of
       which the Unicode Standard says this: "The Low Surrogate Area does  not
       contain  any  character  assignments,  consequently  no  character code
       charts or namelists are provided for this area. Surrogates are reserved
       for  use  with  UTF-16 and then must be used in pairs." The code points
       that are encoded by UTF-16 pairs  are  available  as  independent  code
       points  in  the  UTF-8  encoding.  (In other words, the whole surrogate
       thing is a fudge for UTF-16 which unfortunately messes up UTF-8.)

If an  invalid  UTF-8  string  is  passed  to  PCRE,  an  error  return
       (PCRE_ERROR_BADUTF8) is given. In some situations, you may already know
       that your strings are valid, and therefore want to skip these checks in
       order to improve performance. If you set the PCRE_NO_UTF8_CHECK flag at
       compile time or at run time, PCRE assumes that the pattern  or  subject
       it  is  given  (respectively)  contains only valid UTF-8 codes. In this
       case, it does not diagnose an invalid UTF-8 string.

If you pass an invalid UTF-8 string  when  PCRE_NO_UTF8_CHECK  is  set,
       what  happens  depends on why the string is invalid. If the string con-
       forms to the "old" definition of UTF-8 (RFC 2279), it is processed as a
       string  of  characters  in  the  range 0 to 0x7FFFFFFF. In other words,
       apart from the initial validity test, PCRE (when in UTF-8 mode) handles
       strings  according  to  the more liberal rules of RFC 2279. However, if
       the string does not even conform to RFC 2279, the result is  undefined.
       Your program may crash.

If  you  want  to  process  strings  of  values  in the full range 0 to
       0x7FFFFFFF, encoded in a UTF-8-like manner as per the old RFC, you  can
       set PCRE_NO_UTF8_CHECK to bypass the more restrictive test. However, in
       this situation, you will have to apply your own validity check.

General comments about UTF-8 mode

1. An unbraced hexadecimal escape sequence (such  as  \xb3)  matches  a
       two-byte UTF-8 character if the value is greater than 127.

2.  Octal  numbers  up to \777 are recognized, and match two-byte UTF-8
       characters for values greater than \177.

3. Repeat quantifiers apply to complete UTF-8 characters, not to  indi-
       vidual bytes, for example: \x{100}{3}.

4.  The dot metacharacter matches one UTF-8 character instead of a sin-
       gle byte.

5. The escape sequence \C can be used to match a single byte  in  UTF-8
       mode,  but  its  use can lead to some strange effects. This facility is
       not available in the alternative matching function, pcre_dfa_exec().

6. The character escapes \b, \B, \d, \D, \s, \S, \w, and  \W  correctly
       test  characters of any code value, but the characters that PCRE recog-
       nizes as digits, spaces, or word characters  remain  the  same  set  as
       before, all with values less than 256. This remains true even when PCRE
       includes Unicode property support, because to do otherwise  would  slow
       down  PCRE in many common cases. If you really want to test for a wider
       sense of, say, "digit", you must use Unicode  property  tests  such  as
       \p{Nd}.

7.  Similarly,  characters that match the POSIX named character classes
       are all low-valued characters.

8. However, the Perl 5.10 horizontal and vertical  whitespace  matching
       escapes (\h, \H, \v, and \V) do match all the appropriate Unicode char-
       acters.

9. Case-insensitive matching applies only to  characters  whose  values
       are  less than 128, unless PCRE is built with Unicode property support.
       Even when Unicode property support is available, PCRE  still  uses  its
       own  character  tables when checking the case of low-valued characters,
       so as not to degrade performance.  The Unicode property information  is
       used only for characters with higher values. Even when Unicode property
       support is available, PCRE supports case-insensitive matching only when
       there  is  a  one-to-one  mapping between a letter's cases. There are a
       small number of many-to-one mappings in Unicode;  these  are  not  sup-
       ported by PCRE.

AUTHOR

Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.

Putting  an actual email address here seems to have been a spam magnet,
       so I've taken it away. If you want to email me, use  my  two  initials,
       followed by the two digits 10, at the domain cam.ac.uk.

REVISION

Last updated: 12 April 2008
       Copyright (c) 1997-2008 University of Cambridge.
------------------------------------------------------------------------------

PCREBUILD(3)                                                      PCREBUILD(3)

NAME
       PCRE - Perl-compatible regular expressions

PCRE BUILD-TIME OPTIONS

This  document  describes  the  optional  features  of PCRE that can be
       selected when the library is compiled. It assumes use of the  configure
       script,  where the optional features are selected or deselected by pro-
       viding options to configure before running the make  command.  However,
       the  same  options  can be selected in both Unix-like and non-Unix-like
       environments using the GUI facility of  CMakeSetup  if  you  are  using
       CMake instead of configure to build PCRE.

The complete list of options for configure (which includes the standard
       ones such as the  selection  of  the  installation  directory)  can  be
       obtained by running

./configure --help

The  following  sections  include  descriptions  of options whose names
       begin with --enable or --disable. These settings specify changes to the
       defaults  for  the configure command. Because of the way that configure
       works, --enable and --disable always come in pairs, so  the  complemen-
       tary  option always exists as well, but as it specifies the default, it
       is not described.

C++ SUPPORT

By default, the configure script will search for a C++ compiler and C++
       header files. If it finds them, it automatically builds the C++ wrapper
       library for PCRE. You can disable this by adding

--disable-cpp

to the configure command.

UTF-8 SUPPORT

To build PCRE with support for UTF-8 character strings, add

--enable-utf8

to the configure command. Of itself, this  does  not  make  PCRE  treat
       strings  as UTF-8. As well as compiling PCRE with this option, you also
       have have to set the PCRE_UTF8 option when you call the  pcre_compile()
       function.

UNICODE CHARACTER PROPERTY SUPPORT

UTF-8  support allows PCRE to process character values greater than 255
       in the strings that it handles. On its own, however, it does  not  pro-
       vide any facilities for accessing the properties of such characters. If
       you want to be able to use the pattern escapes \P, \p,  and  \X,  which
       refer to Unicode character properties, you must add

--enable-unicode-properties

to  the configure command. This implies UTF-8 support, even if you have
       not explicitly requested it.

Including Unicode property support adds around 30K  of  tables  to  the
       PCRE  library.  Only  the general category properties such as Lu and Nd
       are supported. Details are given in the pcrepattern documentation.

CODE VALUE OF NEWLINE

By default, PCRE interprets character 10 (linefeed, LF)  as  indicating
       the  end  of  a line. This is the normal newline character on Unix-like
       systems. You can compile PCRE to use character 13 (carriage return, CR)
       instead, by adding

--enable-newline-is-cr

to  the  configure  command.  There  is  also  a --enable-newline-is-lf
       option, which explicitly specifies linefeed as the newline character.

Alternatively, you can specify that line endings are to be indicated by
       the two character sequence CRLF. If you want this, add

--enable-newline-is-crlf

to the configure command. There is a fourth option, specified by

--enable-newline-is-anycrlf

which  causes  PCRE  to recognize any of the three sequences CR, LF, or
       CRLF as indicating a line ending. Finally, a fifth option, specified by

--enable-newline-is-any

causes PCRE to recognize any Unicode newline sequence.

Whatever  line  ending convention is selected when PCRE is built can be
       overridden when the library functions are called. At build time  it  is
       conventional to use the standard for your operating system.

WHAT \R MATCHES

By  default,  the  sequence \R in a pattern matches any Unicode newline
       sequence, whatever has been selected as the line  ending  sequence.  If
       you specify

--enable-bsr-anycrlf

the  default  is changed so that \R matches only CR, LF, or CRLF. What-
       ever is selected when PCRE is built can be overridden when the  library
       functions are called.

BUILDING SHARED AND STATIC LIBRARIES

The  PCRE building process uses libtool to build both shared and static
       Unix libraries by default. You can suppress one of these by adding  one
       of

--disable-shared
         --disable-static

to the configure command, as required.

POSIX MALLOC USAGE

When PCRE is called through the POSIX interface (see the pcreposix doc-
       umentation), additional working storage is  required  for  holding  the
       pointers  to capturing substrings, because PCRE requires three integers
       per substring, whereas the POSIX interface provides only  two.  If  the
       number of expected substrings is small, the wrapper function uses space
       on the stack, because this is faster than using malloc() for each call.
       The default threshold above which the stack is no longer used is 10; it
       can be changed by adding a setting such as

--with-posix-malloc-threshold=20

to the configure command.

HANDLING VERY LARGE PATTERNS

Within a compiled pattern, offset values are used  to  point  from  one
       part  to another (for example, from an opening parenthesis to an alter-
       nation metacharacter). By default, two-byte values are used  for  these
       offsets,  leading  to  a  maximum size for a compiled pattern of around
       64K. This is sufficient to handle all but the most  gigantic  patterns.
       Nevertheless,  some  people do want to process enormous patterns, so it
       is possible to compile PCRE to use three-byte or four-byte  offsets  by
       adding a setting such as

--with-link-size=3

to  the  configure  command.  The value given must be 2, 3, or 4. Using
       longer offsets slows down the operation of PCRE because it has to  load
       additional bytes when handling them.

AVOIDING EXCESSIVE STACK USAGE

When matching with the pcre_exec() function, PCRE implements backtrack-
       ing by making recursive calls to an internal function  called  match().
       In  environments  where  the size of the stack is limited, this can se-
       verely limit PCRE's operation. (The Unix environment does  not  usually
       suffer from this problem, but it may sometimes be necessary to increase
       the maximum stack size.  There is a discussion in the  pcrestack  docu-
       mentation.)  An alternative approach to recursion that uses memory from
       the heap to remember data, instead of using recursive  function  calls,
       has  been  implemented to work round the problem of limited stack size.
       If you want to build a version of PCRE that works this way, add

--disable-stack-for-recursion

to the configure command. With this configuration, PCRE  will  use  the
       pcre_stack_malloc  and pcre_stack_free variables to call memory manage-
       ment functions. By default these point to malloc() and free(), but  you
       can replace the pointers so that your own functions are used.

Separate  functions  are  provided  rather  than  using pcre_malloc and
       pcre_free because the  usage  is  very  predictable:  the  block  sizes
       requested  are  always  the  same,  and  the blocks are always freed in
       reverse order. A calling program might be able to  implement  optimized
       functions  that  perform  better  than  malloc()  and free(). PCRE runs
       noticeably more slowly when built in this way. This option affects only
       the   pcre_exec()   function;   it   is   not   relevant  for  the  the
       pcre_dfa_exec() function.

LIMITING PCRE RESOURCE USAGE

Internally, PCRE has a function called match(), which it calls  repeat-
       edly   (sometimes   recursively)  when  matching  a  pattern  with  the
       pcre_exec() function. By controlling the maximum number of  times  this
       function  may be called during a single matching operation, a limit can
       be placed on the resources used by a single call  to  pcre_exec().  The
       limit  can be changed at run time, as described in the pcreapi documen-
       tation. The default is 10 million, but this can be changed by adding  a
       setting such as

--with-match-limit=500000

to   the   configure  command.  This  setting  has  no  effect  on  the
       pcre_dfa_exec() matching function.

In some environments it is desirable to limit the  depth  of  recursive
       calls of match() more strictly than the total number of calls, in order
       to restrict the maximum amount of stack (or heap,  if  --disable-stack-
       for-recursion is specified) that is used. A second limit controls this;
       it defaults to the value that  is  set  for  --with-match-limit,  which
       imposes  no  additional constraints. However, you can set a lower limit
       by adding, for example,

--with-match-limit-recursion=10000

to the configure command. This value can  also  be  overridden  at  run
       time.

CREATING CHARACTER TABLES AT BUILD TIME

PCRE  uses fixed tables for processing characters whose code values are
       less than 256. By default, PCRE is built with a set of tables that  are
       distributed  in  the  file pcre_chartables.c.dist. These tables are for
       ASCII codes only. If you add

--enable-rebuild-chartables

to the configure command, the distributed tables are  no  longer  used.
       Instead,  a  program  called dftables is compiled and run. This outputs
       the source for new set of tables, created in the default locale of your
       C runtime system. (This method of replacing the tables does not work if
       you are cross compiling, because dftables is run on the local host.  If
       you  need  to  create alternative tables when cross compiling, you will
       have to do so "by hand".)

USING EBCDIC CODE

PCRE assumes by default that it will run in an  environment  where  the
       character  code  is  ASCII  (or Unicode, which is a superset of ASCII).
       This is the case for most computer operating systems.  PCRE  can,  how-
       ever, be compiled to run in an EBCDIC environment by adding

--enable-ebcdic

to the configure command. This setting implies --enable-rebuild-charta-
       bles. You should only use it if you know that  you  are  in  an  EBCDIC
       environment (for example, an IBM mainframe operating system).

PCREGREP OPTIONS FOR COMPRESSED FILE SUPPORT

By default, pcregrep reads all files as plain text. You can build it so
       that it recognizes files whose names end in .gz or .bz2, and reads them
       with libz or libbz2, respectively, by adding one or both of

--enable-pcregrep-libz
         --enable-pcregrep-libbz2

to the configure command. These options naturally require that the rel-
       evant libraries are installed on your system. Configuration  will  fail
       if they are not.

PCRETEST OPTION FOR LIBREADLINE SUPPORT

If you add

--enable-pcretest-libreadline

to  the  configure  command,  pcretest  is  linked with the libreadline
       library, and when its input is from a terminal, it reads it  using  the
       readline() function. This provides line-editing and history facilities.
       Note that libreadline is GPL-licenced, so if you distribute a binary of
       pcretest linked in this way, there may be licensing issues.

Setting  this  option  causes  the -lreadline option to be added to the
       pcretest build. In many operating environments with  a  sytem-installed
       libreadline this is sufficient. However, in some environments (e.g.  if
       an unmodified distribution version of readline is in use),  some  extra
       configuration  may  be necessary. The INSTALL file for libreadline says
       this:

"Readline uses the termcap functions, but does not link with the
         termcap or curses library itself, allowing applications which link
         with readline the to choose an appropriate library."

If your environment has not been set up so that an appropriate  library
       is automatically included, you may need to add something like

LIBS="-ncurses"

immediately before the configure command.

SEE ALSO

pcreapi(3), pcre_config(3).

AUTHOR

Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.

REVISION

Last updated: 13 April 2008
       Copyright (c) 1997-2008 University of Cambridge.
------------------------------------------------------------------------------

PCREMATCHING(3)                                                PCREMATCHING(3)

NAME
       PCRE - Perl-compatible regular expressions

PCRE MATCHING ALGORITHMS

This document describes the two different algorithms that are available
       in PCRE for matching a compiled regular expression against a given sub-
       ject  string.  The  "standard"  algorithm  is  the  one provided by the
       pcre_exec() function.  This works in the same was  as  Perl's  matching
       function, and provides a Perl-compatible matching operation.

An  alternative  algorithm is provided by the pcre_dfa_exec() function;
       this operates in a different way, and is not  Perl-compatible.  It  has
       advantages  and disadvantages compared with the standard algorithm, and
       these are described below.

When there is only one possible way in which a given subject string can
       match  a pattern, the two algorithms give the same answer. A difference
       arises, however, when there are multiple possibilities. For example, if
       the pattern

^<.*>

is matched against the string

there are three possible answers. The standard algorithm finds only one
       of them, whereas the alternative algorithm finds all three.

REGULAR EXPRESSIONS AS TREES

The set of strings that are matched by a regular expression can be rep-
       resented  as  a  tree structure. An unlimited repetition in the pattern
       makes the tree of infinite size, but it is still a tree.  Matching  the
       pattern  to a given subject string (from a given starting point) can be
       thought of as a search of the tree.  There are two  ways  to  search  a
       tree:  depth-first  and  breadth-first, and these correspond to the two
       matching algorithms provided by PCRE.

THE STANDARD MATCHING ALGORITHM

In the terminology of Jeffrey Friedl's book "Mastering Regular  Expres-
       sions",  the  standard  algorithm  is an "NFA algorithm". It conducts a
       depth-first search of the pattern tree. That is, it  proceeds  along  a
       single path through the tree, checking that the subject matches what is
       required. When there is a mismatch, the algorithm  tries  any  alterna-
       tives  at  the  current point, and if they all fail, it backs up to the
       previous branch point in the  tree,  and  tries  the  next  alternative
       branch  at  that  level.  This often involves backing up (moving to the
       left) in the subject string as well.  The  order  in  which  repetition
       branches  are  tried  is controlled by the greedy or ungreedy nature of
       the quantifier.

If a leaf node is reached, a matching string has  been  found,  and  at
       that  point the algorithm stops. Thus, if there is more than one possi-
       ble match, this algorithm returns the first one that it finds.  Whether
       this  is the shortest, the longest, or some intermediate length depends
       on the way the greedy and ungreedy repetition quantifiers are specified
       in the pattern.

Because  it  ends  up  with a single path through the tree, it is rela-
       tively straightforward for this algorithm to keep  track  of  the  sub-
       strings  that  are  matched  by portions of the pattern in parentheses.
       This provides support for capturing parentheses and back references.

THE ALTERNATIVE MATCHING ALGORITHM

This algorithm conducts a breadth-first search of  the  tree.  Starting
       from  the  first  matching  point  in the subject, it scans the subject
       string from left to right, once, character by character, and as it does
       this,  it remembers all the paths through the tree that represent valid
       matches. In Friedl's terminology, this is a kind  of  "DFA  algorithm",
       though  it is not implemented as a traditional finite state machine (it
       keeps multiple states active simultaneously).

The scan continues until either the end of the subject is  reached,  or
       there  are  no more unterminated paths. At this point, terminated paths
       represent the different matching possibilities (if there are none,  the
       match  has  failed).   Thus,  if there is more than one possible match,
       this algorithm finds all of them, and in particular, it finds the long-
       est.  In PCRE, there is an option to stop the algorithm after the first
       match (which is necessarily the shortest) has been found.

Note that all the matches that are found start at the same point in the
       subject. If the pattern

cat(er(pillar)?)

is  matched  against the string "the caterpillar catchment", the result
       will be the three strings "cat", "cater", and "caterpillar" that  start
       at the fourth character of the subject. The algorithm does not automat-
       ically move on to find matches that start at later positions.

There are a number of features of PCRE regular expressions that are not
       supported by the alternative matching algorithm. They are as follows:

1.  Because  the  algorithm  finds  all possible matches, the greedy or
       ungreedy nature of repetition quantifiers is not relevant.  Greedy  and
       ungreedy quantifiers are treated in exactly the same way. However, pos-
       sessive quantifiers can make a difference when what follows could  also
       match what is quantified, for example in a pattern like this:

^a++\w!

This  pattern matches "aaab!" but not "aaa!", which would be matched by
       a non-possessive quantifier. Similarly, if an atomic group is  present,
       it  is matched as if it were a standalone pattern at the current point,
       and the longest match is then "locked in" for the rest of  the  overall
       pattern.

2. When dealing with multiple paths through the tree simultaneously, it
       is not straightforward to keep track of  captured  substrings  for  the
       different  matching  possibilities,  and  PCRE's implementation of this
       algorithm does not attempt to do this. This means that no captured sub-
       strings are available.

3.  Because no substrings are captured, back references within the pat-
       tern are not supported, and cause errors if encountered.

4. For the same reason, conditional expressions that use  a  backrefer-
       ence  as  the  condition or test for a specific group recursion are not
       supported.

5. Because many paths through the tree may be  active,  the  \K  escape
       sequence, which resets the start of the match when encountered (but may
       be on some paths and not on others), is not  supported.  It  causes  an
       error if encountered.

6.  Callouts  are  supported, but the value of the capture_top field is
       always 1, and the value of the capture_last field is always -1.

7. The \C escape sequence, which (in the standard algorithm) matches  a
       single  byte, even in UTF-8 mode, is not supported because the alterna-
       tive algorithm moves through the subject  string  one  character  at  a
       time, for all active paths through the tree.

8.  Except for (*FAIL), the backtracking control verbs such as (*PRUNE)
       are not supported. (*FAIL) is supported, and  behaves  like  a  failing
       negative assertion.

ADVANTAGES OF THE ALTERNATIVE ALGORITHM

Using  the alternative matching algorithm provides the following advan-
       tages:

1. All possible matches (at a single point in the subject) are automat-
       ically  found,  and  in particular, the longest match is found. To find
       more than one match using the standard algorithm, you have to do kludgy
       things with callouts.

2.  There is much better support for partial matching. The restrictions
       on the content of the pattern that apply when using the standard  algo-
       rithm  for  partial matching do not apply to the alternative algorithm.
       For non-anchored patterns, the starting position of a partial match  is
       available.

3.  Because  the  alternative  algorithm  scans the subject string just
       once, and never needs to backtrack, it is possible to  pass  very  long
       subject  strings  to  the matching function in several pieces, checking
       for partial matching each time.

DISADVANTAGES OF THE ALTERNATIVE ALGORITHM

The alternative algorithm suffers from a number of disadvantages:

1. It is substantially slower than  the  standard  algorithm.  This  is
       partly  because  it has to search for all possible matches, but is also
       because it is less susceptible to optimization.

2. Capturing parentheses and back references are not supported.

3. Although atomic groups are supported, their use does not provide the
       performance advantage that it does for the standard algorithm.

AUTHOR

Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.

REVISION

Last updated: 19 April 2008
       Copyright (c) 1997-2008 University of Cambridge.
------------------------------------------------------------------------------

PCREAPI(3)                                                          PCREAPI(3)

NAME
       PCRE - Perl-compatible regular expressions

PCRE NATIVE API

#include <pcre.h>

pcre *pcre_compile(const char *pattern, int options,
            const char **errptr, int *erroffset,
            const unsigned char *tableptr);

pcre *pcre_compile2(const char *pattern, int options,
            int *errorcodeptr,
            const char **errptr, int *erroffset,
            const unsigned char *tableptr);

pcre_extra *pcre_study(const pcre *code, int options,
            const char **errptr);

int pcre_exec(const pcre *code, const pcre_extra *extra,
            const char *subject, int length, int startoffset,
            int options, int *ovector, int ovecsize);

int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
            const char *subject, int length, int startoffset,
            int options, int *ovector, int ovecsize,
            int *workspace, int wscount);

int pcre_copy_named_substring(const pcre *code,
            const char *subject, int *ovector,
            int stringcount, const char *stringname,
            char *buffer, int buffersize);

int pcre_copy_substring(const char *subject, int *ovector,
            int stringcount, int stringnumber, char *buffer,
            int buffersize);

int pcre_get_named_substring(const pcre *code,
            const char *subject, int *ovector,
            int stringcount, const char *stringname,
            const char **stringptr);

int pcre_get_stringnumber(const pcre *code,
            const char *name);

int pcre_get_stringtable_entries(const pcre *code,
            const char *name, char **first, char **last);

int pcre_get_substring(const char *subject, int *ovector,
            int stringcount, int stringnumber,
            const char **stringptr);

int pcre_get_substring_list(const char *subject,
            int *ovector, int stringcount, const char ***listptr);

void pcre_free_substring(const char *stringptr);

void pcre_free_substring_list(const char **stringptr);

const unsigned char *pcre_maketables(void);

int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
            int what, void *where);

int pcre_info(const pcre *code, int *optptr, int *firstcharptr);

int pcre_refcount(pcre *code, int adjust);

int pcre_config(int what, void *where);

char *pcre_version(void);

void *(*pcre_malloc)(size_t);

void (*pcre_free)(void *);

void *(*pcre_stack_malloc)(size_t);

void (*pcre_stack_free)(void *);

int (*pcre_callout)(pcre_callout_block *);

PCRE API OVERVIEW

PCRE has its own native API, which is described in this document. There
       are also some wrapper functions that correspond to  the  POSIX  regular
       expression  API.  These  are  described in the pcreposix documentation.
       Both of these APIs define a set of C function calls. A C++  wrapper  is
       distributed with PCRE. It is documented in the pcrecpp page.

The  native  API  C  function prototypes are defined in the header file
       pcre.h, and on Unix systems the library itself is called  libpcre.   It
       can normally be accessed by adding -lpcre to the command for linking an
       application  that  uses  PCRE.  The  header  file  defines  the  macros
       PCRE_MAJOR  and  PCRE_MINOR to contain the major and minor release num-
       bers for the library.  Applications can use these  to  include  support
       for different releases of PCRE.

The   functions   pcre_compile(),  pcre_compile2(),  pcre_study(),  and
       pcre_exec() are used for compiling and matching regular expressions  in
       a  Perl-compatible  manner. A sample program that demonstrates the sim-
       plest way of using them is provided in the file  called  pcredemo.c  in
       the  source distribution. The pcresample documentation describes how to
       compile and run it.

A second matching function, pcre_dfa_exec(), which is not Perl-compati-
       ble,  is  also provided. This uses a different algorithm for the match-
       ing. The alternative algorithm finds all possible matches (at  a  given
       point  in  the subject), and scans the subject just once. However, this
       algorithm does not return captured substrings. A description of the two
       matching  algorithms and their advantages and disadvantages is given in
       the pcrematching documentation.

In addition to the main compiling and  matching  functions,  there  are
       convenience functions for extracting captured substrings from a subject
       string that is matched by pcre_exec(). They are:

pcre_copy_substring()
         pcre_copy_named_substring()
         pcre_get_substring()
         pcre_get_named_substring()
         pcre_get_substring_list()
         pcre_get_stringnumber()
         pcre_get_stringtable_entries()

pcre_free_substring() and pcre_free_substring_list() are also provided,
       to free the memory used for extracted strings.

The  function  pcre_maketables()  is  used  to build a set of character
       tables  in  the  current  locale   for   passing   to   pcre_compile(),
       pcre_exec(),  or  pcre_dfa_exec(). This is an optional facility that is
       provided for specialist use.  Most  commonly,  no  special  tables  are
       passed,  in  which case internal tables that are generated when PCRE is
       built are used.

The function pcre_fullinfo() is used to find out  information  about  a
       compiled  pattern; pcre_info() is an obsolete version that returns only
       some of the available information, but is retained for  backwards  com-
       patibility.   The function pcre_version() returns a pointer to a string
       containing the version of PCRE and its date of release.

The function pcre_refcount() maintains a  reference  count  in  a  data
       block  containing  a compiled pattern. This is provided for the benefit
       of object-oriented applications.

The global variables pcre_malloc and pcre_free  initially  contain  the
       entry  points  of  the  standard malloc() and free() functions, respec-
       tively. PCRE calls the memory management functions via these variables,
       so  a  calling  program  can replace them if it wishes to intercept the
       calls. This should be done before calling any PCRE functions.

The global variables pcre_stack_malloc  and  pcre_stack_free  are  also
       indirections  to  memory  management functions. These special functions
       are used only when PCRE is compiled to use  the  heap  for  remembering
       data, instead of recursive function calls, when running the pcre_exec()
       function. See the pcrebuild documentation for  details  of  how  to  do
       this.  It  is  a non-standard way of building PCRE, for use in environ-
       ments that have limited stacks. Because of the greater  use  of  memory
       management,  it  runs  more  slowly. Separate functions are provided so
       that special-purpose external code can be  used  for  this  case.  When
       used,  these  functions  are always called in a stack-like manner (last
       obtained, first freed), and always for memory blocks of the same  size.
       There  is  a discussion about PCRE's stack usage in the pcrestack docu-
       mentation.

The global variable pcre_callout initially contains NULL. It can be set
       by  the  caller  to  a "callout" function, which PCRE will then call at
       specified points during a matching operation. Details are given in  the
       pcrecallout documentation.

NEWLINES

PCRE  supports five different conventions for indicating line breaks in
       strings: a single CR (carriage return) character, a  single  LF  (line-
       feed) character, the two-character sequence CRLF, any of the three pre-
       ceding, or any Unicode newline sequence. The Unicode newline  sequences
       are  the  three just mentioned, plus the single characters VT (vertical
       tab, U+000B), FF (formfeed, U+000C), NEL (next line, U+0085), LS  (line
       separator, U+2028), and PS (paragraph separator, U+2029).

Each  of  the first three conventions is used by at least one operating
       system as its standard newline sequence. When PCRE is built, a  default
       can  be  specified.  The default default is LF, which is the Unix stan-
       dard. When PCRE is run, the default can be overridden,  either  when  a
       pattern is compiled, or when it is matched.

At compile time, the newline convention can be specified by the options
       argument of pcre_compile(), or it can be specified by special  text  at
       the start of the pattern itself; this overrides any other settings. See
       the pcrepattern page for details of the special character sequences.

In the PCRE documentation the word "newline" is used to mean "the char-
       acter  or pair of characters that indicate a line break". The choice of
       newline convention affects the handling of  the  dot,  circumflex,  and
       dollar metacharacters, the handling of #-comments in /x mode, and, when
       CRLF is a recognized line ending sequence, the match position  advance-
       ment for a non-anchored pattern. There is more detail about this in the
       section on pcre_exec() options below.

The choice of newline convention does not affect the interpretation  of
       the  \n  or  \r  escape  sequences, nor does it affect what \R matches,
       which is controlled in a similar way, but by separate options.

MULTITHREADING

The PCRE functions can be used in  multi-threading  applications,  with
       the  proviso  that  the  memory  management  functions  pointed  to  by
       pcre_malloc, pcre_free, pcre_stack_malloc, and pcre_stack_free, and the
       callout function pointed to by pcre_callout, are shared by all threads.

The compiled form of a regular expression is not altered during  match-
       ing, so the same compiled pattern can safely be used by several threads
       at once.

SAVING PRECOMPILED PATTERNS FOR LATER USE

The compiled form of a regular expression can be saved and re-used at a
       later  time,  possibly by a different program, and even on a host other
       than the one on which  it  was  compiled.  Details  are  given  in  the
       pcreprecompile  documentation.  However, compiling a regular expression
       with one version of PCRE for use with a different version is not  guar-
       anteed to work and may cause crashes.

CHECKING BUILD-TIME OPTIONS

int pcre_config(int what, void *where);

The  function pcre_config() makes it possible for a PCRE client to dis-
       cover which optional features have been compiled into the PCRE library.
       The  pcrebuild documentation has more details about these optional fea-
       tures.

The first argument for pcre_config() is an  integer,  specifying  which
       information is required; the second argument is a pointer to a variable
       into which the information is  placed.  The  following  information  is
       available:

PCRE_CONFIG_UTF8

The  output is an integer that is set to one if UTF-8 support is avail-
       able; otherwise it is set to zero.

PCRE_CONFIG_UNICODE_PROPERTIES

The output is an integer that is set to  one  if  support  for  Unicode
       character properties is available; otherwise it is set to zero.

PCRE_CONFIG_NEWLINE

The  output  is  an integer whose value specifies the default character
       sequence that is recognized as meaning "newline". The four values  that
       are supported are: 10 for LF, 13 for CR, 3338 for CRLF, -2 for ANYCRLF,
       and -1 for ANY. The default should normally be  the  standard  sequence
       for your operating system.

PCRE_CONFIG_BSR

The output is an integer whose value indicates what character sequences
       the \R escape sequence matches by default. A value of 0 means  that  \R
       matches  any  Unicode  line ending sequence; a value of 1 means that \R
       matches only CR, LF, or CRLF. The default can be overridden when a pat-
       tern is compiled or matched.

PCRE_CONFIG_LINK_SIZE

The  output  is  an  integer that contains the number of bytes used for
       internal linkage in compiled regular expressions. The value is 2, 3, or
       4.  Larger  values  allow larger regular expressions to be compiled, at
       the expense of slower matching. The default value of  2  is  sufficient
       for  all  but  the  most massive patterns, since it allows the compiled
       pattern to be up to 64K in size.

PCRE_CONFIG_POSIX_MALLOC_THRESHOLD

The output is an integer that contains the threshold  above  which  the
       POSIX  interface  uses malloc() for output vectors. Further details are
       given in the pcreposix documentation.

PCRE_CONFIG_MATCH_LIMIT

The output is an integer that gives the default limit for the number of
       internal  matching  function  calls in a pcre_exec() execution. Further
       details are given with pcre_exec() below.

PCRE_CONFIG_MATCH_LIMIT_RECURSION

The output is an integer that gives the default limit for the depth  of
       recursion  when calling the internal matching function in a pcre_exec()
       execution. Further details are given with pcre_exec() below.

PCRE_CONFIG_STACKRECURSE

The output is an integer that is set to one if internal recursion  when
       running pcre_exec() is implemented by recursive function calls that use
       the stack to remember their state. This is the usual way that  PCRE  is
       compiled. The output is zero if PCRE was compiled to use blocks of data
       on the  heap  instead  of  recursive  function  calls.  In  this  case,
       pcre_stack_malloc  and  pcre_stack_free  are  called  to  manage memory
       blocks on the heap, thus avoiding the use of the stack.

COMPILING A PATTERN

pcre *pcre_compile(const char *pattern, int options,
            const char **errptr, int *erroffset,
            const unsigned char *tableptr);

pcre *pcre_compile2(const char *pattern, int options,
            int *errorcodeptr,
            const char **errptr, int *erroffset,
            const unsigned char *tableptr);

Either of the functions pcre_compile() or pcre_compile2() can be called
       to compile a pattern into an internal form. The only difference between
       the two interfaces is that pcre_compile2() has an additional  argument,
       errorcodeptr, via which a numerical error code can be returned.

The pattern is a C string terminated by a binary zero, and is passed in
       the pattern argument. A pointer to a single block  of  memory  that  is
       obtained  via  pcre_malloc is returned. This contains the compiled code
       and related data. The pcre type is defined for the returned block; this
       is a typedef for a structure whose contents are not externally defined.
       It is up to the caller to free the memory (via pcre_free) when it is no
       longer required.

Although  the compiled code of a PCRE regex is relocatable, that is, it
       does not depend on memory location, the complete pcre data block is not
       fully  relocatable, because it may contain a copy of the tableptr argu-
       ment, which is an address (see below).

The options argument contains various bit settings that affect the com-
       pilation.  It  should be zero if no options are required. The available
       options are described below. Some of them, in  particular,  those  that
       are  compatible  with  Perl,  can also be set and unset from within the
       pattern (see the detailed description  in  the  pcrepattern  documenta-
       tion).  For  these options, the contents of the options argument speci-
       fies their initial settings at the start of compilation and  execution.
       The  PCRE_ANCHORED  and PCRE_NEWLINE_xxx options can be set at the time
       of matching as well as at compile time.

If errptr is NULL, pcre_compile() returns NULL immediately.  Otherwise,
       if  compilation  of  a  pattern fails, pcre_compile() returns NULL, and
       sets the variable pointed to by errptr to point to a textual error mes-
       sage. This is a static string that is part of the library. You must not
       try to free it. The offset from the start of the pattern to the charac-
       ter where the error was discovered is placed in the variable pointed to
       by erroffset, which must not be NULL. If it is, an immediate  error  is
       given.

If  pcre_compile2()  is  used instead of pcre_compile(), and the error-
       codeptr argument is not NULL, a non-zero error code number is  returned
       via  this argument in the event of an error. This is in addition to the
       textual error message. Error codes and messages are listed below.

If the final argument, tableptr, is NULL, PCRE uses a  default  set  of
       character  tables  that  are  built  when  PCRE  is compiled, using the
       default C locale. Otherwise, tableptr must be an address  that  is  the
       result  of  a  call to pcre_maketables(). This value is stored with the
       compiled pattern, and used again by pcre_exec(), unless  another  table
       pointer is passed to it. For more discussion, see the section on locale
       support below.

This code fragment shows a typical straightforward  call  to  pcre_com-
       pile():

pcre *re;
         const char *error;
         int erroffset;
         re = pcre_compile(
           "^A.*Z",          /* the pattern */
           0,                /* default options */
           &error,           /* for error message */
           &erroffset,       /* for error offset */
           NULL);            /* use default character tables */

The  following  names  for option bits are defined in the pcre.h header
       file:

PCRE_ANCHORED

If this bit is set, the pattern is forced to be "anchored", that is, it
       is  constrained to match only at the first matching point in the string
       that is being searched (the "subject string"). This effect can also  be
       achieved  by appropriate constructs in the pattern itself, which is the
       only way to do it in Perl.

PCRE_AUTO_CALLOUT

If this bit is set, pcre_compile() automatically inserts callout items,
       all  with  number  255, before each pattern item. For discussion of the
       callout facility, see the pcrecallout documentation.

PCRE_BSR_ANYCRLF
         PCRE_BSR_UNICODE

These options (which are mutually exclusive) control what the \R escape
       sequence  matches.  The choice is either to match only CR, LF, or CRLF,
       or to match any Unicode newline sequence. The default is specified when
       PCRE is built. It can be overridden from within the pattern, or by set-
       ting an option when a compiled pattern is matched.

PCRE_CASELESS

If this bit is set, letters in the pattern match both upper  and  lower
       case  letters.  It  is  equivalent  to  Perl's /i option, and it can be
       changed within a pattern by a (?i) option setting. In UTF-8 mode,  PCRE
       always  understands the concept of case for characters whose values are
       less than 128, so caseless matching is always possible. For  characters
       with  higher  values,  the concept of case is supported if PCRE is com-
       piled with Unicode property support, but not otherwise. If you want  to
       use  caseless  matching  for  characters 128 and above, you must ensure
       that PCRE is compiled with Unicode property support  as  well  as  with
       UTF-8 support.

PCRE_DOLLAR_ENDONLY

If  this bit is set, a dollar metacharacter in the pattern matches only
       at the end of the subject string. Without this option,  a  dollar  also
       matches  immediately before a newline at the end of the string (but not
       before any other newlines). The PCRE_DOLLAR_ENDONLY option  is  ignored
       if  PCRE_MULTILINE  is  set.   There is no equivalent to this option in
       Perl, and no way to set it within a pattern.

PCRE_DOTALL

If this bit is set, a dot metacharater in the pattern matches all char-
       acters,  including  those that indicate newline. Without it, a dot does
       not match when the current position is at a  newline.  This  option  is
       equivalent  to Perl's /s option, and it can be changed within a pattern
       by a (?s) option setting. A negative class such as [^a] always  matches
       newline characters, independent of the setting of this option.

PCRE_DUPNAMES

If  this  bit is set, names used to identify capturing subpatterns need
       not be unique. This can be helpful for certain types of pattern when it
       is  known  that  only  one instance of the named subpattern can ever be
       matched. There are more details of named subpatterns  below;  see  also
       the pcrepattern documentation.

PCRE_EXTENDED

If  this  bit  is  set,  whitespace  data characters in the pattern are
       totally ignored except when escaped or inside a character class. White-
       space does not include the VT character (code 11). In addition, charac-
       ters between an unescaped # outside a character class and the next new-
       line,  inclusive,  are  also  ignored.  This is equivalent to Perl's /x
       option, and it can be changed within a pattern by a  (?x)  option  set-
       ting.

This  option  makes  it possible to include comments inside complicated
       patterns.  Note, however, that this applies only  to  data  characters.
       Whitespace   characters  may  never  appear  within  special  character
       sequences in a pattern, for  example  within  the  sequence  (?(  which
       introduces a conditional subpattern.

PCRE_EXTRA

This  option  was invented in order to turn on additional functionality
       of PCRE that is incompatible with Perl, but it  is  currently  of  very
       little  use. When set, any backslash in a pattern that is followed by a
       letter that has no special meaning  causes  an  error,  thus  reserving
       these  combinations  for  future  expansion.  By default, as in Perl, a
       backslash followed by a letter with no special meaning is treated as  a
       literal.  (Perl can, however, be persuaded to give a warning for this.)
       There are at present no other features controlled by  this  option.  It
       can also be set by a (?X) option setting within a pattern.

PCRE_FIRSTLINE

If  this  option  is  set,  an  unanchored pattern is required to match
       before or at the first  newline  in  the  subject  string,  though  the
       matched text may continue over the newline.

PCRE_JAVASCRIPT_COMPAT

If this option is set, PCRE's behaviour is changed in some ways so that
       it is compatible with JavaScript rather than Perl. The changes  are  as
       follows:

(1)  A  lone  closing square bracket in a pattern causes a compile-time
       error, because this is illegal in JavaScript (by default it is  treated
       as a data character). Thus, the pattern AB]CD becomes illegal when this
       option is set.

(2) At run time, a back reference to an unset subpattern group  matches
       an  empty  string (by default this causes the current matching alterna-
       tive to fail). A pattern such as (\1)(a) succeeds when this  option  is
       set  (assuming  it can find an "a" in the subject), whereas it fails by
       default, for Perl compatibility.

PCRE_MULTILINE

By default, PCRE treats the subject string as consisting  of  a  single
       line  of characters (even if it actually contains newlines). The "start
       of line" metacharacter (^) matches only at the  start  of  the  string,
       while  the  "end  of line" metacharacter ($) matches only at the end of
       the string, or before a terminating newline (unless PCRE_DOLLAR_ENDONLY
       is set). This is the same as Perl.

When  PCRE_MULTILINE  it  is set, the "start of line" and "end of line"
       constructs match immediately following or immediately  before  internal
       newlines  in  the  subject string, respectively, as well as at the very
       start and end. This is equivalent to Perl's /m option, and  it  can  be
       changed within a pattern by a (?m) option setting. If there are no new-
       lines in a subject string, or no occurrences of ^ or $  in  a  pattern,
       setting PCRE_MULTILINE has no effect.

PCRE_NEWLINE_CR
         PCRE_NEWLINE_LF
         PCRE_NEWLINE_CRLF
         PCRE_NEWLINE_ANYCRLF
         PCRE_NEWLINE_ANY

These  options  override the default newline definition that was chosen
       when PCRE was built. Setting the first or the second specifies  that  a
       newline  is  indicated  by a single character (CR or LF, respectively).
       Setting PCRE_NEWLINE_CRLF specifies that a newline is indicated by  the
       two-character  CRLF  sequence.  Setting  PCRE_NEWLINE_ANYCRLF specifies
       that any of the three preceding sequences should be recognized. Setting
       PCRE_NEWLINE_ANY  specifies that any Unicode newline sequence should be
       recognized. The Unicode newline sequences are the three just mentioned,
       plus  the  single  characters  VT (vertical tab, U+000B), FF (formfeed,
       U+000C), NEL (next line, U+0085), LS (line separator, U+2028),  and  PS
       (paragraph  separator,  U+2029).  The  last  two are recognized only in
       UTF-8 mode.

The newline setting in the  options  word  uses  three  bits  that  are
       treated as a number, giving eight possibilities. Currently only six are
       used (default plus the five values above). This means that if  you  set
       more  than one newline option, the combination may or may not be sensi-
       ble. For example, PCRE_NEWLINE_CR with PCRE_NEWLINE_LF is equivalent to
       PCRE_NEWLINE_CRLF,  but other combinations may yield unused numbers and
       cause an error.

The only time that a line break is specially recognized when  compiling
       a  pattern  is  if  PCRE_EXTENDED  is set, and an unescaped # outside a
       character class is encountered. This indicates  a  comment  that  lasts
       until  after the next line break sequence. In other circumstances, line
       break  sequences  are  treated  as  literal  data,   except   that   in
       PCRE_EXTENDED mode, both CR and LF are treated as whitespace characters
       and are therefore ignored.

The newline option that is set at compile time becomes the default that
       is  used for pcre_exec() and pcre_dfa_exec(), but it can be overridden.

PCRE_NO_AUTO_CAPTURE

If this option is set, it disables the use of numbered capturing paren-
       theses  in the pattern. Any opening parenthesis that is not followed by
       ? behaves as if it were followed by ?: but named parentheses can  still
       be  used  for  capturing  (and  they acquire numbers in the usual way).
       There is no equivalent of this option in Perl.

PCRE_UNGREEDY

This option inverts the "greediness" of the quantifiers  so  that  they
       are  not greedy by default, but become greedy if followed by "?". It is
       not compatible with Perl. It can also be set by a (?U)  option  setting
       within the pattern.

PCRE_UTF8

This  option  causes PCRE to regard both the pattern and the subject as
       strings of UTF-8 characters instead of single-byte  character  strings.
       However,  it is available only when PCRE is built to include UTF-8 sup-
       port. If not, the use of this option provokes an error. Details of  how
       this  option  changes the behaviour of PCRE are given in the section on
       UTF-8 support in the main pcre page.

PCRE_NO_UTF8_CHECK

When PCRE_UTF8 is set, the validity of the pattern as a UTF-8 string is
       automatically  checked.  There  is  a  discussion about the validity of
       UTF-8 strings in the main pcre page. If an invalid  UTF-8  sequence  of
       bytes  is  found,  pcre_compile() returns an error. If you already know
       that your pattern is valid, and you want to skip this check for perfor-
       mance  reasons,  you  can set the PCRE_NO_UTF8_CHECK option. When it is
       set, the effect of passing an invalid UTF-8  string  as  a  pattern  is
       undefined.  It  may  cause your program to crash. Note that this option
       can also be passed to pcre_exec() and pcre_dfa_exec(), to suppress  the
       UTF-8 validity checking of subject strings.

COMPILATION ERROR CODES

The  following  table  lists  the  error  codes than may be returned by
       pcre_compile2(), along with the error messages that may be returned  by
       both  compiling functions. As PCRE has developed, some error codes have
       fallen out of use. To avoid confusion, they have not been re-used.

0  no error
          1  \ at end of pattern
          2  \c at end of pattern
          3  unrecognized character follows \
          4  numbers out of order in {} quantifier
          5  number too big in {} quantifier
          6  missing terminating ] for character class
          7  invalid escape sequence in character class
          8  range out of order in character class
          9  nothing to repeat
         10  [this code is not in use]
         11  internal error: unexpected repeat
         12  unrecognized character after (? or (?-
         13  POSIX named classes are supported only within a class
         14  missing )
         15  reference to non-existent subpattern
         16  erroffset passed as NULL
         17  unknown option bit(s) set
         18  missing ) after comment
         19  [this code is not in use]
         20  regular expression is too large
         21  failed to get memory
         22  unmatched parentheses
         23  internal error: code overflow
         24  unrecognized character after (?<
         25  lookbehind assertion is not fixed length
         26  malformed number or name after (?(
         27  conditional group contains more than two branches
         28  assertion expected after (?(
         29  (?R or (?[+-]digits must be followed by )
         30  unknown POSIX class name
         31  POSIX collating elements are not supported
         32  this version of PCRE is not compiled with PCRE_UTF8 support
         33  [this code is not in use]
         34  character value in \x{...} sequence is too large
         35  invalid condition (?(0)
         36  \C not allowed in lookbehind assertion
         37  PCRE does not support \L, \l, \N, \U, or \u
         38  number after (?C is > 255
         39  closing ) for (?C expected
         40  recursive call could loop indefinitely
         41  unrecognized character after (?P
         42  syntax error in subpattern name (missing terminator)
         43  two named subpatterns have the same name
         44  invalid UTF-8 string
         45  support for \P, \p, and \X has not been compiled
         46  malformed \P or \p sequence
         47  unknown property name after \P or \p
         48  subpattern name is too long (maximum 32 characters)
         49  too many named subpatterns (maximum 10000)
         50  [this code is not in use]
         51  octal value is greater than \377 (not in UTF-8 mode)
         52  internal error: overran compiling workspace
         53  internal  error:  previously-checked  referenced  subpattern  not
       found
         54  DEFINE group contains more than one branch
         55  repeating a DEFINE group is not allowed
         56  inconsistent NEWLINE options
         57  \g is not followed by a braced, angle-bracketed, or quoted
               name/number or by a plain number
         58  a numbered reference must not be zero
         59  (*VERB) with an argument is not supported
         60  (*VERB) not recognized
         61  number is too big
         62  subpattern name expected
         63  digit expected after (?+
         64  ] is an invalid data character in JavaScript compatibility mode

The  numbers  32  and 10000 in errors 48 and 49 are defaults; different
       values may be used if the limits were changed when PCRE was built.

STUDYING A PATTERN

pcre_extra *pcre_study(const pcre *code, int options
            const char **errptr);

If a compiled pattern is going to be used several times,  it  is  worth
       spending more time analyzing it in order to speed up the time taken for
       matching. The function pcre_study() takes a pointer to a compiled  pat-
       tern as its first argument. If studying the pattern produces additional
       information that will help speed up matching,  pcre_study()  returns  a
       pointer  to a pcre_extra block, in which the study_data field points to
       the results of the study.

The  returned  value  from  pcre_study()  can  be  passed  directly  to
       pcre_exec().  However,  a  pcre_extra  block also contains other fields
       that can be set by the caller before the block  is  passed;  these  are
       described below in the section on matching a pattern.

If  studying  the  pattern  does not produce any additional information
       pcre_study() returns NULL. In that circumstance, if the calling program
       wants  to  pass  any of the other fields to pcre_exec(), it must set up
       its own pcre_extra block.

The second argument of pcre_study() contains option bits.  At  present,
       no options are defined, and this argument should always be zero.

The  third argument for pcre_study() is a pointer for an error message.
       If studying succeeds (even if no data is  returned),  the  variable  it
       points  to  is  set  to NULL. Otherwise it is set to point to a textual
       error message. This is a static string that is part of the library. You
       must  not  try  to  free it. You should test the error pointer for NULL
       after calling pcre_study(), to be sure that it has run successfully.

This is a typical call to pcre_study():

pcre_extra *pe;
         pe = pcre_study(
           re,             /* result of pcre_compile() */
           0,              /* no options exist */
           &error);        /* set to NULL or points to a message */

At present, studying a pattern is useful only for non-anchored patterns
       that  do not have a single fixed starting character. A bitmap of possi-
       ble starting bytes is created.

LOCALE SUPPORT

PCRE handles caseless matching, and determines whether  characters  are
       letters,  digits, or whatever, by reference to a set of tables, indexed
       by character value. When running in UTF-8 mode, this  applies  only  to
       characters  with  codes  less than 128. Higher-valued codes never match
       escapes such as \w or \d, but can be tested with \p if  PCRE  is  built
       with  Unicode  character property support. The use of locales with Uni-
       code is discouraged. If you are handling characters with codes  greater
       than  128, you should either use UTF-8 and Unicode, or use locales, but
       not try to mix the two.

PCRE contains an internal set of tables that are used  when  the  final
       argument  of  pcre_compile()  is  NULL.  These  are sufficient for many
       applications.  Normally, the internal tables recognize only ASCII char-
       acters. However, when PCRE is built, it is possible to cause the inter-
       nal tables to be rebuilt in the default "C" locale of the local system,
       which may cause them to be different.

The  internal tables can always be overridden by tables supplied by the
       application that calls PCRE. These may be created in a different locale
       from  the  default.  As more and more applications change to using Uni-
       code, the need for this locale support is expected to die away.

External tables are built by calling  the  pcre_maketables()  function,
       which  has no arguments, in the relevant locale. The result can then be
       passed to pcre_compile() or pcre_exec()  as  often  as  necessary.  For
       example,  to  build  and use tables that are appropriate for the French
       locale (where accented characters with  values  greater  than  128  are
       treated as letters), the following code could be used:

setlocale(LC_CTYPE, "fr_FR");
         tables = pcre_maketables();
         re = pcre_compile(..., tables);

The  locale  name "fr_FR" is used on Linux and other Unix-like systems;
       if you are using Windows, the name for the French locale is "french".

When pcre_maketables() runs, the tables are built  in  memory  that  is
       obtained  via  pcre_malloc. It is the caller's responsibility to ensure
       that the memory containing the tables remains available for as long  as
       it is needed.

The pointer that is passed to pcre_compile() is saved with the compiled
       pattern, and the same tables are used via this pointer by  pcre_study()
       and normally also by pcre_exec(). Thus, by default, for any single pat-
       tern, compilation, studying and matching all happen in the same locale,
       but different patterns can be compiled in different locales.

It  is  possible to pass a table pointer or NULL (indicating the use of
       the internal tables) to pcre_exec(). Although  not  intended  for  this
       purpose,  this facility could be used to match a pattern in a different
       locale from the one in which it was compiled. Passing table pointers at
       run time is discussed below in the section on matching a pattern.

INFORMATION ABOUT A PATTERN

int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
            int what, void *where);

The  pcre_fullinfo() function returns information about a compiled pat-
       tern. It replaces the obsolete pcre_info() function, which is neverthe-
       less retained for backwards compability (and is documented below).

The  first  argument  for  pcre_fullinfo() is a pointer to the compiled
       pattern. The second argument is the result of pcre_study(), or NULL  if
       the  pattern  was not studied. The third argument specifies which piece
       of information is required, and the fourth argument is a pointer  to  a
       variable  to  receive  the  data. The yield of the function is zero for
       success, or one of the following negative numbers:

PCRE_ERROR_NULL       the argument code was NULL
                               the argument where was NULL
         PCRE_ERROR_BADMAGIC   the "magic number" was not found
         PCRE_ERROR_BADOPTION  the value of what was invalid

The "magic number" is placed at the start of each compiled  pattern  as
       an  simple check against passing an arbitrary memory pointer. Here is a
       typical call of pcre_fullinfo(), to obtain the length of  the  compiled
       pattern:

int rc;
         size_t length;
         rc = pcre_fullinfo(
           re,               /* result of pcre_compile() */
           pe,               /* result of pcre_study(), or NULL */
           PCRE_INFO_SIZE,   /* what is required */
           &length);         /* where to put the data */

The  possible  values for the third argument are defined in pcre.h, and
       are as follows:

PCRE_INFO_BACKREFMAX

Return the number of the highest back reference  in  the  pattern.  The
       fourth  argument  should  point to an int variable. Zero is returned if
       there are no back references.

PCRE_INFO_CAPTURECOUNT

Return the number of capturing subpatterns in the pattern.  The  fourth
       argument should point to an int variable.

PCRE_INFO_DEFAULT_TABLES

Return  a pointer to the internal default character tables within PCRE.
       The fourth argument should point to an unsigned char *  variable.  This
       information call is provided for internal use by the pcre_study() func-
       tion. External callers can cause PCRE to use  its  internal  tables  by
       passing a NULL table pointer.

PCRE_INFO_FIRSTBYTE

Return  information  about  the first byte of any matched string, for a
       non-anchored pattern. The fourth argument should point to an int  vari-
       able.  (This option used to be called PCRE_INFO_FIRSTCHAR; the old name
       is still recognized for backwards compatibility.)

If there is a fixed first byte, for example, from  a  pattern  such  as
       (cat|cow|coyote), its value is returned. Otherwise, if either

(a)  the pattern was compiled with the PCRE_MULTILINE option, and every
       branch starts with "^", or

(b) every branch of the pattern starts with ".*" and PCRE_DOTALL is not
       set (if it were set, the pattern would be anchored),

-1  is  returned, indicating that the pattern matches only at the start
       of a subject string or after any newline within the  string.  Otherwise
       -2 is returned. For anchored patterns, -2 is returned.

PCRE_INFO_FIRSTTABLE

If  the pattern was studied, and this resulted in the construction of a
       256-bit table indicating a fixed set of bytes for the first byte in any
       matching  string, a pointer to the table is returned. Otherwise NULL is
       returned. The fourth argument should point to an unsigned char *  vari-
       able.

PCRE_INFO_HASCRORLF

Return  1  if  the  pattern  contains any explicit matches for CR or LF
       characters, otherwise 0. The fourth argument should  point  to  an  int
       variable.  An explicit match is either a literal CR or LF character, or
       \r or \n.

PCRE_INFO_JCHANGED

Return 1 if the (?J) or (?-J) option setting is used  in  the  pattern,
       otherwise  0. The fourth argument should point to an int variable. (?J)
       and (?-J) set and unset the local PCRE_DUPNAMES option, respectively.

PCRE_INFO_LASTLITERAL

Return the value of the rightmost literal byte that must exist  in  any
       matched  string,  other  than  at  its  start,  if such a byte has been
       recorded. The fourth argument should point to an int variable. If there
       is  no such byte, -1 is returned. For anchored patterns, a last literal
       byte is recorded only if it follows something of variable  length.  For
       example, for the pattern /^a\d+z\d+/ the returned value is "z", but for
       /^a\dz\d/ the returned value is -1.

PCRE_INFO_NAMECOUNT
         PCRE_INFO_NAMEENTRYSIZE
         PCRE_INFO_NAMETABLE

PCRE supports the use of named as well as numbered capturing  parenthe-
       ses.  The names are just an additional way of identifying the parenthe-
       ses, which still acquire numbers. Several convenience functions such as
       pcre_get_named_substring()  are  provided  for extracting captured sub-
       strings by name. It is also possible to extract the data  directly,  by
       first  converting  the  name to a number in order to access the correct
       pointers in the output vector (described with pcre_exec() below). To do
       the  conversion,  you  need  to  use  the  name-to-number map, which is
       described by these three values.

The map consists of a number of fixed-size entries. PCRE_INFO_NAMECOUNT
       gives the number of entries, and PCRE_INFO_NAMEENTRYSIZE gives the size
       of each entry; both of these  return  an  int  value.  The  entry  size
       depends  on the length of the longest name. PCRE_INFO_NAMETABLE returns
       a pointer to the first entry of the table  (a  pointer  to  char).  The
       first two bytes of each entry are the number of the capturing parenthe-
       sis, most significant byte first. The rest of the entry is  the  corre-
       sponding  name,  zero  terminated. The names are in alphabetical order.
       When PCRE_DUPNAMES is set, duplicate names are in order of their paren-
       theses  numbers.  For  example,  consider the following pattern (assume
       PCRE_EXTENDED is  set,  so  white  space  -  including  newlines  -  is
       ignored):

(?<date> (?<year>(\d\d)?\d\d) -
         (?<month>\d\d) - (?<day>\d\d) )

There  are  four  named subpatterns, so the table has four entries, and
       each entry in the table is eight bytes long. The table is  as  follows,
       with non-printing bytes shows in hexadecimal, and undefined bytes shown
       as ??:

00 01 d  a  t  e  00 ??
         00 05 d  a  y  00 ?? ??
         00 04 m  o  n  t  h  00
         00 02 y  e  a  r  00 ??

When writing code to extract data  from  named  subpatterns  using  the
       name-to-number  map,  remember that the length of the entries is likely
       to be different for each compiled pattern.

PCRE_INFO_OKPARTIAL

Return 1 if the pattern can be used for partial matching, otherwise  0.
       The  fourth  argument  should point to an int variable. The pcrepartial
       documentation lists the restrictions that apply to patterns  when  par-
       tial matching is used.

PCRE_INFO_OPTIONS

Return  a  copy of the options with which the pattern was compiled. The
       fourth argument should point to an unsigned long  int  variable.  These
       option bits are those specified in the call to pcre_compile(), modified
       by any top-level option settings at the start of the pattern itself. In
       other  words,  they are the options that will be in force when matching
       starts. For example, if the pattern /(?im)abc(?-i)d/ is  compiled  with
       the  PCRE_EXTENDED option, the result is PCRE_CASELESS, PCRE_MULTILINE,
       and PCRE_EXTENDED.

A pattern is automatically anchored by PCRE if  all  of  its  top-level
       alternatives begin with one of the following:

^     unless PCRE_MULTILINE is set
         \A    always
         \G    always
         .*    if PCRE_DOTALL is set and there are no back
                 references to the subpattern in which .* appears

For such patterns, the PCRE_ANCHORED bit is set in the options returned
       by pcre_fullinfo().

PCRE_INFO_SIZE

Return the size of the compiled pattern, that is, the  value  that  was
       passed as the argument to pcre_malloc() when PCRE was getting memory in
       which to place the compiled data. The fourth argument should point to a
       size_t variable.

PCRE_INFO_STUDYSIZE

Return the size of the data block pointed to by the study_data field in
       a pcre_extra block. That is,  it  is  the  value  that  was  passed  to
       pcre_malloc() when PCRE was getting memory into which to place the data
       created by pcre_study(). The fourth argument should point to  a  size_t
       variable.

OBSOLETE INFO FUNCTION

int pcre_info(const pcre *code, int *optptr, int *firstcharptr);

The  pcre_info()  function is now obsolete because its interface is too
       restrictive to return all the available data about a compiled  pattern.
       New   programs   should  use  pcre_fullinfo()  instead.  The  yield  of
       pcre_info() is the number of capturing subpatterns, or one of the  fol-
       lowing negative numbers:

PCRE_ERROR_NULL       the argument code was NULL
         PCRE_ERROR_BADMAGIC   the "magic number" was not found

If  the  optptr  argument is not NULL, a copy of the options with which
       the pattern was compiled is placed in the integer  it  points  to  (see
       PCRE_INFO_OPTIONS above).

If  the  pattern  is  not anchored and the firstcharptr argument is not
       NULL, it is used to pass back information about the first character  of
       any matched string (see PCRE_INFO_FIRSTBYTE above).

REFERENCE COUNTS

int pcre_refcount(pcre *code, int adjust);

The  pcre_refcount()  function is used to maintain a reference count in
       the data block that contains a compiled pattern. It is provided for the
       benefit  of  applications  that  operate  in an object-oriented manner,
       where different parts of the application may be using the same compiled
       pattern, but you want to free the block when they are all done.

When a pattern is compiled, the reference count field is initialized to
       zero.  It is changed only by calling this function, whose action is  to
       add  the  adjust  value  (which may be positive or negative) to it. The
       yield of the function is the new value. However, the value of the count
       is  constrained to lie between 0 and 65535, inclusive. If the new value
       is outside these limits, it is forced to the appropriate limit value.

Except when it is zero, the reference count is not correctly  preserved
       if  a  pattern  is  compiled on one host and then transferred to a host
       whose byte-order is different. (This seems a highly unlikely scenario.)

MATCHING A PATTERN: THE TRADITIONAL FUNCTION

int pcre_exec(const pcre *code, const pcre_extra *extra,
            const char *subject, int length, int startoffset,
            int options, int *ovector, int ovecsize);

The  function pcre_exec() is called to match a subject string against a
       compiled pattern, which is passed in the code argument. If the  pattern
       has been studied, the result of the study should be passed in the extra
       argument. This function is the main matching facility of  the  library,
       and it operates in a Perl-like manner. For specialist use there is also
       an alternative matching function, which is described below in the  sec-
       tion about the pcre_dfa_exec() function.

In  most applications, the pattern will have been compiled (and option-
       ally studied) in the same process that calls pcre_exec().  However,  it
       is possible to save compiled patterns and study data, and then use them
       later in different processes, possibly even on different hosts.  For  a
       discussion about this, see the pcreprecompile documentation.

Here is an example of a simple call to pcre_exec():

int rc;
         int ovector[30];
         rc = pcre_exec(
           re,             /* result of pcre_compile() */
           NULL,           /* we didn't study the pattern */
           "some string",  /* the subject string */
           11,             /* the length of the subject string */
           0,              /* start at offset 0 in the subject */
           0,              /* default options */
           ovector,        /* vector of integers for substring information */
           30);            /* number of elements (NOT size in bytes) */

Extra data for pcre_exec()

If  the  extra argument is not NULL, it must point to a pcre_extra data
       block. The pcre_study() function returns such a block (when it  doesn't
       return  NULL), but you can also create one for yourself, and pass addi-
       tional information in it. The pcre_extra block contains  the  following
       fields (not necessarily in this order):

unsigned long int flags;
         void *study_data;
         unsigned long int match_limit;
         unsigned long int match_limit_recursion;
         void *callout_data;
         const unsigned char *tables;

The  flags  field  is a bitmap that specifies which of the other fields
       are set. The flag bits are:

PCRE_EXTRA_STUDY_DATA
         PCRE_EXTRA_MATCH_LIMIT
         PCRE_EXTRA_MATCH_LIMIT_RECURSION
         PCRE_EXTRA_CALLOUT_DATA
         PCRE_EXTRA_TABLES

Other flag bits should be set to zero. The study_data field is  set  in
       the  pcre_extra  block  that is returned by pcre_study(), together with
       the appropriate flag bit. You should not set this yourself, but you may
       add  to  the  block by setting the other fields and their corresponding
       flag bits.

The match_limit field provides a means of preventing PCRE from using up
       a  vast amount of resources when running patterns that are not going to
       match, but which have a very large number  of  possibilities  in  their
       search  trees.  The  classic  example  is  the  use of nested unlimited
       repeats.

Internally, PCRE uses a function called match() which it calls  repeat-
       edly  (sometimes  recursively). The limit set by match_limit is imposed
       on the number of times this function is called during  a  match,  which
       has  the  effect  of  limiting the amount of backtracking that can take
       place. For patterns that are not anchored, the count restarts from zero
       for each position in the subject string.

The  default  value  for  the  limit can be set when PCRE is built; the
       default default is 10 million, which handles all but the  most  extreme
       cases.  You  can  override  the  default by suppling pcre_exec() with a
       pcre_extra    block    in    which    match_limit    is    set,     and
       PCRE_EXTRA_MATCH_LIMIT  is  set  in  the  flags  field. If the limit is
       exceeded, pcre_exec() returns PCRE_ERROR_MATCHLIMIT.

The match_limit_recursion field is similar to match_limit, but  instead
       of limiting the total number of times that match() is called, it limits
       the depth of recursion. The recursion depth is a  smaller  number  than
       the  total number of calls, because not all calls to match() are recur-
       sive.  This limit is of use only if it is set smaller than match_limit.

Limiting  the  recursion  depth  limits the amount of stack that can be
       used, or, when PCRE has been compiled to use memory on the heap instead
       of the stack, the amount of heap memory that can be used.

The  default  value  for  match_limit_recursion can be set when PCRE is
       built; the default default  is  the  same  value  as  the  default  for
       match_limit.  You can override the default by suppling pcre_exec() with
       a  pcre_extra  block  in  which  match_limit_recursion  is   set,   and
       PCRE_EXTRA_MATCH_LIMIT_RECURSION  is  set  in  the  flags field. If the
       limit is exceeded, pcre_exec() returns PCRE_ERROR_RECURSIONLIMIT.

The pcre_callout field is used in conjunction with the  "callout"  fea-
       ture, which is described in the pcrecallout documentation.

The  tables  field  is  used  to  pass  a  character  tables pointer to
       pcre_exec(); this overrides the value that is stored with the  compiled
       pattern.  A  non-NULL value is stored with the compiled pattern only if
       custom tables were supplied to pcre_compile() via  its  tableptr  argu-
       ment.  If NULL is passed to pcre_exec() using this mechanism, it forces
       PCRE's internal tables to be used. This facility is  helpful  when  re-
       using  patterns  that  have been saved after compiling with an external
       set of tables, because the external tables  might  be  at  a  different
       address  when  pcre_exec() is called. See the pcreprecompile documenta-
       tion for a discussion of saving compiled patterns for later use.

Option bits for pcre_exec()

The unused bits of the options argument for pcre_exec() must  be  zero.
       The  only  bits  that  may  be set are PCRE_ANCHORED, PCRE_NEWLINE_xxx,
       PCRE_NOTBOL,   PCRE_NOTEOL,   PCRE_NOTEMPTY,   PCRE_NO_UTF8_CHECK   and
       PCRE_PARTIAL.

PCRE_ANCHORED

The  PCRE_ANCHORED  option  limits pcre_exec() to matching at the first
       matching position. If a pattern was  compiled  with  PCRE_ANCHORED,  or
       turned  out to be anchored by virtue of its contents, it cannot be made
       unachored at matching time.

PCRE_BSR_ANYCRLF
         PCRE_BSR_UNICODE

These options (which are mutually exclusive) control what the \R escape
       sequence  matches.  The choice is either to match only CR, LF, or CRLF,
       or to match any Unicode newline sequence. These  options  override  the
       choice that was made or defaulted when the pattern was compiled.

PCRE_NEWLINE_CR
         PCRE_NEWLINE_LF
         PCRE_NEWLINE_CRLF
         PCRE_NEWLINE_ANYCRLF
         PCRE_NEWLINE_ANY

These  options  override  the  newline  definition  that  was chosen or
       defaulted when the pattern was compiled. For details, see the  descrip-
       tion  of  pcre_compile()  above.  During  matching,  the newline choice
       affects the behaviour of the dot, circumflex,  and  dollar  metacharac-
       ters.  It may also alter the way the match position is advanced after a
       match failure for an unanchored pattern.

When PCRE_NEWLINE_CRLF, PCRE_NEWLINE_ANYCRLF,  or  PCRE_NEWLINE_ANY  is
       set,  and a match attempt for an unanchored pattern fails when the cur-
       rent position is at a  CRLF  sequence,  and  the  pattern  contains  no
       explicit  matches  for  CR  or  LF  characters,  the  match position is
       advanced by two characters instead of one, in other words, to after the
       CRLF.

The above rule is a compromise that makes the most common cases work as
       expected. For example, if the  pattern  is  .+A  (and  the  PCRE_DOTALL
       option is not set), it does not match the string "\r\nA" because, after
       failing at the start, it skips both the CR and the LF before  retrying.
       However,  the  pattern  [\r\n]A does match that string, because it con-
       tains an explicit CR or LF reference, and so advances only by one char-
       acter after the first failure.

An explicit match for CR of LF is either a literal appearance of one of
       those characters, or one of the \r or  \n  escape  sequences.  Implicit
       matches  such  as [^X] do not count, nor does \s (which includes CR and
       LF in the characters that it matches).

Notwithstanding the above, anomalous effects may still occur when  CRLF
       is a valid newline sequence and explicit \r or \n escapes appear in the
       pattern.

PCRE_NOTBOL

This option specifies that first character of the subject string is not
       the  beginning  of  a  line, so the circumflex metacharacter should not
       match before it. Setting this without PCRE_MULTILINE (at compile  time)
       causes  circumflex  never to match. This option affects only the behav-
       iour of the circumflex metacharacter. It does not affect \A.

PCRE_NOTEOL

This option specifies that the end of the subject string is not the end
       of  a line, so the dollar metacharacter should not match it nor (except
       in multiline mode) a newline immediately before it. Setting this  with-
       out PCRE_MULTILINE (at compile time) causes dollar never to match. This
       option affects only the behaviour of the dollar metacharacter. It  does
       not affect \Z or \z.

PCRE_NOTEMPTY

An empty string is not considered to be a valid match if this option is
       set. If there are alternatives in the pattern, they are tried.  If  all
       the  alternatives  match  the empty string, the entire match fails. For
       example, if the pattern

a?b?

is applied to a string not beginning with "a" or "b",  it  matches  the
       empty  string at the start of the subject. With PCRE_NOTEMPTY set, this
       match is not valid, so PCRE searches further into the string for occur-
       rences of "a" or "b".

Perl has no direct equivalent of PCRE_NOTEMPTY, but it does make a spe-
       cial case of a pattern match of the empty  string  within  its  split()
       function,  and  when  using  the /g modifier. It is possible to emulate
       Perl's behaviour after matching a null string by first trying the match
       again at the same offset with PCRE_NOTEMPTY and PCRE_ANCHORED, and then
       if that fails by advancing the starting offset (see below)  and  trying
       an ordinary match again. There is some code that demonstrates how to do
       this in the pcredemo.c sample program.

PCRE_NO_UTF8_CHECK

When PCRE_UTF8 is set at compile time, the validity of the subject as a
       UTF-8 string is automatically checked when pcre_exec() is  subsequently
       called.   The  value  of  startoffset is also checked to ensure that it
       points to the start of a UTF-8 character. There is a  discussion  about
       the  validity  of  UTF-8 strings in the section on UTF-8 support in the
       main pcre page. If  an  invalid  UTF-8  sequence  of  bytes  is  found,
       pcre_exec()  returns  the error PCRE_ERROR_BADUTF8. If startoffset con-
       tains a value that does not point to the start of a UTF-8 character (or
       to the end of the subject), PCRE_ERROR_BADUTF8_OFFSET is returned.

If  you  already  know that your subject is valid, and you want to skip
       these   checks   for   performance   reasons,   you   can    set    the
       PCRE_NO_UTF8_CHECK  option  when calling pcre_exec(). You might want to
       do this for the second and subsequent calls to pcre_exec() if  you  are
       making  repeated  calls  to  find  all  the matches in a single subject
       string. However, you should be  sure  that  the  value  of  startoffset
       points  to  the start of a UTF-8 character (or the end of the subject).
       When PCRE_NO_UTF8_CHECK is set, the effect of passing an invalid  UTF-8
       string  as  a  subject or an invalid value of startoffset is undefined.
       Your program may crash.

PCRE_PARTIAL

This option turns on the  partial  matching  feature.  If  the  subject
       string  fails to match the pattern, but at some point during the match-
       ing process the end of the subject was reached (that  is,  the  subject
       partially  matches  the  pattern and the failure to match occurred only
       because there were not enough subject characters), pcre_exec()  returns
       PCRE_ERROR_PARTIAL  instead of PCRE_ERROR_NOMATCH. When PCRE_PARTIAL is
       used, there are restrictions on what may appear in the  pattern.  These
       are discussed in the pcrepartial documentation.

The string to be matched by pcre_exec()

The  subject string is passed to pcre_exec() as a pointer in subject, a
       length (in bytes) in length, and a starting byte offset in startoffset.
       If this is  negative  or  greater  than  the  length  of  the  subject,
       pcre_exec() returns PCRE_ERROR_BADOFFSET.

In UTF-8 mode, the byte offset must point to the start of a UTF-8 char-
       acter (or the end of the subject). Unlike the pattern string, the  sub-
       ject  may  contain binary zero bytes. When the starting offset is zero,
       the search for a match starts at the beginning of the subject, and this
       is by far the most common case.

A  non-zero  starting offset is useful when searching for another match
       in the same subject by calling pcre_exec() again after a previous  suc-
       cess.   Setting  startoffset differs from just passing over a shortened
       string and setting PCRE_NOTBOL in the case of  a  pattern  that  begins
       with any kind of lookbehind. For example, consider the pattern

\Biss\B

which  finds  occurrences  of "iss" in the middle of words. (\B matches
       only if the current position in the subject is not  a  word  boundary.)
       When  applied  to the string "Mississipi" the first call to pcre_exec()
       finds the first occurrence. If pcre_exec() is called  again  with  just
       the  remainder  of  the  subject,  namely  "issipi", it does not match,
       because \B is always false at the start of the subject, which is deemed
       to  be  a  word  boundary. However, if pcre_exec() is passed the entire
       string again, but with startoffset set to 4, it finds the second occur-
       rence  of "iss" because it is able to look behind the starting point to
       discover that it is preceded by a letter.

If a non-zero starting offset is passed when the pattern  is  anchored,
       one attempt to match at the given offset is made. This can only succeed
       if the pattern does not require the match to be at  the  start  of  the
       subject.

How pcre_exec() returns captured substrings

In  general, a pattern matches a certain portion of the subject, and in
       addition, further substrings from the subject  may  be  picked  out  by
       parts  of  the  pattern.  Following the usage in Jeffrey Friedl's book,
       this is called "capturing" in what follows, and the  phrase  "capturing
       subpattern"  is  used for a fragment of a pattern that picks out a sub-
       string. PCRE supports several other kinds of  parenthesized  subpattern
       that do not cause substrings to be captured.

Captured substrings are returned to the caller via a vector of integers
       whose address is passed in ovector. The number of elements in the  vec-
       tor  is  passed in ovecsize, which must be a non-negative number. Note:
       this argument is NOT the size of ovector in bytes.

The first two-thirds of the vector is used to pass back  captured  sub-
       strings,  each  substring using a pair of integers. The remaining third
       of the vector is used as workspace by pcre_exec() while  matching  cap-
       turing  subpatterns, and is not available for passing back information.
       The number passed in ovecsize should always be a multiple of three.  If
       it is not, it is rounded down.

When  a  match  is successful, information about captured substrings is
       returned in pairs of integers, starting at the  beginning  of  ovector,
       and  continuing  up  to two-thirds of its length at the most. The first
       element of each pair is set to the byte offset of the  first  character
       in  a  substring, and the second is set to the byte offset of the first
       character after the end of a substring. Note: these values  are  always
       byte offsets, even in UTF-8 mode. They are not character counts.

The  first  pair  of  integers, ovector[0] and ovector[1], identify the
       portion of the subject string matched by the entire pattern.  The  next
       pair  is  used for the first capturing subpattern, and so on. The value
       returned by pcre_exec() is one more than the highest numbered pair that
       has  been  set.  For example, if two substrings have been captured, the
       returned value is 3. If there are no capturing subpatterns, the  return
       value from a successful match is 1, indicating that just the first pair
       of offsets has been set.

If a capturing subpattern is matched repeatedly, it is the last portion
       of the string that it matched that is returned.

If  the vector is too small to hold all the captured substring offsets,
       it is used as far as possible (up to two-thirds of its length), and the
       function  returns  a value of zero. If the substring offsets are not of
       interest, pcre_exec() may be called with ovector  passed  as  NULL  and
       ovecsize  as zero. However, if the pattern contains back references and
       the ovector is not big enough to remember the related substrings,  PCRE
       has  to  get additional memory for use during matching. Thus it is usu-
       ally advisable to supply an ovector.

The pcre_info() function can be used to find  out  how  many  capturing
       subpatterns  there  are  in  a  compiled pattern. The smallest size for
       ovector that will allow for n captured substrings, in addition  to  the
       offsets of the substring matched by the whole pattern, is (n+1)*3.

It  is  possible for capturing subpattern number n+1 to match some part
       of the subject when subpattern n has not been used at all. For example,
       if  the  string  "abc"  is  matched against the pattern (a|(z))(bc) the
       return from the function is 4, and subpatterns 1 and 3 are matched, but
       2  is  not.  When  this happens, both values in the offset pairs corre-
       sponding to unused subpatterns are set to -1.

Offset values that correspond to unused subpatterns at the end  of  the
       expression  are  also  set  to  -1. For example, if the string "abc" is
       matched against the pattern (abc)(x(yz)?)? subpatterns 2 and 3 are  not
       matched.  The  return  from the function is 2, because the highest used
       capturing subpattern number is 1. However, you can refer to the offsets
       for  the  second  and third capturing subpatterns if you wish (assuming
       the vector is large enough, of course).

Some convenience functions are provided  for  extracting  the  captured
       substrings as separate strings. These are described below.

Error return values from pcre_exec()

If  pcre_exec()  fails, it returns a negative number. The following are
       defined in the header file:

PCRE_ERROR_NOMATCH        (-1)

The subject string did not match the pattern.

PCRE_ERROR_NULL           (-2)

Either code or subject was passed as NULL,  or  ovector  was  NULL  and
       ovecsize was not zero.

PCRE_ERROR_BADOPTION      (-3)

An unrecognized bit was set in the options argument.

PCRE_ERROR_BADMAGIC       (-4)

PCRE  stores a 4-byte "magic number" at the start of the compiled code,
       to catch the case when it is passed a junk pointer and to detect when a
       pattern that was compiled in an environment of one endianness is run in
       an environment with the other endianness. This is the error  that  PCRE
       gives when the magic number is not present.

PCRE_ERROR_UNKNOWN_OPCODE (-5)

While running the pattern match, an unknown item was encountered in the
       compiled pattern. This error could be caused by a bug  in  PCRE  or  by
       overwriting of the compiled pattern.

PCRE_ERROR_NOMEMORY       (-6)

If  a  pattern contains back references, but the ovector that is passed
       to pcre_exec() is not big enough to remember the referenced substrings,
       PCRE  gets  a  block of memory at the start of matching to use for this
       purpose. If the call via pcre_malloc() fails, this error is given.  The
       memory is automatically freed at the end of matching.

PCRE_ERROR_NOSUBSTRING    (-7)

This  error is used by the pcre_copy_substring(), pcre_get_substring(),
       and  pcre_get_substring_list()  functions  (see  below).  It  is  never
       returned by pcre_exec().

PCRE_ERROR_MATCHLIMIT     (-8)

The  backtracking  limit,  as  specified  by the match_limit field in a
       pcre_extra structure (or defaulted) was reached.  See  the  description
       above.

PCRE_ERROR_CALLOUT        (-9)

This error is never generated by pcre_exec() itself. It is provided for
       use by callout functions that want to yield a distinctive  error  code.
       See the pcrecallout documentation for details.

PCRE_ERROR_BADUTF8        (-10)

A  string  that contains an invalid UTF-8 byte sequence was passed as a
       subject.

PCRE_ERROR_BADUTF8_OFFSET (-11)

The UTF-8 byte sequence that was passed as a subject was valid, but the
       value  of startoffset did not point to the beginning of a UTF-8 charac-
       ter.

PCRE_ERROR_PARTIAL        (-12)

The subject string did not match, but it did match partially.  See  the
       pcrepartial documentation for details of partial matching.

PCRE_ERROR_BADPARTIAL     (-13)

The  PCRE_PARTIAL  option  was  used with a compiled pattern containing
       items that are not supported for partial matching. See the  pcrepartial
       documentation for details of partial matching.

PCRE_ERROR_INTERNAL       (-14)

An  unexpected  internal error has occurred. This error could be caused
       by a bug in PCRE or by overwriting of the compiled pattern.

PCRE_ERROR_BADCOUNT       (-15)

This error is given if the value of the ovecsize argument is  negative.

PCRE_ERROR_RECURSIONLIMIT (-21)

The internal recursion limit, as specified by the match_limit_recursion
       field in a pcre_extra structure (or defaulted)  was  reached.  See  the
       description above.

PCRE_ERROR_BADNEWLINE     (-23)

An invalid combination of PCRE_NEWLINE_xxx options was given.

PCRE_ERROR_BADOFFSET      (-24)

The value of startoffset was negative or greater than the length of the
       subject, that is, the value in length.

Error numbers -16 to -20 and -22 are not used by pcre_exec().

EXTRACTING CAPTURED SUBSTRINGS BY NUMBER

int pcre_copy_substring(const char *subject, int *ovector,
            int stringcount, int stringnumber, char *buffer,
            int buffersize);

int pcre_get_substring(const char *subject, int *ovector,
            int stringcount, int stringnumber,
            const char **stringptr);

int pcre_get_substring_list(const char *subject,
            int *ovector, int stringcount, const char ***listptr);

Captured  substrings  can  be  accessed  directly  by using the offsets
       returned by pcre_exec() in  ovector.  For  convenience,  the  functions
       pcre_copy_substring(),    pcre_get_substring(),    and    pcre_get_sub-
       string_list() are provided for extracting captured substrings  as  new,
       separate,  zero-terminated strings. These functions identify substrings
       by number. The next section describes functions  for  extracting  named
       substrings.

A  substring that contains a binary zero is correctly extracted and has
       a further zero added on the end, but the result is not, of course, a  C
       string.   However,  you  can  process such a string by referring to the
       length that is  returned  by  pcre_copy_substring()  and  pcre_get_sub-
       string().  Unfortunately, the interface to pcre_get_substring_list() is
       not adequate for handling strings containing binary zeros, because  the
       end of the final string is not independently indicated.

The  first  three  arguments  are the same for all three of these func-
       tions: subject is the subject string that has  just  been  successfully
       matched, ovector is a pointer to the vector of integer offsets that was
       passed to pcre_exec(), and stringcount is the number of substrings that
       were  captured  by  the match, including the substring that matched the
       entire regular expression. This is the value returned by pcre_exec() if
       it  is greater than zero. If pcre_exec() returned zero, indicating that
       it ran out of space in ovector, the value passed as stringcount  should
       be the number of elements in the vector divided by three.

The  functions pcre_copy_substring() and pcre_get_substring() extract a
       single substring, whose number is given as  stringnumber.  A  value  of
       zero  extracts  the  substring that matched the entire pattern, whereas
       higher values  extract  the  captured  substrings.  For  pcre_copy_sub-
       string(),  the  string  is  placed  in buffer, whose length is given by
       buffersize, while for pcre_get_substring() a new  block  of  memory  is
       obtained  via  pcre_malloc,  and its address is returned via stringptr.
       The yield of the function is the length of the  string,  not  including
       the terminating zero, or one of these error codes:

PCRE_ERROR_NOMEMORY       (-6)

The  buffer  was too small for pcre_copy_substring(), or the attempt to
       get memory failed for pcre_get_substring().

PCRE_ERROR_NOSUBSTRING    (-7)

There is no substring whose number is stringnumber.

The pcre_get_substring_list()  function  extracts  all  available  sub-
       strings  and  builds  a list of pointers to them. All this is done in a
       single block of memory that is obtained via pcre_malloc. The address of
       the  memory  block  is returned via listptr, which is also the start of
       the list of string pointers. The end of the list is marked  by  a  NULL
       pointer.  The  yield  of  the function is zero if all went well, or the
       error code

PCRE_ERROR_NOMEMORY       (-6)

if the attempt to get the memory block failed.

When any of these functions encounter a substring that is unset,  which
       can  happen  when  capturing subpattern number n+1 matches some part of
       the subject, but subpattern n has not been used at all, they return  an
       empty string. This can be distinguished from a genuine zero-length sub-
       string by inspecting the appropriate offset in ovector, which is  nega-
       tive for unset substrings.

The  two convenience functions pcre_free_substring() and pcre_free_sub-
       string_list() can be used to free the memory  returned  by  a  previous
       call  of  pcre_get_substring()  or  pcre_get_substring_list(),  respec-
       tively. They do nothing more than  call  the  function  pointed  to  by
       pcre_free,  which  of course could be called directly from a C program.
       However, PCRE is used in some situations where it is linked via a  spe-
       cial   interface  to  another  programming  language  that  cannot  use
       pcre_free directly; it is for these cases that the functions  are  pro-
       vided.

EXTRACTING CAPTURED SUBSTRINGS BY NAME

int pcre_get_stringnumber(const pcre *code,
            const char *name);

int pcre_copy_named_substring(const pcre *code,
            const char *subject, int *ovector,
            int stringcount, const char *stringname,
            char *buffer, int buffersize);

int pcre_get_named_substring(const pcre *code,
            const char *subject, int *ovector,
            int stringcount, const char *stringname,
            const char **stringptr);

To  extract a substring by name, you first have to find associated num-
       ber.  For example, for this pattern

(a+)b(?<xxx>\d+)...

the number of the subpattern called "xxx" is 2. If the name is known to
       be unique (PCRE_DUPNAMES was not set), you can find the number from the
       name by calling pcre_get_stringnumber(). The first argument is the com-
       piled pattern, and the second is the name. The yield of the function is
       the subpattern number, or PCRE_ERROR_NOSUBSTRING (-7) if  there  is  no
       subpattern of that name.

Given the number, you can extract the substring directly, or use one of
       the functions described in the previous section. For convenience, there
       are also two functions that do the whole job.

Most    of    the    arguments   of   pcre_copy_named_substring()   and
       pcre_get_named_substring() are the same  as  those  for  the  similarly
       named  functions  that extract by number. As these are described in the
       previous section, they are not re-described here. There  are  just  two
       differences:

First,  instead  of a substring number, a substring name is given. Sec-
       ond, there is an extra argument, given at the start, which is a pointer
       to  the compiled pattern. This is needed in order to gain access to the
       name-to-number translation table.

These functions call pcre_get_stringnumber(), and if it succeeds,  they
       then  call  pcre_copy_substring() or pcre_get_substring(), as appropri-
       ate. NOTE: If PCRE_DUPNAMES is set and there are duplicate  names,  the
       behaviour may not be what you want (see the next section).

DUPLICATE SUBPATTERN NAMES

int pcre_get_stringtable_entries(const pcre *code,
            const char *name, char **first, char **last);

When  a  pattern  is  compiled with the PCRE_DUPNAMES option, names for
       subpatterns are not required to  be  unique.  Normally,  patterns  with
       duplicate  names  are such that in any one match, only one of the named
       subpatterns participates. An example is shown in the pcrepattern  docu-
       mentation.

When    duplicates   are   present,   pcre_copy_named_substring()   and
       pcre_get_named_substring() return the first substring corresponding  to
       the  given  name  that  is set. If none are set, PCRE_ERROR_NOSUBSTRING
       (-7) is returned; no  data  is  returned.  The  pcre_get_stringnumber()
       function  returns one of the numbers that are associated with the name,
       but it is not defined which it is.

If you want to get full details of all captured substrings for a  given
       name,  you  must  use  the pcre_get_stringtable_entries() function. The
       first argument is the compiled pattern, and the second is the name. The
       third  and  fourth  are  pointers to variables which are updated by the
       function. After it has run, they point to the first and last entries in
       the  name-to-number  table  for  the  given  name.  The function itself
       returns the length of each entry,  or  PCRE_ERROR_NOSUBSTRING  (-7)  if
       there  are none. The format of the table is described above in the sec-
       tion entitled Information about a  pattern.   Given  all  the  relevant
       entries  for the name, you can extract each of their numbers, and hence
       the captured data, if any.

FINDING ALL POSSIBLE MATCHES

The traditional matching function uses a  similar  algorithm  to  Perl,
       which stops when it finds the first match, starting at a given point in
       the subject. If you want to find all possible matches, or  the  longest
       possible  match,  consider using the alternative matching function (see
       below) instead. If you cannot use the alternative function,  but  still
       need  to  find all possible matches, you can kludge it up by making use
       of the callout facility, which is described in the pcrecallout documen-
       tation.

What you have to do is to insert a callout right at the end of the pat-
       tern.  When your callout function is called, extract and save the  cur-
       rent  matched  substring.  Then  return  1, which forces pcre_exec() to
       backtrack and try other alternatives. Ultimately, when it runs  out  of
       matches, pcre_exec() will yield PCRE_ERROR_NOMATCH.

MATCHING A PATTERN: THE ALTERNATIVE FUNCTION

The  function  pcre_dfa_exec()  is  called  to  match  a subject string
       against a compiled pattern, using a matching algorithm that  scans  the
       subject  string  just  once, and does not backtrack. This has different
       characteristics to the normal algorithm, and  is  not  compatible  with
       Perl.  Some  of the features of PCRE patterns are not supported. Never-
       theless, there are times when this kind of matching can be useful.  For
       a discussion of the two matching algorithms, see the pcrematching docu-
       mentation.

The arguments for the pcre_dfa_exec() function  are  the  same  as  for
       pcre_exec(), plus two extras. The ovector argument is used in a differ-
       ent way, and this is described below. The other  common  arguments  are
       used  in  the  same way as for pcre_exec(), so their description is not
       repeated here.

The two additional arguments provide workspace for  the  function.  The
       workspace  vector  should  contain at least 20 elements. It is used for
       keeping  track  of  multiple  paths  through  the  pattern  tree.  More
       workspace  will  be  needed for patterns and subjects where there are a
       lot of potential matches.

Here is an example of a simple call to pcre_dfa_exec():

int rc;
         int ovector[10];
         int wspace[20];
         rc = pcre_dfa_exec(
           re,             /* result of pcre_compile() */
           NULL,           /* we didn't study the pattern */
           "some string",  /* the subject string */
           11,             /* the length of the subject string */
           0,              /* start at offset 0 in the subject */
           0,              /* default options */
           ovector,        /* vector of integers for substring information */
           10,             /* number of elements (NOT size in bytes) */
           wspace,         /* working space vector */
           20);            /* number of elements (NOT size in bytes) */

Option bits for pcre_dfa_exec()

The unused bits of the options argument  for  pcre_dfa_exec()  must  be
       zero.  The  only  bits  that  may  be  set are PCRE_ANCHORED, PCRE_NEW-
       LINE_xxx, PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY,  PCRE_NO_UTF8_CHECK,
       PCRE_PARTIAL, PCRE_DFA_SHORTEST, and PCRE_DFA_RESTART. All but the last
       three of these are the same as for pcre_exec(), so their description is
       not repeated here.

PCRE_PARTIAL

This  has  the  same general effect as it does for pcre_exec(), but the
       details  are  slightly  different.  When  PCRE_PARTIAL   is   set   for
       pcre_dfa_exec(),  the  return code PCRE_ERROR_NOMATCH is converted into
       PCRE_ERROR_PARTIAL if the end of the subject  is  reached,  there  have
       been no complete matches, but there is still at least one matching pos-
       sibility. The portion of the string that provided the partial match  is
       set as the first matching string.

PCRE_DFA_SHORTEST

Setting  the  PCRE_DFA_SHORTEST option causes the matching algorithm to
       stop as soon as it has found one match. Because of the way the alterna-
       tive  algorithm  works, this is necessarily the shortest possible match
       at the first possible matching point in the subject string.

PCRE_DFA_RESTART

When pcre_dfa_exec()  is  called  with  the  PCRE_PARTIAL  option,  and
       returns  a  partial  match, it is possible to call it again, with addi-
       tional subject characters, and have it continue with  the  same  match.
       The  PCRE_DFA_RESTART  option requests this action; when it is set, the
       workspace and wscount options must reference the same vector as  before
       because  data  about  the  match so far is left in them after a partial
       match. There is more discussion of this  facility  in  the  pcrepartial
       documentation.

Successful returns from pcre_dfa_exec()

When  pcre_dfa_exec()  succeeds, it may have matched more than one sub-
       string in the subject. Note, however, that all the matches from one run
       of  the  function  start  at the same point in the subject. The shorter
       matches are all initial substrings of the longer matches. For  example,
       if the pattern

<.*>

is matched against the string

This is <something> <something else> <something further> no more

the three matched strings are

On  success,  the  yield of the function is a number greater than zero,
       which is the number of matched substrings.  The  substrings  themselves
       are  returned  in  ovector. Each string uses two elements; the first is
       the offset to the start, and the second is the offset to  the  end.  In
       fact,  all  the  strings  have the same start offset. (Space could have
       been saved by giving this only once, but it was decided to retain  some
       compatibility  with  the  way pcre_exec() returns data, even though the
       meaning of the strings is different.)

The strings are returned in reverse order of length; that is, the long-
       est  matching  string is given first. If there were too many matches to
       fit into ovector, the yield of the function is zero, and the vector  is
       filled with the longest matches.

Error returns from pcre_dfa_exec()

The  pcre_dfa_exec()  function returns a negative number when it fails.
       Many of the errors are the same  as  for  pcre_exec(),  and  these  are
       described  above.   There are in addition the following errors that are
       specific to pcre_dfa_exec():

PCRE_ERROR_DFA_UITEM      (-16)

This return is given if pcre_dfa_exec() encounters an item in the  pat-
       tern  that  it  does not support, for instance, the use of \C or a back
       reference.

PCRE_ERROR_DFA_UCOND      (-17)

This return is given if pcre_dfa_exec()  encounters  a  condition  item
       that  uses  a back reference for the condition, or a test for recursion
       in a specific group. These are not supported.

PCRE_ERROR_DFA_UMLIMIT    (-18)

This return is given if pcre_dfa_exec() is called with an  extra  block
       that contains a setting of the match_limit field. This is not supported
       (it is meaningless).

PCRE_ERROR_DFA_WSSIZE     (-19)

This return is given if  pcre_dfa_exec()  runs  out  of  space  in  the
       workspace vector.

PCRE_ERROR_DFA_RECURSE    (-20)

When  a  recursive subpattern is processed, the matching function calls
       itself recursively, using private vectors for  ovector  and  workspace.
       This  error  is  given  if  the output vector is not large enough. This
       should be extremely rare, as a vector of size 1000 is used.

SEE ALSO

pcrebuild(3), pcrecallout(3), pcrecpp(3)(3), pcrematching(3),  pcrepar-
       tial(3),  pcreposix(3), pcreprecompile(3), pcresample(3), pcrestack(3).

AUTHOR

Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.

REVISION

Last updated: 24 August 2008
       Copyright (c) 1997-2008 University of Cambridge.
------------------------------------------------------------------------------

PCRECALLOUT(3)                                                  PCRECALLOUT(3)

NAME
       PCRE - Perl-compatible regular expressions

PCRE CALLOUTS

int (*pcre_callout)(pcre_callout_block *);

PCRE provides a feature called "callout", which is a means of temporar-
       ily passing control to the caller of PCRE  in  the  middle  of  pattern
       matching.  The  caller of PCRE provides an external function by putting
       its entry point in the global variable pcre_callout. By  default,  this
       variable contains NULL, which disables all calling out.

Within  a  regular  expression,  (?C) indicates the points at which the
       external function is to be called.  Different  callout  points  can  be
       identified  by  putting  a number less than 256 after the letter C. The
       default value is zero.  For  example,  this  pattern  has  two  callout
       points:

(?C1)abc(?C2)def

If  the  PCRE_AUTO_CALLOUT  option  bit  is  set when pcre_compile() is
       called, PCRE automatically  inserts  callouts,  all  with  number  255,
       before  each  item in the pattern. For example, if PCRE_AUTO_CALLOUT is
       used with the pattern

A(\d{2}|--)

it is processed as if it were

(?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)

Notice that there is a callout before and after  each  parenthesis  and
       alternation  bar.  Automatic  callouts  can  be  used  for tracking the
       progress of pattern matching. The pcretest command has an  option  that
       sets  automatic callouts; when it is used, the output indicates how the
       pattern is matched. This is useful information when you are  trying  to
       optimize the performance of a particular pattern.

MISSING CALLOUTS

You  should  be  aware  that,  because of optimizations in the way PCRE
       matches patterns, callouts sometimes do not happen. For example, if the
       pattern is

ab(?C4)cd

PCRE knows that any matching string must contain the letter "d". If the
       subject string is "abyz", the lack of "d" means that  matching  doesn't
       ever  start,  and  the  callout is never reached. However, with "abyd",
       though the result is still no match, the callout is obeyed.

THE CALLOUT INTERFACE

During matching, when PCRE reaches a callout point, the external  func-
       tion  defined by pcre_callout is called (if it is set). This applies to
       both the pcre_exec() and the pcre_dfa_exec()  matching  functions.  The
       only  argument  to  the callout function is a pointer to a pcre_callout
       block. This structure contains the following fields:

int          version;
         int          callout_number;
         int         *offset_vector;
         const char  *subject;
         int          subject_length;
         int          start_match;
         int          current_position;
         int          capture_top;
         int          capture_last;
         void        *callout_data;
         int          pattern_position;
         int          next_item_length;

The version field is an integer containing the version  number  of  the
       block  format. The initial version was 0; the current version is 1. The
       version number will change again in future  if  additional  fields  are
       added, but the intention is never to remove any of the existing fields.

The callout_number field contains the number of the  callout,  as  com-
       piled  into  the pattern (that is, the number after ?C for manual call-
       outs, and 255 for automatically generated callouts).

The offset_vector field is a pointer to the vector of offsets that  was
       passed   by   the   caller  to  pcre_exec()  or  pcre_dfa_exec().  When
       pcre_exec() is used, the contents can be inspected in order to  extract
       substrings  that  have  been  matched  so  far,  in the same way as for
       extracting substrings after a match has completed. For  pcre_dfa_exec()
       this field is not useful.

The subject and subject_length fields contain copies of the values that
       were passed to pcre_exec().

The start_match field normally contains the offset within  the  subject
       at  which  the  current  match  attempt started. However, if the escape
       sequence \K has been encountered, this value is changed to reflect  the
       modified  starting  point.  If the pattern is not anchored, the callout
       function may be called several times from the same point in the pattern
       for different starting points in the subject.

The  current_position  field  contains the offset within the subject of
       the current match pointer.

When the pcre_exec() function is used, the capture_top  field  contains
       one  more than the number of the highest numbered captured substring so
       far. If no substrings have been captured, the value of  capture_top  is
       one.  This  is always the case when pcre_dfa_exec() is used, because it
       does not support captured substrings.

The capture_last field contains the number of the  most  recently  cap-
       tured  substring. If no substrings have been captured, its value is -1.
       This is always the case when pcre_dfa_exec() is used.

The callout_data field contains a value that is passed  to  pcre_exec()
       or  pcre_dfa_exec() specifically so that it can be passed back in call-
       outs. It is passed in the pcre_callout field  of  the  pcre_extra  data
       structure.  If  no such data was passed, the value of callout_data in a
       pcre_callout block is NULL. There is a description  of  the  pcre_extra
       structure in the pcreapi documentation.

The  pattern_position field is present from version 1 of the pcre_call-
       out structure. It contains the offset to the next item to be matched in
       the pattern string.

The  next_item_length field is present from version 1 of the pcre_call-
       out structure. It contains the length of the next item to be matched in
       the  pattern  string. When the callout immediately precedes an alterna-
       tion bar, a closing parenthesis, or the end of the pattern, the  length
       is  zero.  When the callout precedes an opening parenthesis, the length
       is that of the entire subpattern.

The pattern_position and next_item_length fields are intended  to  help
       in  distinguishing between different automatic callouts, which all have
       the same callout number. However, they are set for all callouts.

RETURN VALUES

The external callout function returns an integer to PCRE. If the  value
       is  zero,  matching  proceeds  as  normal. If the value is greater than
       zero, matching fails at the current point, but  the  testing  of  other
       matching possibilities goes ahead, just as if a lookahead assertion had
       failed. If the value is less than zero, the  match  is  abandoned,  and
       pcre_exec() (or pcre_dfa_exec()) returns the negative value.

Negative   values   should   normally   be   chosen  from  the  set  of
       PCRE_ERROR_xxx values. In particular, PCRE_ERROR_NOMATCH forces a stan-
       dard  "no  match"  failure.   The  error  number  PCRE_ERROR_CALLOUT is
       reserved for use by callout functions; it will never be  used  by  PCRE
       itself.

AUTHOR

Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.

REVISION

Last updated: 29 May 2007
       Copyright (c) 1997-2007 University of Cambridge.
------------------------------------------------------------------------------

PCRECOMPAT(3)                                                    PCRECOMPAT(3)

NAME
       PCRE - Perl-compatible regular expressions

DIFFERENCES BETWEEN PCRE AND PERL

This  document describes the differences in the ways that PCRE and Perl
       handle regular expressions. The differences described here  are  mainly
       with  respect  to  Perl 5.8, though PCRE versions 7.0 and later contain
       some features that are expected to be in the forthcoming Perl 5.10.

1. PCRE has only a subset of Perl's UTF-8 and Unicode support.  Details
       of  what  it does have are given in the section on UTF-8 support in the
       main pcre page.

2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl
       permits  them,  but they do not mean what you might think. For example,
       (?!a){3} does not assert that the next three characters are not "a". It
       just asserts that the next character is not "a" three times.

3.  Capturing  subpatterns  that occur inside negative lookahead asser-
       tions are counted, but their entries in the offsets  vector  are  never
       set.  Perl sets its numerical variables from any such patterns that are
       matched before the assertion fails to match something (thereby succeed-
       ing),  but  only  if the negative lookahead assertion contains just one
       branch.

4. Though binary zero characters are supported in the  subject  string,
       they are not allowed in a pattern string because it is passed as a nor-
       mal C string, terminated by zero. The escape sequence \0 can be used in
       the pattern to represent a binary zero.

5.  The  following Perl escape sequences are not supported: \l, \u, \L,
       \U, and \N. In fact these are implemented by Perl's general string-han-
       dling  and are not part of its pattern matching engine. If any of these
       are encountered by PCRE, an error is generated.

6. The Perl escape sequences \p, \P, and \X are supported only if  PCRE
       is  built  with Unicode character property support. The properties that
       can be tested with \p and \P are limited to the general category  prop-
       erties  such  as  Lu and Nd, script names such as Greek or Han, and the
       derived properties Any and L&.

7. PCRE does support the \Q...\E escape for quoting substrings. Charac-
       ters  in  between  are  treated as literals. This is slightly different
       from Perl in that $ and @ are  also  handled  as  literals  inside  the
       quotes.  In Perl, they cause variable interpolation (but of course PCRE
       does not have variables). Note the following examples:

Pattern            PCRE matches      Perl matches

\Qabc$xyz\E        abc$xyz           abc followed by the
                                                  contents of $xyz
           \Qabc\$xyz\E       abc\$xyz          abc\$xyz
           \Qabc\E\$\Qxyz\E   abc$xyz           abc$xyz

The \Q...\E sequence is recognized both inside  and  outside  character
       classes.

8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
       constructions. However, there is support for recursive  patterns.  This
       is  not available in Perl 5.8, but will be in Perl 5.10. Also, the PCRE
       "callout" feature allows an external function to be called during  pat-
       tern matching. See the pcrecallout documentation for details.

9.  Subpatterns  that  are  called  recursively or as "subroutines" are
       always treated as atomic groups in  PCRE.  This  is  like  Python,  but
       unlike Perl.

10.  There are some differences that are concerned with the settings of
       captured strings when part of  a  pattern  is  repeated.  For  example,
       matching  "aba"  against  the  pattern  /^(a(b)?)+$/  in Perl leaves $2
       unset, but in PCRE it is set to "b".

11.  PCRE  does  support  Perl  5.10's  backtracking  verbs  (*ACCEPT),
       (*FAIL),  (*F),  (*COMMIT), (*PRUNE), (*SKIP), and (*THEN), but only in
       the forms without an  argument.  PCRE  does  not  support  (*MARK).  If
       (*ACCEPT)  is within capturing parentheses, PCRE does not set that cap-
       ture group; this is different to Perl.

12. PCRE provides some extensions to the Perl regular expression facil-
       ities.   Perl  5.10  will  include new features that are not in earlier
       versions, some of which (such as named parentheses) have been  in  PCRE
       for some time. This list is with respect to Perl 5.10:

(a)  Although  lookbehind  assertions  must match fixed length strings,
       each alternative branch of a lookbehind assertion can match a different
       length of string. Perl requires them all to have the same length.

(b)  If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
       meta-character matches only at the very end of the string.

(c) If PCRE_EXTRA is set, a backslash followed by a letter with no spe-
       cial meaning is faulted. Otherwise, like Perl, the backslash is quietly
       ignored.  (Perl can be made to issue a warning.)

(d) If PCRE_UNGREEDY is set, the greediness of the  repetition  quanti-
       fiers is inverted, that is, by default they are not greedy, but if fol-
       lowed by a question mark they are.

(e) PCRE_ANCHORED can be used at matching time to force a pattern to be
       tried only at the first matching position in the subject string.

(f)  The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and PCRE_NO_AUTO_CAP-
       TURE options for pcre_exec() have no Perl equivalents.

(g) The \R escape sequence can be restricted to match only CR,  LF,  or
       CRLF by the PCRE_BSR_ANYCRLF option.

(h) The callout facility is PCRE-specific.

(i) The partial matching facility is PCRE-specific.

(j) Patterns compiled by PCRE can be saved and re-used at a later time,
       even on different hosts that have the other endianness.

(k) The alternative matching function (pcre_dfa_exec())  matches  in  a
       different way and is not Perl-compatible.

(l)  PCRE  recognizes some special sequences such as (*CR) at the start
       of a pattern that set overall options that cannot be changed within the
       pattern.

AUTHOR

Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.

REVISION

Last updated: 11 September 2007
       Copyright (c) 1997-2007 University of Cambridge.
------------------------------------------------------------------------------

PCREPATTERN(3)                                                  PCREPATTERN(3)

NAME
       PCRE - Perl-compatible regular expressions

PCRE REGULAR EXPRESSION DETAILS

The  syntax and semantics of the regular expressions that are supported
       by PCRE are described in detail below. There is a quick-reference  syn-
       tax summary in the pcresyntax page. PCRE tries to match Perl syntax and
       semantics as closely as it can. PCRE  also  supports  some  alternative
       regular  expression  syntax (which does not conflict with the Perl syn-
       tax) in order to provide some compatibility with regular expressions in
       Python, .NET, and Oniguruma.

Perl's  regular expressions are described in its own documentation, and
       regular expressions in general are covered in a number of  books,  some
       of  which  have  copious  examples. Jeffrey Friedl's "Mastering Regular
       Expressions", published by  O'Reilly,  covers  regular  expressions  in
       great  detail.  This  description  of  PCRE's  regular  expressions  is
       intended as reference material.

The original operation of PCRE was on strings of  one-byte  characters.
       However,  there is now also support for UTF-8 character strings. To use
       this, you must build PCRE to  include  UTF-8  support,  and  then  call
       pcre_compile()  with  the  PCRE_UTF8  option.  How this affects pattern
       matching is mentioned in several places below. There is also a  summary
       of  UTF-8  features  in  the  section on UTF-8 support in the main pcre
       page.

The remainder of this document discusses the  patterns  that  are  sup-
       ported  by  PCRE when its main matching function, pcre_exec(), is used.
       From  release  6.0,   PCRE   offers   a   second   matching   function,
       pcre_dfa_exec(),  which matches using a different algorithm that is not
       Perl-compatible. Some of the features discussed below are not available
       when  pcre_dfa_exec()  is used. The advantages and disadvantages of the
       alternative function, and how it differs from the normal function,  are
       discussed in the pcrematching page.

NEWLINE CONVENTIONS

PCRE  supports five different conventions for indicating line breaks in
       strings: a single CR (carriage return) character, a  single  LF  (line-
       feed) character, the two-character sequence CRLF, any of the three pre-
       ceding, or any Unicode newline sequence. The pcreapi page  has  further
       discussion  about newlines, and shows how to set the newline convention
       in the options arguments for the compiling and matching functions.

It is also possible to specify a newline convention by starting a  pat-
       tern string with one of the following five sequences:

(*CR)        carriage return
         (*LF)        linefeed
         (*CRLF)      carriage return, followed by linefeed
         (*ANYCRLF)   any of the three above
         (*ANY)       all Unicode newline sequences

These override the default and the options given to pcre_compile(). For
       example, on a Unix system where LF is the default newline sequence, the
       pattern

(*CR)a.b

changes the convention to CR. That pattern matches "a\nb" because LF is
       no longer a newline. Note that these special settings,  which  are  not
       Perl-compatible,  are  recognized  only at the very start of a pattern,
       and that they must be in upper case.  If  more  than  one  of  them  is
       present, the last one is used.

The  newline  convention  does  not  affect what the \R escape sequence
       matches. By default, this is any Unicode  newline  sequence,  for  Perl
       compatibility.  However, this can be changed; see the description of \R
       in the section entitled "Newline sequences" below. A change of \R  set-
       ting can be combined with a change of newline convention.

CHARACTERS AND METACHARACTERS

A  regular  expression  is  a pattern that is matched against a subject
       string from left to right. Most characters stand for  themselves  in  a
       pattern,  and  match  the corresponding characters in the subject. As a
       trivial example, the pattern

The quick brown fox

matches a portion of a subject string that is identical to itself. When
       caseless  matching is specified (the PCRE_CASELESS option), letters are
       matched independently of case. In UTF-8 mode, PCRE  always  understands
       the  concept  of case for characters whose values are less than 128, so
       caseless matching is always possible. For characters with  higher  val-
       ues,  the concept of case is supported if PCRE is compiled with Unicode
       property support, but not otherwise.   If  you  want  to  use  caseless
       matching  for  characters  128  and above, you must ensure that PCRE is
       compiled with Unicode property support as well as with UTF-8 support.

The power of regular expressions comes  from  the  ability  to  include
       alternatives  and  repetitions in the pattern. These are encoded in the
       pattern by the use of metacharacters, which do not stand for themselves
       but instead are interpreted in some special way.

There  are  two different sets of metacharacters: those that are recog-
       nized anywhere in the pattern except within square brackets, and  those
       that  are  recognized  within square brackets. Outside square brackets,
       the metacharacters are as follows:

\      general escape character with several uses
         ^      assert start of string (or line, in multiline mode)
         $      assert end of string (or line, in multiline mode)
         .      match any character except newline (by default)
         [      start character class definition
         |      start of alternative branch
         (      start subpattern
         )      end subpattern
         ?      extends the meaning of (
                also 0 or 1 quantifier
                also quantifier minimizer
         *      0 or more quantifier
         +      1 or more quantifier
                also "possessive quantifier"
         {      start min/max quantifier

Part of a pattern that is in square brackets  is  called  a  "character
       class". In a character class the only metacharacters are:

\      general escape character
         ^      negate the class, but only if the first character
         -      indicates character range
         [      POSIX character class (only if followed by POSIX
                  syntax)
         ]      terminates the character class

The  following sections describe the use of each of the metacharacters.

BACKSLASH

The backslash character has several uses. Firstly, if it is followed by
       a  non-alphanumeric  character,  it takes away any special meaning that
       character may have. This  use  of  backslash  as  an  escape  character
       applies both inside and outside character classes.

For  example,  if  you want to match a * character, you write \* in the
       pattern.  This escaping action applies whether  or  not  the  following
       character  would  otherwise be interpreted as a metacharacter, so it is
       always safe to precede a non-alphanumeric  with  backslash  to  specify
       that  it stands for itself. In particular, if you want to match a back-
       slash, you write \\.

If a pattern is compiled with the PCRE_EXTENDED option,  whitespace  in
       the  pattern (other than in a character class) and characters between a
       # outside a character class and the next newline are ignored. An escap-
       ing  backslash  can  be  used to include a whitespace or # character as
       part of the pattern.

If you want to remove the special meaning from a  sequence  of  charac-
       ters,  you can do so by putting them between \Q and \E. This is differ-
       ent from Perl in that $ and  @  are  handled  as  literals  in  \Q...\E
       sequences  in  PCRE, whereas in Perl, $ and @ cause variable interpola-
       tion. Note the following examples:

Pattern            PCRE matches   Perl matches

\Qabc$xyz\E        abc$xyz        abc followed by the
                                             contents of $xyz
         \Qabc\$xyz\E       abc\$xyz       abc\$xyz
         \Qabc\E\$\Qxyz\E   abc$xyz        abc$xyz

The \Q...\E sequence is recognized both inside  and  outside  character
       classes.

Non-printing characters

A second use of backslash provides a way of encoding non-printing char-
       acters in patterns in a visible manner. There is no restriction on  the
       appearance  of non-printing characters, apart from the binary zero that
       terminates a pattern, but when a pattern  is  being  prepared  by  text
       editing,  it  is  usually  easier  to  use  one of the following escape
       sequences than the binary character it represents:

\a        alarm, that is, the BEL character (hex 07)
         \cx       "control-x", where x is any character
         \e        escape (hex 1B)
         \f        formfeed (hex 0C)
         \n        linefeed (hex 0A)
         \r        carriage return (hex 0D)
         \t        tab (hex 09)
         \ddd      character with octal code ddd, or backreference
         \xhh      character with hex code hh
         \x{hhh..} character with hex code hhh..

The precise effect of \cx is as follows: if x is a lower  case  letter,
       it  is converted to upper case. Then bit 6 of the character (hex 40) is
       inverted.  Thus \cz becomes hex 1A, but \c{ becomes hex 3B,  while  \c;
       becomes hex 7B.

After  \x, from zero to two hexadecimal digits are read (letters can be
       in upper or lower case). Any number of hexadecimal  digits  may  appear
       between  \x{  and  },  but the value of the character code must be less
       than 256 in non-UTF-8 mode, and less than 2**31 in UTF-8 mode. That is,
       the  maximum value in hexadecimal is 7FFFFFFF. Note that this is bigger
       than the largest Unicode code point, which is 10FFFF.

If characters other than hexadecimal digits appear between \x{  and  },
       or if there is no terminating }, this form of escape is not recognized.
       Instead, the initial \x will be  interpreted  as  a  basic  hexadecimal
       escape,  with  no  following  digits, giving a character whose value is
       zero.

Characters whose value is less than 256 can be defined by either of the
       two  syntaxes  for  \x. There is no difference in the way they are han-
       dled. For example, \xdc is exactly the same as \x{dc}.

After \0 up to two further octal digits are read. If  there  are  fewer
       than  two  digits,  just  those  that  are  present  are used. Thus the
       sequence \0\x\07 specifies two binary zeros followed by a BEL character
       (code  value 7). Make sure you supply two digits after the initial zero
       if the pattern character that follows is itself an octal digit.

The handling of a backslash followed by a digit other than 0 is compli-
       cated.  Outside a character class, PCRE reads it and any following dig-
       its as a decimal number. If the number is less than  10,  or  if  there
       have been at least that many previous capturing left parentheses in the
       expression, the entire  sequence  is  taken  as  a  back  reference.  A
       description  of how this works is given later, following the discussion
       of parenthesized subpatterns.

Inside a character class, or if the decimal number is  greater  than  9
       and  there have not been that many capturing subpatterns, PCRE re-reads
       up to three octal digits following the backslash, and uses them to gen-
       erate  a data character. Any subsequent digits stand for themselves. In
       non-UTF-8 mode, the value of a character specified  in  octal  must  be
       less  than  \400.  In  UTF-8 mode, values up to \777 are permitted. For
       example:

\040   is another way of writing a space
         \40    is the same, provided there are fewer than 40
                   previous capturing subpatterns
         \7     is always a back reference
         \11    might be a back reference, or another way of
                   writing a tab
         \011   is always a tab
         \0113  is a tab followed by the character "3"
         \113   might be a back reference, otherwise the
                   character with octal code 113
         \377   might be a back reference, otherwise
                   the byte consisting entirely of 1 bits
         \81    is either a back reference, or a binary zero
                   followed by the two characters "8" and "1"

Note that octal values of 100 or greater must not be  introduced  by  a
       leading zero, because no more than three octal digits are ever read.

All the sequences that define a single character value can be used both
       inside and outside character classes. In addition, inside  a  character
       class,  the  sequence \b is interpreted as the backspace character (hex
       08), and the sequences \R and \X are interpreted as the characters  "R"
       and  "X", respectively. Outside a character class, these sequences have
       different meanings (see below).

Absolute and relative back references

The sequence \g followed by an unsigned or a negative  number,  option-
       ally  enclosed  in braces, is an absolute or relative back reference. A
       named back reference can be coded as \g{name}. Back references are dis-
       cussed later, following the discussion of parenthesized subpatterns.

Absolute and relative subroutine calls

For  compatibility with Oniguruma, the non-Perl syntax \g followed by a
       name or a number enclosed either in angle brackets or single quotes, is
       an  alternative  syntax for referencing a subpattern as a "subroutine".
       Details are discussed later.   Note  that  \g{...}  (Perl  syntax)  and
       \g<...>  (Oniguruma  syntax)  are  not synonymous. The former is a back
       reference; the latter is a subroutine call.

Generic character types

Another use of backslash is for specifying generic character types. The
       following are always recognized:

\d     any decimal digit
         \D     any character that is not a decimal digit
         \h     any horizontal whitespace character
         \H     any character that is not a horizontal whitespace character
         \s     any whitespace character
         \S     any character that is not a whitespace character
         \v     any vertical whitespace character
         \V     any character that is not a vertical whitespace character
         \w     any "word" character
         \W     any "non-word" character

Each pair of escape sequences partitions the complete set of characters
       into two disjoint sets. Any given character matches one, and only  one,
       of each pair.

These character type sequences can appear both inside and outside char-
       acter classes. They each match one character of the  appropriate  type.
       If  the current matching point is at the end of the subject string, all
       of them fail, since there is no character to match.

For compatibility with Perl, \s does not match the VT  character  (code
       11).   This makes it different from the the POSIX "space" class. The \s
       characters are HT (9), LF (10), FF (12), CR (13), and  space  (32).  If
       "use locale;" is included in a Perl script, \s may match the VT charac-
       ter. In PCRE, it never does.

In UTF-8 mode, characters with values greater than 128 never match  \d,
       \s, or \w, and always match \D, \S, and \W. This is true even when Uni-
       code character property support is available.  These  sequences  retain
       their original meanings from before UTF-8 support was available, mainly
       for efficiency reasons.

The sequences \h, \H, \v, and \V are Perl 5.10 features. In contrast to
       the  other  sequences, these do match certain high-valued codepoints in
       UTF-8 mode.  The horizontal space characters are:

U+0009     Horizontal tab
         U+0020     Space
         U+00A0     Non-break space
         U+1680     Ogham space mark
         U+180E     Mongolian vowel separator
         U+2000     En quad
         U+2001     Em quad
         U+2002     En space
         U+2003     Em space
         U+2004     Three-per-em space
         U+2005     Four-per-em space
         U+2006     Six-per-em space
         U+2007     Figure space
         U+2008     Punctuation space
         U+2009     Thin space
         U+200A     Hair space
         U+202F     Narrow no-break space
         U+205F     Medium mathematical space
         U+3000     Ideographic space

The vertical space characters are:

U+000A     Linefeed
         U+000B     Vertical tab
         U+000C     Formfeed
         U+000D     Carriage return
         U+0085     Next line
         U+2028     Line separator
         U+2029     Paragraph separator

A "word" character is an underscore or any character less than 256 that
       is  a  letter  or  digit.  The definition of letters and digits is con-
       trolled by PCRE's low-valued character tables, and may vary if  locale-
       specific  matching is taking place (see "Locale support" in the pcreapi
       page). For example, in a French locale such  as  "fr_FR"  in  Unix-like
       systems,  or "french" in Windows, some character codes greater than 128
       are used for accented letters, and these are matched by \w. The use  of
       locales with Unicode is discouraged.

Newline sequences

Outside  a  character class, by default, the escape sequence \R matches
       any Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8
       mode \R is equivalent to the following:

(?>\r\n|\n|\x0b|\f|\r|\x85)

This  is  an  example  of an "atomic group", details of which are given
       below.  This particular group matches either the two-character sequence
       CR  followed  by  LF,  or  one  of  the single characters LF (linefeed,
       U+000A), VT (vertical tab, U+000B), FF (formfeed, U+000C), CR (carriage
       return, U+000D), or NEL (next line, U+0085). The two-character sequence
       is treated as a single unit that cannot be split.

In UTF-8 mode, two additional characters whose codepoints  are  greater
       than 255 are added: LS (line separator, U+2028) and PS (paragraph sepa-
       rator, U+2029).  Unicode character property support is not  needed  for
       these characters to be recognized.

It is possible to restrict \R to match only CR, LF, or CRLF (instead of
       the complete set  of  Unicode  line  endings)  by  setting  the  option
       PCRE_BSR_ANYCRLF either at compile time or when the pattern is matched.
       (BSR is an abbrevation for "backslash R".) This can be made the default
       when  PCRE  is  built;  if this is the case, the other behaviour can be
       requested via the PCRE_BSR_UNICODE option.   It  is  also  possible  to
       specify  these  settings  by  starting a pattern string with one of the
       following sequences:

(*BSR_ANYCRLF)   CR, LF, or CRLF only
         (*BSR_UNICODE)   any Unicode newline sequence

These override the default and the options given to pcre_compile(), but
       they can be overridden by options given to pcre_exec(). Note that these
       special settings, which are not Perl-compatible, are recognized only at
       the  very  start  of a pattern, and that they must be in upper case. If
       more than one of them is present, the last one is  used.  They  can  be
       combined  with  a  change of newline convention, for example, a pattern
       can start with:

(*ANY)(*BSR_ANYCRLF)

Inside a character class, \R matches the letter "R".

Unicode character properties

When PCRE is built with Unicode character property support, three addi-
       tional  escape sequences that match characters with specific properties
       are available.  When not in UTF-8 mode, these sequences are  of  course
       limited  to  testing characters whose codepoints are less than 256, but
       they do work in this mode.  The extra escape sequences are:

\p{xx}   a character with the xx property
         \P{xx}   a character without the xx property
         \X       an extended Unicode sequence

The property names represented by xx above are limited to  the  Unicode
       script names, the general category properties, and "Any", which matches
       any character (including newline). Other properties such as "InMusical-
       Symbols"  are  not  currently supported by PCRE. Note that \P{Any} does
       not match any characters, so always causes a match failure.

Sets of Unicode characters are defined as belonging to certain scripts.
       A  character from one of these sets can be matched using a script name.
       For example:

\p{Greek}
         \P{Han}

Those that are not part of an identified script are lumped together  as
       "Common". The current list of scripts is:

Arabic,  Armenian,  Balinese,  Bengali,  Bopomofo,  Braille,  Buginese,
       Buhid,  Canadian_Aboriginal,  Cherokee,  Common,   Coptic,   Cuneiform,
       Cypriot, Cyrillic, Deseret, Devanagari, Ethiopic, Georgian, Glagolitic,
       Gothic, Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew,  Hira-
       gana,  Inherited,  Kannada,  Katakana,  Kharoshthi,  Khmer, Lao, Latin,
       Limbu,  Linear_B,  Malayalam,  Mongolian,  Myanmar,  New_Tai_Lue,  Nko,
       Ogham,  Old_Italic,  Old_Persian, Oriya, Osmanya, Phags_Pa, Phoenician,
       Runic,  Shavian,  Sinhala,  Syloti_Nagri,  Syriac,  Tagalog,  Tagbanwa,
       Tai_Le, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh, Ugaritic, Yi.

Each  character has exactly one general category property, specified by
       a two-letter abbreviation. For compatibility with Perl, negation can be
       specified  by  including a circumflex between the opening brace and the
       property name. For example, \p{^Lu} is the same as \P{Lu}.

If only one letter is specified with \p or \P, it includes all the gen-
       eral  category properties that start with that letter. In this case, in
       the absence of negation, the curly brackets in the escape sequence  are
       optional; these two examples have the same effect:

\p{L}
         \pL

The following general category property codes are supported:

C     Other
         Cc    Control
         Cf    Format
         Cn    Unassigned
         Co    Private use
         Cs    Surrogate

L     Letter
         Ll    Lower case letter
         Lm    Modifier letter
         Lo    Other letter
         Lt    Title case letter
         Lu    Upper case letter

M     Mark
         Mc    Spacing mark
         Me    Enclosing mark
         Mn    Non-spacing mark

N     Number
         Nd    Decimal number
         Nl    Letter number
         No    Other number

P     Punctuation
         Pc    Connector punctuation
         Pd    Dash punctuation
         Pe    Close punctuation
         Pf    Final punctuation
         Pi    Initial punctuation
         Po    Other punctuation
         Ps    Open punctuation

S     Symbol
         Sc    Currency symbol
         Sk    Modifier symbol
         Sm    Mathematical symbol
         So    Other symbol

Z     Separator
         Zl    Line separator
         Zp    Paragraph separator
         Zs    Space separator

The  special property L& is also supported: it matches a character that
       has the Lu, Ll, or Lt property, in other words, a letter  that  is  not
       classified as a modifier or "other".

The  Cs  (Surrogate)  property  applies only to characters in the range
       U+D800 to U+DFFF. Such characters are not valid in UTF-8  strings  (see
       RFC 3629) and so cannot be tested by PCRE, unless UTF-8 validity check-
       ing has been turned off (see the discussion  of  PCRE_NO_UTF8_CHECK  in
       the pcreapi page).

The  long  synonyms  for  these  properties that Perl supports (such as
       \p{Letter}) are not supported by PCRE, nor is it  permitted  to  prefix
       any of these properties with "Is".

No character that is in the Unicode table has the Cn (unassigned) prop-
       erty.  Instead, this property is assumed for any code point that is not
       in the Unicode table.

Specifying  caseless  matching  does not affect these escape sequences.
       For example, \p{Lu} always matches only upper case letters.

The \X escape matches any number of Unicode  characters  that  form  an
       extended Unicode sequence. \X is equivalent to

(?>\PM\pM*)

That  is,  it matches a character without the "mark" property, followed
       by zero or more characters with the "mark"  property,  and  treats  the
       sequence  as  an  atomic group (see below).  Characters with the "mark"
       property are typically accents that  affect  the  preceding  character.
       None  of  them  have  codepoints less than 256, so in non-UTF-8 mode \X
       matches any one character.

Matching characters by Unicode property is not fast, because  PCRE  has
       to  search  a  structure  that  contains data for over fifteen thousand
       characters. That is why the traditional escape sequences such as \d and
       \w do not use Unicode properties in PCRE.

Resetting the match start

The escape sequence \K, which is a Perl 5.10 feature, causes any previ-
       ously matched characters not  to  be  included  in  the  final  matched
       sequence. For example, the pattern:

foo\Kbar

matches  "foobar",  but reports that it has matched "bar". This feature
       is similar to a lookbehind assertion (described  below).   However,  in
       this  case, the part of the subject before the real match does not have
       to be of fixed length, as lookbehind assertions do. The use of \K  does
       not  interfere  with  the setting of captured substrings.  For example,
       when the pattern

(foo)\Kbar

matches "foobar", the first substring is still set to "foo".

Simple assertions

The final use of backslash is for certain simple assertions. An  asser-
       tion  specifies a condition that has to be met at a particular point in
       a match, without consuming any characters from the subject string.  The
       use  of subpatterns for more complicated assertions is described below.
       The backslashed assertions are:

\b     matches at a word boundary
         \B     matches when not at a word boundary
         \A     matches at the start of the subject
         \Z     matches at the end of the subject
                 also matches before a newline at the end of the subject
         \z     matches only at the end of the subject
         \G     matches at the first matching position in the subject

These assertions may not appear in character classes (but note that  \b
       has a different meaning, namely the backspace character, inside a char-
       acter class).

A word boundary is a position in the subject string where  the  current
       character  and  the previous character do not both match \w or \W (i.e.
       one matches \w and the other matches \W), or the start or  end  of  the
       string if the first or last character matches \w, respectively.

The  \A,  \Z,  and \z assertions differ from the traditional circumflex
       and dollar (described in the next section) in that they only ever match
       at  the  very start and end of the subject string, whatever options are
       set. Thus, they are independent of multiline mode. These  three  asser-
       tions are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options, which
       affect only the behaviour of the circumflex and dollar  metacharacters.
       However,  if the startoffset argument of pcre_exec() is non-zero, indi-
       cating that matching is to start at a point other than the beginning of
       the  subject,  \A  can never match. The difference between \Z and \z is
       that \Z matches before a newline at the end of the string as well as at
       the very end, whereas \z matches only at the end.

The  \G assertion is true only when the current matching position is at
       the start point of the match, as specified by the startoffset  argument
       of  pcre_exec().  It  differs  from \A when the value of startoffset is
       non-zero. By calling pcre_exec() multiple times with appropriate  argu-
       ments, you can mimic Perl's /g option, and it is in this kind of imple-
       mentation where \G can be useful.

Note, however, that PCRE's interpretation of \G, as the  start  of  the
       current match, is subtly different from Perl's, which defines it as the
       end of the previous match. In Perl, these can  be  different  when  the
       previously  matched  string was empty. Because PCRE does just one match
       at a time, it cannot reproduce this behaviour.

If all the alternatives of a pattern begin with \G, the  expression  is
       anchored to the starting match position, and the "anchored" flag is set
       in the compiled regular expression.

CIRCUMFLEX AND DOLLAR

Outside a character class, in the default matching mode, the circumflex
       character  is  an  assertion  that is true only if the current matching
       point is at the start of the subject string. If the  startoffset  argu-
       ment  of  pcre_exec()  is  non-zero,  circumflex can never match if the
       PCRE_MULTILINE option is unset. Inside a  character  class,  circumflex
       has an entirely different meaning (see below).

Circumflex  need  not be the first character of the pattern if a number
       of alternatives are involved, but it should be the first thing in  each
       alternative  in  which  it appears if the pattern is ever to match that
       branch. If all possible alternatives start with a circumflex, that  is,
       if  the  pattern  is constrained to match only at the start of the sub-
       ject, it is said to be an "anchored" pattern.  (There  are  also  other
       constructs that can cause a pattern to be anchored.)

A  dollar  character  is  an assertion that is true only if the current
       matching point is at the end of  the  subject  string,  or  immediately
       before a newline at the end of the string (by default). Dollar need not
       be the last character of the pattern if a number  of  alternatives  are
       involved,  but  it  should  be  the last item in any branch in which it
       appears. Dollar has no special meaning in a character class.

The meaning of dollar can be changed so that it  matches  only  at  the
       very  end  of  the string, by setting the PCRE_DOLLAR_ENDONLY option at
       compile time. This does not affect the \Z assertion.

The meanings of the circumflex and dollar characters are changed if the
       PCRE_MULTILINE  option  is  set.  When  this  is the case, a circumflex
       matches immediately after internal newlines as well as at the start  of
       the  subject  string.  It  does not match after a newline that ends the
       string. A dollar matches before any newlines in the string, as well  as
       at  the very end, when PCRE_MULTILINE is set. When newline is specified
       as the two-character sequence CRLF, isolated CR and  LF  characters  do
       not indicate newlines.

For  example, the pattern /^abc$/ matches the subject string "def\nabc"
       (where \n represents a newline) in multiline mode, but  not  otherwise.
       Consequently,  patterns  that  are anchored in single line mode because
       all branches start with ^ are not anchored in  multiline  mode,  and  a
       match  for  circumflex  is  possible  when  the startoffset argument of
       pcre_exec() is non-zero. The PCRE_DOLLAR_ENDONLY option is  ignored  if
       PCRE_MULTILINE is set.

Note  that  the sequences \A, \Z, and \z can be used to match the start
       and end of the subject in both modes, and if all branches of a  pattern
       start  with  \A it is always anchored, whether or not PCRE_MULTILINE is
       set.

FULL STOP (PERIOD, DOT)

Outside a character class, a dot in the pattern matches any one charac-
       ter  in  the subject string except (by default) a character that signi-
       fies the end of a line. In UTF-8 mode, the  matched  character  may  be
       more than one byte long.

When  a line ending is defined as a single character, dot never matches
       that character; when the two-character sequence CRLF is used, dot  does
       not  match  CR  if  it  is immediately followed by LF, but otherwise it
       matches all characters (including isolated CRs and LFs). When any  Uni-
       code  line endings are being recognized, dot does not match CR or LF or
       any of the other line ending characters.

The behaviour of dot with regard to newlines can  be  changed.  If  the
       PCRE_DOTALL  option  is  set,  a dot matches any one character, without
       exception. If the two-character sequence CRLF is present in the subject
       string, it takes two dots to match it.

The  handling of dot is entirely independent of the handling of circum-
       flex and dollar, the only relationship being  that  they  both  involve
       newlines. Dot has no special meaning in a character class.

MATCHING A SINGLE BYTE

Outside a character class, the escape sequence \C matches any one byte,
       both in and out of UTF-8 mode. Unlike a  dot,  it  always  matches  any
       line-ending  characters.  The  feature  is provided in Perl in order to
       match individual bytes in UTF-8 mode. Because it breaks up UTF-8  char-
       acters  into individual bytes, what remains in the string may be a mal-
       formed UTF-8 string. For this reason, the \C escape  sequence  is  best
       avoided.

PCRE  does  not  allow \C to appear in lookbehind assertions (described
       below), because in UTF-8 mode this would make it impossible  to  calcu-
       late the length of the lookbehind.

SQUARE BRACKETS AND CHARACTER CLASSES

An opening square bracket introduces a character class, terminated by a
       closing square bracket. A closing square bracket on its own is not spe-
       cial. If a closing square bracket is required as a member of the class,
       it should be the first data character in the class  (after  an  initial
       circumflex, if present) or escaped with a backslash.

A  character  class matches a single character in the subject. In UTF-8
       mode, the character may occupy more than one byte. A matched  character
       must be in the set of characters defined by the class, unless the first
       character in the class definition is a circumflex, in  which  case  the
       subject  character  must  not  be in the set defined by the class. If a
       circumflex is actually required as a member of the class, ensure it  is
       not the first character, or escape it with a backslash.

For  example, the character class [aeiou] matches any lower case vowel,
       while [^aeiou] matches any character that is not a  lower  case  vowel.
       Note that a circumflex is just a convenient notation for specifying the
       characters that are in the class by enumerating those that are  not.  A
       class  that starts with a circumflex is not an assertion: it still con-
       sumes a character from the subject string, and therefore  it  fails  if
       the current pointer is at the end of the string.

In  UTF-8 mode, characters with values greater than 255 can be included
       in a class as a literal string of bytes, or by using the  \x{  escaping
       mechanism.

When  caseless  matching  is set, any letters in a class represent both
       their upper case and lower case versions, so for  example,  a  caseless
       [aeiou]  matches  "A"  as well as "a", and a caseless [^aeiou] does not
       match "A", whereas a caseful version would. In UTF-8 mode, PCRE  always
       understands  the  concept  of case for characters whose values are less
       than 128, so caseless matching is always possible. For characters  with
       higher  values,  the  concept  of case is supported if PCRE is compiled
       with Unicode property support, but not otherwise.  If you want  to  use
       caseless  matching  for  characters 128 and above, you must ensure that
       PCRE is compiled with Unicode property support as well  as  with  UTF-8
       support.

Characters  that  might  indicate  line breaks are never treated in any
       special way  when  matching  character  classes,  whatever  line-ending
       sequence  is  in  use,  and  whatever  setting  of  the PCRE_DOTALL and
       PCRE_MULTILINE options is used. A class such as [^a] always matches one
       of these characters.

The  minus (hyphen) character can be used to specify a range of charac-
       ters in a character  class.  For  example,  [d-m]  matches  any  letter
       between  d  and  m,  inclusive.  If  a minus character is required in a
       class, it must be escaped with a backslash  or  appear  in  a  position
       where  it cannot be interpreted as indicating a range, typically as the
       first or last character in the class.

It is not possible to have the literal character "]" as the end charac-
       ter  of a range. A pattern such as [W-]46] is interpreted as a class of
       two characters ("W" and "-") followed by a literal string "46]", so  it
       would  match  "W46]"  or  "-46]". However, if the "]" is escaped with a
       backslash it is interpreted as the end of range, so [W-\]46] is  inter-
       preted  as a class containing a range followed by two other characters.
       The octal or hexadecimal representation of "]" can also be used to  end
       a range.

Ranges  operate in the collating sequence of character values. They can
       also  be  used  for  characters  specified  numerically,  for   example
       [\000-\037].  In UTF-8 mode, ranges can include characters whose values
       are greater than 255, for example [\x{100}-\x{2ff}].

If a range that includes letters is used when caseless matching is set,
       it matches the letters in either case. For example, [W-c] is equivalent
       to [][\\^_`wxyzabc], matched caselessly,  and  in  non-UTF-8  mode,  if
       character  tables  for  a French locale are in use, [\xc8-\xcb] matches
       accented E characters in both cases. In UTF-8 mode, PCRE  supports  the
       concept  of  case for characters with values greater than 128 only when
       it is compiled with Unicode property support.

The character types \d, \D, \p, \P, \s, \S, \w, and \W may also  appear
       in  a  character  class,  and add the characters that they match to the
       class. For example, [\dABCDEF] matches any hexadecimal digit. A circum-
       flex  can  conveniently  be used with the upper case character types to
       specify a more restricted set of characters  than  the  matching  lower
       case  type.  For example, the class [^\W_] matches any letter or digit,
       but not underscore.

The only metacharacters that are recognized in  character  classes  are
       backslash,  hyphen  (only  where  it can be interpreted as specifying a
       range), circumflex (only at the start), opening  square  bracket  (only
       when  it can be interpreted as introducing a POSIX class name - see the
       next section), and the terminating  closing  square  bracket.  However,
       escaping other non-alphanumeric characters does no harm.

POSIX CHARACTER CLASSES

Perl supports the POSIX notation for character classes. This uses names
       enclosed by [: and :] within the enclosing square brackets.  PCRE  also
       supports this notation. For example,

[01[:alpha:]%]

matches "0", "1", any alphabetic character, or "%". The supported class
       names are

alnum    letters and digits
         alpha    letters
         ascii    character codes 0 - 127
         blank    space or tab only
         cntrl    control characters
         digit    decimal digits (same as \d)
         graph    printing characters, excluding space
         lower    lower case letters
         print    printing characters, including space
         punct    printing characters, excluding letters and digits
         space    white space (not quite the same as \s)
         upper    upper case letters
         word     "word" characters (same as \w)
         xdigit   hexadecimal digits

The "space" characters are HT (9), LF (10), VT (11), FF (12), CR  (13),
       and  space  (32). Notice that this list includes the VT character (code
       11). This makes "space" different to \s, which does not include VT (for
       Perl compatibility).

The  name  "word"  is  a Perl extension, and "blank" is a GNU extension
       from Perl 5.8. Another Perl extension is negation, which  is  indicated
       by a ^ character after the colon. For example,

[12[:^digit:]]

matches  "1", "2", or any non-digit. PCRE (and Perl) also recognize the
       POSIX syntax [.ch.] and [=ch=] where "ch" is a "collating element", but
       these are not supported, and an error is given if they are encountered.

In UTF-8 mode, characters with values greater than 128 do not match any
       of the POSIX character classes.

VERTICAL BAR

Vertical  bar characters are used to separate alternative patterns. For
       example, the pattern

gilbert|sullivan

matches either "gilbert" or "sullivan". Any number of alternatives  may
       appear,  and  an  empty  alternative  is  permitted (matching the empty
       string). The matching process tries each alternative in turn, from left
       to  right, and the first one that succeeds is used. If the alternatives
       are within a subpattern (defined below), "succeeds" means matching  the
       rest  of the main pattern as well as the alternative in the subpattern.

INTERNAL OPTION SETTING

The settings of the  PCRE_CASELESS,  PCRE_MULTILINE,  PCRE_DOTALL,  and
       PCRE_EXTENDED  options  (which are Perl-compatible) can be changed from
       within the pattern by  a  sequence  of  Perl  option  letters  enclosed
       between "(?" and ")".  The option letters are

i  for PCRE_CASELESS
         m  for PCRE_MULTILINE
         s  for PCRE_DOTALL
         x  for PCRE_EXTENDED

For example, (?im) sets caseless, multiline matching. It is also possi-
       ble to unset these options by preceding the letter with a hyphen, and a
       combined  setting and unsetting such as (?im-sx), which sets PCRE_CASE-
       LESS and PCRE_MULTILINE while unsetting PCRE_DOTALL and  PCRE_EXTENDED,
       is  also  permitted.  If  a  letter  appears  both before and after the
       hyphen, the option is unset.

The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and  PCRE_EXTRA
       can  be changed in the same way as the Perl-compatible options by using
       the characters J, U and X respectively.

When an option change occurs at top level (that is, not inside  subpat-
       tern  parentheses),  the change applies to the remainder of the pattern
       that follows.  If the change is placed right at the start of a pattern,
       PCRE extracts it into the global options (and it will therefore show up
       in data extracted by the pcre_fullinfo() function).

An option change within a subpattern (see below for  a  description  of
       subpatterns) affects only that part of the current pattern that follows
       it, so

(a(?i)b)c

matches abc and aBc and no other strings (assuming PCRE_CASELESS is not
       used).   By  this means, options can be made to have different settings
       in different parts of the pattern. Any changes made in one  alternative
       do  carry  on  into subsequent branches within the same subpattern. For
       example,

(a(?i)b|c)

matches "ab", "aB", "c", and "C", even though  when  matching  "C"  the
       first  branch  is  abandoned before the option setting. This is because
       the effects of option settings happen at compile time. There  would  be
       some very weird behaviour otherwise.

Note:  There  are  other  PCRE-specific  options that can be set by the
       application when the compile or match functions  are  called.  In  some
       cases  the  pattern  can  contain special leading sequences to override
       what the application has set or what has been  defaulted.  Details  are
       given in the section entitled "Newline sequences" above.

SUBPATTERNS

Subpatterns are delimited by parentheses (round brackets), which can be
       nested.  Turning part of a pattern into a subpattern does two things:

1. It localizes a set of alternatives. For example, the pattern

cat(aract|erpillar|)

matches one of the words "cat", "cataract", or  "caterpillar".  Without
       the  parentheses,  it  would  match  "cataract", "erpillar" or an empty
       string.

2. It sets up the subpattern as  a  capturing  subpattern.  This  means
       that,  when  the  whole  pattern  matches,  that portion of the subject
       string that matched the subpattern is passed back to the caller via the
       ovector  argument  of pcre_exec(). Opening parentheses are counted from
       left to right (starting from 1) to obtain  numbers  for  the  capturing
       subpatterns.

For  example,  if the string "the red king" is matched against the pat-
       tern

the ((red|white) (king|queen))

the captured substrings are "red king", "red", and "king", and are num-
       bered 1, 2, and 3, respectively.

The  fact  that  plain  parentheses  fulfil two functions is not always
       helpful.  There are often times when a grouping subpattern is  required
       without  a capturing requirement. If an opening parenthesis is followed
       by a question mark and a colon, the subpattern does not do any  captur-
       ing,  and  is  not  counted when computing the number of any subsequent
       capturing subpatterns. For example, if the string "the white queen"  is
       matched against the pattern

the ((?:red|white) (king|queen))

the captured substrings are "white queen" and "queen", and are numbered
       1 and 2. The maximum number of capturing subpatterns is 65535.

As a convenient shorthand, if any option settings are required  at  the
       start  of  a  non-capturing  subpattern,  the option letters may appear
       between the "?" and the ":". Thus the two patterns

(?i:saturday|sunday)
         (?:(?i)saturday|sunday)

match exactly the same set of strings. Because alternative branches are
       tried  from  left  to right, and options are not reset until the end of
       the subpattern is reached, an option setting in one branch does  affect
       subsequent  branches,  so  the above patterns match "SUNDAY" as well as
       "Saturday".

DUPLICATE SUBPATTERN NUMBERS

Perl 5.10 introduced a feature whereby each alternative in a subpattern
       uses  the same numbers for its capturing parentheses. Such a subpattern
       starts with (?| and is itself a non-capturing subpattern. For  example,
       consider this pattern:

(?|(Sat)ur|(Sun))day

Because  the two alternatives are inside a (?| group, both sets of cap-
       turing parentheses are numbered one. Thus, when  the  pattern  matches,
       you  can  look  at captured substring number one, whichever alternative
       matched. This construct is useful when you want to  capture  part,  but
       not all, of one of a number of alternatives. Inside a (?| group, paren-
       theses are numbered as usual, but the number is reset at the  start  of
       each  branch. The numbers of any capturing buffers that follow the sub-
       pattern start after the highest number used in any branch. The  follow-
       ing  example  is taken from the Perl documentation.  The numbers under-
       neath show in which buffer the captured content will be stored.

# before  ---------------branch-reset----------- after
         / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
         # 1            2         2  3        2     3     4

A backreference or a recursive call to  a  numbered  subpattern  always
       refers to the first one in the pattern with the given number.

An  alternative approach to using this "branch reset" feature is to use
       duplicate named subpatterns, as described in the next section.

NAMED SUBPATTERNS

Identifying capturing parentheses by number is simple, but  it  can  be
       very  hard  to keep track of the numbers in complicated regular expres-
       sions. Furthermore, if an  expression  is  modified,  the  numbers  may
       change.  To help with this difficulty, PCRE supports the naming of sub-
       patterns. This feature was not added to Perl until release 5.10. Python
       had  the  feature earlier, and PCRE introduced it at release 4.0, using
       the Python syntax. PCRE now supports both the Perl and the Python  syn-
       tax.

In  PCRE,  a subpattern can be named in one of three ways: (?<name>...)
       or (?'name'...) as in Perl, or (?P<name>...) as in  Python.  References
       to capturing parentheses from other parts of the pattern, such as back-
       references, recursion, and conditions, can be made by name as  well  as
       by number.

Names  consist  of  up  to  32 alphanumeric characters and underscores.
       Named capturing parentheses are still  allocated  numbers  as  well  as
       names,  exactly as if the names were not present. The PCRE API provides
       function calls for extracting the name-to-number translation table from
       a compiled pattern. There is also a convenience function for extracting
       a captured substring by name.

By default, a name must be unique within a pattern, but it is  possible
       to relax this constraint by setting the PCRE_DUPNAMES option at compile
       time. This can be useful for patterns where only one  instance  of  the
       named  parentheses  can  match. Suppose you want to match the name of a
       weekday, either as a 3-letter abbreviation or as the full name, and  in
       both cases you want to extract the abbreviation. This pattern (ignoring
       the line breaks) does the job:

There are five capturing substrings, but only one is ever set  after  a
       match.  (An alternative way of solving this problem is to use a "branch
       reset" subpattern, as described in the previous section.)

The convenience function for extracting the data by  name  returns  the
       substring  for  the first (and in this example, the only) subpattern of
       that name that matched. This saves searching  to  find  which  numbered
       subpattern  it  was. If you make a reference to a non-unique named sub-
       pattern from elsewhere in the pattern, the one that corresponds to  the
       lowest  number  is used. For further details of the interfaces for han-
       dling named subpatterns, see the pcreapi documentation.

REPETITION

Repetition is specified by quantifiers, which can  follow  any  of  the
       following items:

a literal data character
         the dot metacharacter
         the \C escape sequence
         the \X escape sequence (in UTF-8 mode with Unicode properties)
         the \R escape sequence
         an escape such as \d that matches a single character
         a character class
         a back reference (see next section)
         a parenthesized subpattern (unless it is an assertion)

The  general repetition quantifier specifies a minimum and maximum num-
       ber of permitted matches, by giving the two numbers in  curly  brackets
       (braces),  separated  by  a comma. The numbers must be less than 65536,
       and the first must be less than or equal to the second. For example:

z{2,4}

matches "zz", "zzz", or "zzzz". A closing brace on its  own  is  not  a
       special  character.  If  the second number is omitted, but the comma is
       present, there is no upper limit; if the second number  and  the  comma
       are  both omitted, the quantifier specifies an exact number of required
       matches. Thus

[aeiou]{3,}

matches at least 3 successive vowels, but may match many more, while

\d{8}

matches exactly 8 digits. An opening curly bracket that  appears  in  a
       position  where a quantifier is not allowed, or one that does not match
       the syntax of a quantifier, is taken as a literal character. For  exam-
       ple, {,6} is not a quantifier, but a literal string of four characters.

In UTF-8 mode, quantifiers apply to UTF-8  characters  rather  than  to
       individual bytes. Thus, for example, \x{100}{2} matches two UTF-8 char-
       acters, each of which is represented by a two-byte sequence. Similarly,
       when Unicode property support is available, \X{3} matches three Unicode
       extended sequences, each of which may be several bytes long  (and  they
       may be of different lengths).

The quantifier {0} is permitted, causing the expression to behave as if
       the previous item and the quantifier were not present. This may be use-
       ful  for  subpatterns that are referenced as subroutines from elsewhere
       in the pattern. Items other than subpatterns that have a {0} quantifier
       are omitted from the compiled pattern.

For  convenience, the three most common quantifiers have single-charac-
       ter abbreviations:

*    is equivalent to {0,}
         +    is equivalent to {1,}
         ?    is equivalent to {0,1}

It is possible to construct infinite loops by  following  a  subpattern
       that can match no characters with a quantifier that has no upper limit,
       for example:

(a?)*

Earlier versions of Perl and PCRE used to give an error at compile time
       for  such  patterns. However, because there are cases where this can be
       useful, such patterns are now accepted, but if any  repetition  of  the
       subpattern  does in fact match no characters, the loop is forcibly bro-
       ken.

By default, the quantifiers are "greedy", that is, they match  as  much
       as  possible  (up  to  the  maximum number of permitted times), without
       causing the rest of the pattern to fail. The classic example  of  where
       this gives problems is in trying to match comments in C programs. These
       appear between /* and */ and within the comment,  individual  *  and  /
       characters  may  appear. An attempt to match C comments by applying the
       pattern

/\*.*\*/

to the string

/* first comment */  not comment  /* second comment */

fails, because it matches the entire string owing to the greediness  of
       the .*  item.

However,  if  a quantifier is followed by a question mark, it ceases to
       be greedy, and instead matches the minimum number of times possible, so
       the pattern

/\*.*?\*/

does  the  right  thing with the C comments. The meaning of the various
       quantifiers is not otherwise changed,  just  the  preferred  number  of
       matches.   Do  not  confuse this use of question mark with its use as a
       quantifier in its own right. Because it has two uses, it can  sometimes
       appear doubled, as in

\d??\d

which matches one digit by preference, but can match two if that is the
       only way the rest of the pattern matches.

If the PCRE_UNGREEDY option is set (an option that is not available  in
       Perl),  the  quantifiers are not greedy by default, but individual ones
       can be made greedy by following them with a  question  mark.  In  other
       words, it inverts the default behaviour.

When  a  parenthesized  subpattern  is quantified with a minimum repeat
       count that is greater than 1 or with a limited maximum, more memory  is
       required  for  the  compiled  pattern, in proportion to the size of the
       minimum or maximum.

If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equiv-
       alent  to  Perl's  /s) is set, thus allowing the dot to match newlines,
       the pattern is implicitly anchored, because whatever  follows  will  be
       tried  against every character position in the subject string, so there
       is no point in retrying the overall match at  any  position  after  the
       first.  PCRE  normally treats such a pattern as though it were preceded
       by \A.

In cases where it is known that the subject  string  contains  no  new-
       lines,  it  is  worth setting PCRE_DOTALL in order to obtain this opti-
       mization, or alternatively using ^ to indicate anchoring explicitly.

However, there is one situation where the optimization cannot be  used.
       When  .*   is  inside  capturing  parentheses that are the subject of a
       backreference elsewhere in the pattern, a match at the start  may  fail
       where a later one succeeds. Consider, for example:

(.*)abc\1

If  the subject is "xyz123abc123" the match point is the fourth charac-
       ter. For this reason, such a pattern is not implicitly anchored.

When a capturing subpattern is repeated, the value captured is the sub-
       string that matched the final iteration. For example, after

(tweedle[dume]{3}\s*)+

has matched "tweedledum tweedledee" the value of the captured substring
       is "tweedledee". However, if there are  nested  capturing  subpatterns,
       the  corresponding captured values may have been set in previous itera-
       tions. For example, after

/(a|(b))+/

matches "aba" the value of the second captured substring is "b".

ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS

With both maximizing ("greedy") and minimizing ("ungreedy"  or  "lazy")
       repetition,  failure  of what follows normally causes the repeated item
       to be re-evaluated to see if a different number of repeats  allows  the
       rest  of  the pattern to match. Sometimes it is useful to prevent this,
       either to change the nature of the match, or to cause it  fail  earlier
       than  it otherwise might, when the author of the pattern knows there is
       no point in carrying on.

Consider, for example, the pattern \d+foo when applied to  the  subject
       line

123456bar

After matching all 6 digits and then failing to match "foo", the normal
       action of the matcher is to try again with only 5 digits  matching  the
       \d+  item,  and  then  with  4,  and  so on, before ultimately failing.
       "Atomic grouping" (a term taken from Jeffrey  Friedl's  book)  provides
       the  means for specifying that once a subpattern has matched, it is not
       to be re-evaluated in this way.

If we use atomic grouping for the previous example, the  matcher  gives
       up  immediately  on failing to match "foo" the first time. The notation
       is a kind of special parenthesis, starting with (?> as in this example:

(?>\d+)foo

This  kind  of  parenthesis "locks up" the  part of the pattern it con-
       tains once it has matched, and a failure further into  the  pattern  is
       prevented  from  backtracking into it. Backtracking past it to previous
       items, however, works as normal.

An alternative description is that a subpattern of  this  type  matches
       the  string  of  characters  that an identical standalone pattern would
       match, if anchored at the current point in the subject string.

Atomic grouping subpatterns are not capturing subpatterns. Simple cases
       such as the above example can be thought of as a maximizing repeat that
       must swallow everything it can. So, while both \d+ and  \d+?  are  pre-
       pared  to  adjust  the number of digits they match in order to make the
       rest of the pattern match, (?>\d+) can only match an entire sequence of
       digits.

Atomic  groups in general can of course contain arbitrarily complicated
       subpatterns, and can be nested. However, when  the  subpattern  for  an
       atomic group is just a single repeated item, as in the example above, a
       simpler notation, called a "possessive quantifier" can  be  used.  This
       consists  of  an  additional  + character following a quantifier. Using
       this notation, the previous example can be rewritten as

\d++foo

Note that a possessive quantifier can be used with an entire group, for
       example:

(abc|xyz){2,3}+

Possessive   quantifiers   are   always  greedy;  the  setting  of  the
       PCRE_UNGREEDY option is ignored. They are a convenient notation for the
       simpler  forms  of atomic group. However, there is no difference in the
       meaning of a possessive quantifier and  the  equivalent  atomic  group,
       though  there  may  be a performance difference; possessive quantifiers
       should be slightly faster.

The possessive quantifier syntax is an extension to the Perl  5.8  syn-
       tax.   Jeffrey  Friedl  originated the idea (and the name) in the first
       edition of his book. Mike McCloskey liked it, so implemented it when he
       built  Sun's Java package, and PCRE copied it from there. It ultimately
       found its way into Perl at release 5.10.

PCRE has an optimization that automatically "possessifies" certain sim-
       ple  pattern  constructs.  For  example, the sequence A+B is treated as
       A++B because there is no point in backtracking into a sequence  of  A's
       when B must follow.

When  a  pattern  contains an unlimited repeat inside a subpattern that
       can itself be repeated an unlimited number of  times,  the  use  of  an
       atomic  group  is  the  only way to avoid some failing matches taking a
       very long time indeed. The pattern

(\D+|<\d+>)*[!?]

matches an unlimited number of substrings that either consist  of  non-
       digits,  or  digits  enclosed in <>, followed by either ! or ?. When it
       matches, it runs quickly. However, if it is applied to

aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

it takes a long time before reporting  failure.  This  is  because  the
       string  can be divided between the internal \D+ repeat and the external
       * repeat in a large number of ways, and all  have  to  be  tried.  (The
       example  uses  [!?]  rather than a single character at the end, because
       both PCRE and Perl have an optimization that allows  for  fast  failure
       when  a single character is used. They remember the last single charac-
       ter that is required for a match, and fail early if it is  not  present
       in  the  string.)  If  the pattern is changed so that it uses an atomic
       group, like this:

((?>\D+)|<\d+>)*[!?]

sequences of non-digits cannot be broken, and failure happens  quickly.

BACK REFERENCES

Outside a character class, a backslash followed by a digit greater than
       0 (and possibly further digits) is a back reference to a capturing sub-
       pattern  earlier  (that is, to its left) in the pattern, provided there
       have been that many previous capturing left parentheses.

However, if the decimal number following the backslash is less than 10,
       it  is  always  taken  as a back reference, and causes an error only if
       there are not that many capturing left parentheses in the  entire  pat-
       tern.  In  other words, the parentheses that are referenced need not be
       to the left of the reference for numbers less than 10. A "forward  back
       reference"  of  this  type can make sense when a repetition is involved
       and the subpattern to the right has participated in an  earlier  itera-
       tion.

It  is  not  possible to have a numerical "forward back reference" to a
       subpattern whose number is 10 or  more  using  this  syntax  because  a
       sequence  such  as  \50 is interpreted as a character defined in octal.
       See the subsection entitled "Non-printing characters" above for further
       details  of  the  handling of digits following a backslash. There is no
       such problem when named parentheses are used. A back reference  to  any
       subpattern is possible using named parentheses (see below).

Another  way  of  avoiding  the ambiguity inherent in the use of digits
       following a backslash is to use the \g escape sequence, which is a fea-
       ture  introduced  in  Perl  5.10.  This  escape  must be followed by an
       unsigned number or a negative number, optionally  enclosed  in  braces.
       These examples are all identical:

(ring), \1
         (ring), \g1
         (ring), \g{1}

An  unsigned number specifies an absolute reference without the ambigu-
       ity that is present in the older syntax. It is also useful when literal
       digits follow the reference. A negative number is a relative reference.
       Consider this example:

(abc(def)ghi)\g{-1}

The sequence \g{-1} is a reference to the most recently started captur-
       ing  subpattern  before \g, that is, is it equivalent to \2. Similarly,
       \g{-2} would be equivalent to \1. The use of relative references can be
       helpful  in  long  patterns,  and  also in patterns that are created by
       joining together fragments that contain references within themselves.

A back reference matches whatever actually matched the  capturing  sub-
       pattern  in  the  current subject string, rather than anything matching
       the subpattern itself (see "Subpatterns as subroutines" below for a way
       of doing that). So the pattern

(sens|respons)e and \1ibility

matches  "sense and sensibility" and "response and responsibility", but
       not "sense and responsibility". If caseful matching is in force at  the
       time  of the back reference, the case of letters is relevant. For exam-
       ple,

((?i)rah)\s+\1

matches "rah rah" and "RAH RAH", but not "RAH  rah",  even  though  the
       original capturing subpattern is matched caselessly.

There  are  several  different ways of writing back references to named
       subpatterns. The .NET syntax \k{name} and the Perl syntax  \k<name>  or
       \k'name'  are supported, as is the Python syntax (?P=name). Perl 5.10's
       unified back reference syntax, in which \g can be used for both numeric
       and  named  references,  is  also supported. We could rewrite the above
       example in any of the following ways:

(?<p1>(?i)rah)\s+\k<p1>
         (?'p1'(?i)rah)\s+\k{p1}
         (?P<p1>(?i)rah)\s+(?P=p1)
         (?<p1>(?i)rah)\s+\g{p1}

A subpattern that is referenced by  name  may  appear  in  the  pattern
       before or after the reference.

There  may be more than one back reference to the same subpattern. If a
       subpattern has not actually been used in a particular match,  any  back
       references to it always fail. For example, the pattern

(a|(bc))\2

always  fails if it starts to match "a" rather than "bc". Because there
       may be many capturing parentheses in a pattern,  all  digits  following
       the  backslash  are taken as part of a potential back reference number.
       If the pattern continues with a digit character, some delimiter must be
       used  to  terminate  the back reference. If the PCRE_EXTENDED option is
       set, this can be whitespace.  Otherwise an  empty  comment  (see  "Com-
       ments" below) can be used.

A  back reference that occurs inside the parentheses to which it refers
       fails when the subpattern is first used, so, for example,  (a\1)  never
       matches.   However,  such references can be useful inside repeated sub-
       patterns. For example, the pattern

(a|b\1)+

matches any number of "a"s and also "aba", "ababbaa" etc. At each iter-
       ation  of  the  subpattern,  the  back  reference matches the character
       string corresponding to the previous iteration. In order  for  this  to
       work,  the  pattern must be such that the first iteration does not need
       to match the back reference. This can be done using alternation, as  in
       the example above, or by a quantifier with a minimum of zero.

ASSERTIONS

An  assertion  is  a  test on the characters following or preceding the
       current matching point that does not actually consume  any  characters.
       The  simple  assertions  coded  as  \b, \B, \A, \G, \Z, \z, ^ and $ are
       described above.

More complicated assertions are coded as  subpatterns.  There  are  two
       kinds:  those  that  look  ahead of the current position in the subject
       string, and those that look  behind  it.  An  assertion  subpattern  is
       matched  in  the  normal way, except that it does not cause the current
       matching position to be changed.

Assertion subpatterns are not capturing subpatterns,  and  may  not  be
       repeated,  because  it  makes no sense to assert the same thing several
       times. If any kind of assertion contains capturing  subpatterns  within
       it,  these are counted for the purposes of numbering the capturing sub-
       patterns in the whole pattern.  However, substring capturing is carried
       out  only  for  positive assertions, because it does not make sense for
       negative assertions.

Lookahead assertions

Lookahead assertions start with (?= for positive assertions and (?! for
       negative assertions. For example,

\w+(?=;)

matches  a word followed by a semicolon, but does not include the semi-
       colon in the match, and

foo(?!bar)

matches any occurrence of "foo" that is not  followed  by  "bar".  Note
       that the apparently similar pattern

(?!foo)bar

does  not  find  an  occurrence  of "bar" that is preceded by something
       other than "foo"; it finds any occurrence of "bar" whatsoever,  because
       the assertion (?!foo) is always true when the next three characters are
       "bar". A lookbehind assertion is needed to achieve the other effect.

If you want to force a matching failure at some point in a pattern, the
       most  convenient  way  to  do  it  is with (?!) because an empty string
       always matches, so an assertion that requires there not to be an  empty
       string must always fail.

Lookbehind assertions

Lookbehind  assertions start with (?<= for positive assertions and (?<!
       for negative assertions. For example,

(?<!foo)bar

does find an occurrence of "bar" that is not  preceded  by  "foo".  The
       contents  of  a  lookbehind  assertion are restricted such that all the
       strings it matches must have a fixed length. However, if there are sev-
       eral  top-level  alternatives,  they  do  not all have to have the same
       fixed length. Thus

(?<=bullock|donkey)

is permitted, but

(?<!dogs?|cats?)

causes an error at compile time. Branches that match  different  length
       strings  are permitted only at the top level of a lookbehind assertion.
       This is an extension compared with  Perl  (at  least  for  5.8),  which
       requires  all branches to match the same length of string. An assertion
       such as

(?<=ab(c|de))

is not permitted, because its single top-level  branch  can  match  two
       different  lengths,  but  it is acceptable if rewritten to use two top-
       level branches:

(?<=abc|abde)

In some cases, the Perl 5.10 escape sequence \K (see above) can be used
       instead  of  a lookbehind assertion; this is not restricted to a fixed-
       length.

The implementation of lookbehind assertions is, for  each  alternative,
       to  temporarily  move the current position back by the fixed length and
       then try to match. If there are insufficient characters before the cur-
       rent position, the assertion fails.

PCRE does not allow the \C escape (which matches a single byte in UTF-8
       mode) to appear in lookbehind assertions, because it makes it  impossi-
       ble  to  calculate the length of the lookbehind. The \X and \R escapes,
       which can match different numbers of bytes, are also not permitted.

Possessive quantifiers can  be  used  in  conjunction  with  lookbehind
       assertions  to  specify  efficient  matching  at the end of the subject
       string. Consider a simple pattern such as

abcd$

when applied to a long string that does  not  match.  Because  matching
       proceeds from left to right, PCRE will look for each "a" in the subject
       and then see if what follows matches the rest of the  pattern.  If  the
       pattern is specified as

^.*abcd$

the  initial .* matches the entire string at first, but when this fails
       (because there is no following "a"), it backtracks to match all but the
       last  character,  then all but the last two characters, and so on. Once
       again the search for "a" covers the entire string, from right to  left,
       so we are no better off. However, if the pattern is written as

^.*+(?<=abcd)

there  can  be  no backtracking for the .*+ item; it can match only the
       entire string. The subsequent lookbehind assertion does a  single  test
       on  the last four characters. If it fails, the match fails immediately.
       For long strings, this approach makes a significant difference  to  the
       processing time.

Using multiple assertions

Several assertions (of any sort) may occur in succession. For example,

(?<=\d{3})(?<!999)foo

matches  "foo" preceded by three digits that are not "999". Notice that
       each of the assertions is applied independently at the  same  point  in
       the  subject  string.  First  there  is a check that the previous three
       characters are all digits, and then there is  a  check  that  the  same
       three characters are not "999".  This pattern does not match "foo" pre-
       ceded by six characters, the first of which are  digits  and  the  last
       three  of  which  are not "999". For example, it doesn't match "123abc-
       foo". A pattern to do that is

(?<=\d{3}...)(?<!999)foo

This time the first assertion looks at the  preceding  six  characters,
       checking that the first three are digits, and then the second assertion
       checks that the preceding three characters are not "999".

Assertions can be nested in any combination. For example,

(?<=(?<!foo)bar)baz

matches an occurrence of "baz" that is preceded by "bar" which in  turn
       is not preceded by "foo", while

(?<=\d{3}(?!999)...)foo

is  another pattern that matches "foo" preceded by three digits and any
       three characters that are not "999".

CONDITIONAL SUBPATTERNS

It is possible to cause the matching process to obey a subpattern  con-
       ditionally  or to choose between two alternative subpatterns, depending
       on the result of an assertion, or whether a previous capturing  subpat-
       tern  matched  or not. The two possible forms of conditional subpattern
       are

(?(condition)yes-pattern)
         (?(condition)yes-pattern|no-pattern)

If the condition is satisfied, the yes-pattern is used;  otherwise  the
       no-pattern  (if  present)  is used. If there are more than two alterna-
       tives in the subpattern, a compile-time error occurs.

There are four kinds of condition: references  to  subpatterns,  refer-
       ences to recursion, a pseudo-condition called DEFINE, and assertions.

Checking for a used subpattern by number

If  the  text between the parentheses consists of a sequence of digits,
       the condition is true if the capturing subpattern of  that  number  has
       previously  matched.  An  alternative notation is to precede the digits
       with a plus or minus sign. In this case, the subpattern number is rela-
       tive rather than absolute.  The most recently opened parentheses can be
       referenced by (?(-1), the next most recent by (?(-2),  and  so  on.  In
       looping constructs it can also make sense to refer to subsequent groups
       with constructs such as (?(+2).

Consider the following pattern, which  contains  non-significant  white
       space to make it more readable (assume the PCRE_EXTENDED option) and to
       divide it into three parts for ease of discussion:

( $ )?    [^()]+    (?(1) $ )

The first part matches an optional opening  parenthesis,  and  if  that
       character is present, sets it as the first captured substring. The sec-
       ond part matches one or more characters that are not  parentheses.  The
       third part is a conditional subpattern that tests whether the first set
       of parentheses matched or not. If they did, that is, if subject started
       with an opening parenthesis, the condition is true, and so the yes-pat-
       tern is executed and a  closing  parenthesis  is  required.  Otherwise,
       since  no-pattern  is  not  present, the subpattern matches nothing. In
       other words,  this  pattern  matches  a  sequence  of  non-parentheses,
       optionally enclosed in parentheses.

If  you  were  embedding  this pattern in a larger one, you could use a
       relative reference:

...other stuff... ( $ )?    [^()]+    (?(-1) $ ) ...

This makes the fragment independent of the parentheses  in  the  larger
       pattern.

Checking for a used subpattern by name

Perl  uses  the  syntax  (?(<name>)...) or (?('name')...) to test for a
       used subpattern by name. For compatibility  with  earlier  versions  of
       PCRE,  which  had this facility before Perl, the syntax (?(name)...) is
       also recognized. However, there is a possible ambiguity with this  syn-
       tax,  because  subpattern  names  may  consist entirely of digits. PCRE
       looks first for a named subpattern; if it cannot find one and the  name
       consists  entirely  of digits, PCRE looks for a subpattern of that num-
       ber, which must be greater than zero. Using subpattern names that  con-
       sist entirely of digits is not recommended.

Rewriting the above example to use a named subpattern gives this:

(?<OPEN> $ )?    [^()]+    (?(<OPEN>) $ )

Checking for pattern recursion

If the condition is the string (R), and there is no subpattern with the
       name R, the condition is true if a recursive call to the whole  pattern
       or any subpattern has been made. If digits or a name preceded by amper-
       sand follow the letter R, for example:

(?(R3)...) or (?(R&name)...)

the condition is true if the most recent recursion is into the  subpat-
       tern  whose  number or name is given. This condition does not check the
       entire recursion stack.

At "top level", all these recursion test conditions are  false.  Recur-
       sive patterns are described below.

Defining subpatterns for use by reference only

If  the  condition  is  the string (DEFINE), and there is no subpattern
       with the name DEFINE, the condition is  always  false.  In  this  case,
       there  may  be  only  one  alternative  in the subpattern. It is always
       skipped if control reaches this point  in  the  pattern;  the  idea  of
       DEFINE  is that it can be used to define "subroutines" that can be ref-
       erenced from elsewhere. (The use of "subroutines" is described  below.)
       For  example,  a pattern to match an IPv4 address could be written like
       this (ignore whitespace and line breaks):

(?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
         \b (?&byte) (\.(?&byte)){3} \b

The first part of the pattern is a DEFINE group inside which a  another
       group  named "byte" is defined. This matches an individual component of
       an IPv4 address (a number less than 256). When  matching  takes  place,
       this  part  of  the pattern is skipped because DEFINE acts like a false
       condition.

The rest of the pattern uses references to the named group to match the
       four  dot-separated  components of an IPv4 address, insisting on a word
       boundary at each end.

Assertion conditions

If the condition is not in any of the above  formats,  it  must  be  an
       assertion.   This may be a positive or negative lookahead or lookbehind
       assertion. Consider  this  pattern,  again  containing  non-significant
       white space, and with the two alternatives on the second line:

(?(?=[^a-z]*[a-z])
         \d{2}-[a-z]{3}-\d{2}  |  \d{2}-\d{2}-\d{2} )

The  condition  is  a  positive  lookahead  assertion  that  matches an
       optional sequence of non-letters followed by a letter. In other  words,
       it  tests  for the presence of at least one letter in the subject. If a
       letter is found, the subject is matched against the first  alternative;
       otherwise  it  is  matched  against  the  second.  This pattern matches
       strings in one of the two forms dd-aaa-dd or dd-dd-dd,  where  aaa  are
       letters and dd are digits.

COMMENTS

The  sequence (?# marks the start of a comment that continues up to the
       next closing parenthesis. Nested parentheses  are  not  permitted.  The
       characters  that make up a comment play no part in the pattern matching
       at all.

If the PCRE_EXTENDED option is set, an unescaped # character outside  a
       character  class  introduces  a  comment  that continues to immediately
       after the next newline in the pattern.

RECURSIVE PATTERNS

Consider the problem of matching a string in parentheses, allowing  for
       unlimited  nested  parentheses.  Without the use of recursion, the best
       that can be done is to use a pattern that  matches  up  to  some  fixed
       depth  of  nesting.  It  is not possible to handle an arbitrary nesting
       depth.

For some time, Perl has provided a facility that allows regular expres-
       sions  to recurse (amongst other things). It does this by interpolating
       Perl code in the expression at run time, and the code can refer to  the
       expression itself. A Perl pattern using code interpolation to solve the
       parentheses problem can be created like this:

$re = qr{$ (?: (?>[^()]+) | (?p{$re}) )* $}x;

The (?p{...}) item interpolates Perl code at run time, and in this case
       refers recursively to the pattern in which it appears.

Obviously, PCRE cannot support the interpolation of Perl code. Instead,
       it supports special syntax for recursion of  the  entire  pattern,  and
       also  for  individual  subpattern  recursion. After its introduction in
       PCRE and Python, this kind of recursion was  introduced  into  Perl  at
       release 5.10.

A  special  item  that consists of (? followed by a number greater than
       zero and a closing parenthesis is a recursive call of the subpattern of
       the  given  number, provided that it occurs inside that subpattern. (If
       not, it is a "subroutine" call, which is described  in  the  next  sec-
       tion.)  The special item (?R) or (?0) is a recursive call of the entire
       regular expression.

In PCRE (like Python, but unlike Perl), a recursive subpattern call  is
       always treated as an atomic group. That is, once it has matched some of
       the subject string, it is never re-entered, even if it contains untried
       alternatives and there is a subsequent matching failure.

This  PCRE  pattern  solves  the nested parentheses problem (assume the
       PCRE_EXTENDED option is set so that white space is ignored):

$ ( (?>[^()]+) | (?R) )* $

First it matches an opening parenthesis. Then it matches any number  of
       substrings  which  can  either  be  a sequence of non-parentheses, or a
       recursive match of the pattern itself (that is, a  correctly  parenthe-
       sized substring).  Finally there is a closing parenthesis.

If  this  were  part of a larger pattern, you would not want to recurse
       the entire pattern, so instead you could use this:

( $ ( (?>[^()]+) | (?1) )* $ )

We have put the pattern into parentheses, and caused the  recursion  to
       refer to them instead of the whole pattern.

In  a  larger  pattern,  keeping  track  of  parenthesis numbers can be
       tricky. This is made easier by the use of relative references. (A  Perl
       5.10  feature.)   Instead  of  (?1)  in the pattern above you can write
       (?-2) to refer to the second most recently opened parentheses preceding
       the  recursion.  In  other  words,  a  negative number counts capturing
       parentheses leftwards from the point at which it is encountered.

It is also possible to refer to  subsequently  opened  parentheses,  by
       writing  references  such  as (?+2). However, these cannot be recursive
       because the reference is not inside the  parentheses  that  are  refer-
       enced.  They  are  always  "subroutine" calls, as described in the next
       section.

An alternative approach is to use named parentheses instead.  The  Perl
       syntax  for  this  is (?&name); PCRE's earlier syntax (?P>name) is also
       supported. We could rewrite the above example as follows:

(?<pn> $ ( (?>[^()]+) | (?&pn) )* $ )

If there is more than one subpattern with the same name,  the  earliest
       one is used.

This  particular  example pattern that we have been looking at contains
       nested unlimited repeats, and so the use of atomic grouping for  match-
       ing  strings  of non-parentheses is important when applying the pattern
       to strings that do not match. For example, when this pattern is applied
       to

(aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()

it  yields "no match" quickly. However, if atomic grouping is not used,
       the match runs for a very long time indeed because there  are  so  many
       different  ways  the  + and * repeats can carve up the subject, and all
       have to be tested before failure can be reported.

At the end of a match, the values set for any capturing subpatterns are
       those from the outermost level of the recursion at which the subpattern
       value is set.  If you want to obtain  intermediate  values,  a  callout
       function  can be used (see below and the pcrecallout documentation). If
       the pattern above is matched against

(ab(cd)ef)

the value for the capturing parentheses is  "ef",  which  is  the  last
       value  taken  on at the top level. If additional parentheses are added,
       giving

$ ( ( (?>[^()]+) | (?R) )* ) $
            ^                        ^
            ^                        ^

the string they capture is "ab(cd)ef", the contents of  the  top  level
       parentheses.  If there are more than 15 capturing parentheses in a pat-
       tern, PCRE has to obtain extra memory to store data during a recursion,
       which  it  does  by  using pcre_malloc, freeing it via pcre_free after-
       wards. If  no  memory  can  be  obtained,  the  match  fails  with  the
       PCRE_ERROR_NOMEMORY error.

Do  not  confuse  the (?R) item with the condition (R), which tests for
       recursion.  Consider this pattern, which matches text in  angle  brack-
       ets,  allowing for arbitrary nesting. Only digits are allowed in nested
       brackets (that is, when recursing), whereas any characters are  permit-
       ted at the outer level.

< (?: (?(R) \d++  | [^<>]*+) | (?R)) * >

In  this  pattern, (?(R) is the start of a conditional subpattern, with
       two different alternatives for the recursive and  non-recursive  cases.
       The (?R) item is the actual recursive call.

SUBPATTERNS AS SUBROUTINES

If the syntax for a recursive subpattern reference (either by number or
       by name) is used outside the parentheses to which it refers,  it  oper-
       ates  like a subroutine in a programming language. The "called" subpat-
       tern may be defined before or after the reference. A numbered reference
       can be absolute or relative, as in these examples:

(...(absolute)...)...(?2)...
         (...(relative)...)...(?-1)...
         (...(?+1)...(relative)...

An earlier example pointed out that the pattern

(sens|respons)e and \1ibility

matches  "sense and sensibility" and "response and responsibility", but
       not "sense and responsibility". If instead the pattern

(sens|respons)e and (?1)ibility

is used, it does match "sense and responsibility" as well as the  other
       two  strings.  Another  example  is  given  in the discussion of DEFINE
       above.

Like recursive subpatterns, a "subroutine" call is always treated as an
       atomic  group. That is, once it has matched some of the subject string,
       it is never re-entered, even if it contains  untried  alternatives  and
       there is a subsequent matching failure.

When  a  subpattern is used as a subroutine, processing options such as
       case-independence are fixed when the subpattern is defined. They cannot
       be changed for different calls. For example, consider this pattern:

(abc)(?i:(?-1))

It  matches  "abcabc". It does not match "abcABC" because the change of
       processing option does not affect the called subpattern.

ONIGURUMA SUBROUTINE SYNTAX

For compatibility with Oniguruma, the non-Perl syntax \g followed by  a
       name or a number enclosed either in angle brackets or single quotes, is
       an alternative syntax for referencing a  subpattern  as  a  subroutine,
       possibly  recursively. Here are two of the examples used above, rewrit-
       ten using this syntax:

(?<pn> $ ( (?>[^()]+) | \g<pn> )* $ )
         (sens|respons)e and \g'1'ibility

PCRE supports an extension to Oniguruma: if a number is preceded  by  a
       plus or a minus sign it is taken as a relative reference. For example:

(abc)(?i:\g<-1>)

Note  that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are not
       synonymous. The former is a back reference; the latter is a  subroutine
       call.

CALLOUTS

Perl has a feature whereby using the sequence (?{...}) causes arbitrary
       Perl code to be obeyed in the middle of matching a regular  expression.
       This makes it possible, amongst other things, to extract different sub-
       strings that match the same pair of parentheses when there is a repeti-
       tion.

PCRE provides a similar feature, but of course it cannot obey arbitrary
       Perl code. The feature is called "callout". The caller of PCRE provides
       an  external function by putting its entry point in the global variable
       pcre_callout.  By default, this variable contains NULL, which  disables
       all calling out.

Within  a  regular  expression,  (?C) indicates the points at which the
       external function is to be called. If you want  to  identify  different
       callout  points, you can put a number less than 256 after the letter C.
       The default value is zero.  For example, this pattern has  two  callout
       points:

(?C1)abc(?C2)def

If the PCRE_AUTO_CALLOUT flag is passed to pcre_compile(), callouts are
       automatically installed before each item in the pattern. They  are  all
       numbered 255.

During matching, when PCRE reaches a callout point (and pcre_callout is
       set), the external function is called. It is provided with  the  number
       of  the callout, the position in the pattern, and, optionally, one item
       of data originally supplied by the caller of pcre_exec().  The  callout
       function  may cause matching to proceed, to backtrack, or to fail alto-
       gether. A complete description of the interface to the callout function
       is given in the pcrecallout documentation.

BACKTRACKING CONTROL

Perl  5.10 introduced a number of "Special Backtracking Control Verbs",
       which are described in the Perl documentation as "experimental and sub-
       ject  to  change or removal in a future version of Perl". It goes on to
       say: "Their usage in production code should be noted to avoid  problems
       during upgrades." The same remarks apply to the PCRE features described
       in this section.

Since these verbs are specifically related  to  backtracking,  most  of
       them  can  be  used  only  when  the  pattern  is  to  be matched using
       pcre_exec(), which uses a backtracking algorithm. With the exception of
       (*FAIL), which behaves like a failing negative assertion, they cause an
       error if encountered by pcre_dfa_exec().

The new verbs make use of what was previously invalid syntax: an  open-
       ing parenthesis followed by an asterisk. In Perl, they are generally of
       the form (*VERB:ARG) but PCRE does not support the use of arguments, so
       its  general  form is just (*VERB). Any number of these verbs may occur
       in a pattern. There are two kinds:

Verbs that act immediately

The following verbs act as soon as they are encountered:

(*ACCEPT)

This verb causes the match to end successfully, skipping the  remainder
       of  the pattern. When inside a recursion, only the innermost pattern is
       ended immediately. PCRE differs  from  Perl  in  what  happens  if  the
       (*ACCEPT)  is inside capturing parentheses. In Perl, the data so far is
       captured: in PCRE no data is captured. For example:

A(A|B(*ACCEPT)|C)D

This matches "AB", "AAD", or "ACD", but when it matches "AB",  no  data
       is captured.

(*FAIL) or (*F)

This  verb  causes the match to fail, forcing backtracking to occur. It
       is equivalent to (?!) but easier to read. The Perl documentation  notes
       that  it  is  probably  useful only when combined with (?{}) or (??{}).
       Those are, of course, Perl features that are not present in  PCRE.  The
       nearest  equivalent is the callout feature, as for example in this pat-
       tern:

a+(?C)(*FAIL)

A match with the string "aaaa" always fails, but the callout  is  taken
       before each backtrack happens (in this example, 10 times).

Verbs that act after backtracking

The following verbs do nothing when they are encountered. Matching con-
       tinues with what follows, but if there is no subsequent match, a  fail-
       ure  is  forced.   The  verbs  differ  in  exactly what kind of failure
       occurs.

(*COMMIT)

This verb causes the whole match to fail outright if the  rest  of  the
       pattern  does  not match. Even if the pattern is unanchored, no further
       attempts to find a match by advancing the start point take place.  Once
       (*COMMIT)  has been passed, pcre_exec() is committed to finding a match
       at the current starting point, or not at all. For example:

a+(*COMMIT)b

This matches "xxaab" but not "aacaab". It can be thought of as  a  kind
       of dynamic anchor, or "I've started, so I must finish."

(*PRUNE)

This  verb causes the match to fail at the current position if the rest
       of the pattern does not match. If the pattern is unanchored, the normal
       "bumpalong"  advance to the next starting character then happens. Back-
       tracking can occur as usual to the left of (*PRUNE), or  when  matching
       to  the right of (*PRUNE), but if there is no match to the right, back-
       tracking cannot cross (*PRUNE).  In simple cases, the use  of  (*PRUNE)
       is just an alternative to an atomic group or possessive quantifier, but
       there are some uses of (*PRUNE) that cannot be expressed in  any  other
       way.

(*SKIP)

This  verb  is like (*PRUNE), except that if the pattern is unanchored,
       the "bumpalong" advance is not to the next character, but to the  posi-
       tion  in  the  subject where (*SKIP) was encountered. (*SKIP) signifies
       that whatever text was matched leading up to it cannot  be  part  of  a
       successful match. Consider:

a+(*SKIP)b

If  the  subject  is  "aaaac...",  after  the first match attempt fails
       (starting at the first character in the  string),  the  starting  point
       skips on to start the next attempt at "c". Note that a possessive quan-
       tifer does not have the same effect in this example; although it  would
       suppress  backtracking  during  the  first  match  attempt,  the second
       attempt would start at the second character instead of skipping  on  to
       "c".

(*THEN)

This verb causes a skip to the next alternation if the rest of the pat-
       tern does not match. That is, it cancels pending backtracking, but only
       within  the  current  alternation.  Its name comes from the observation
       that it can be used for a pattern-based if-then-else block:

( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...

If the COND1 pattern matches, FOO is tried (and possibly further  items
       after  the  end  of  the group if FOO succeeds); on failure the matcher
       skips to the second alternative and tries COND2,  without  backtracking
       into  COND1.  If  (*THEN)  is  used outside of any alternation, it acts
       exactly like (*PRUNE).

SEE ALSO

pcreapi(3), pcrecallout(3), pcrematching(3), pcre(3).

AUTHOR

Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.

REVISION

Last updated: 19 April 2008
       Copyright (c) 1997-2008 University of Cambridge.
------------------------------------------------------------------------------

PCRESYNTAX(3)                                                    PCRESYNTAX(3)

NAME
       PCRE - Perl-compatible regular expressions

PCRE REGULAR EXPRESSION SYNTAX SUMMARY

The  full syntax and semantics of the regular expressions that are sup-
       ported by PCRE are described in  the  pcrepattern  documentation.  This
       document contains just a quick-reference summary of the syntax.

QUOTING

\x         where x is non-alphanumeric is a literal x
         \Q...\E    treat enclosed characters as literal

CHARACTERS

\a         alarm, that is, the BEL character (hex 07)
         \cx        "control-x", where x is any character
         \e         escape (hex 1B)
         \f         formfeed (hex 0C)
         \n         newline (hex 0A)
         \r         carriage return (hex 0D)
         \t         tab (hex 09)
         \ddd       character with octal code ddd, or backreference
         \xhh       character with hex code hh
         \x{hhh..}  character with hex code hhh..

CHARACTER TYPES

.          any character except newline;
                      in dotall mode, any character whatsoever
         \C         one byte, even in UTF-8 mode (best avoided)
         \d         a decimal digit
         \D         a character that is not a decimal digit
         \h         a horizontal whitespace character
         \H         a character that is not a horizontal whitespace character
         \p{xx}     a character with the xx property
         \P{xx}     a character without the xx property
         \R         a newline sequence
         \s         a whitespace character
         \S         a character that is not a whitespace character
         \v         a vertical whitespace character
         \V         a character that is not a vertical whitespace character
         \w         a "word" character
         \W         a "non-word" character
         \X         an extended Unicode sequence

In PCRE, \d, \D, \s, \S, \w, and \W recognize only ASCII characters.

GENERAL CATEGORY PROPERTY CODES FOR \p and \P

C          Other
         Cc         Control
         Cf         Format
         Cn         Unassigned
         Co         Private use
         Cs         Surrogate

L          Letter
         Ll         Lower case letter
         Lm         Modifier letter
         Lo         Other letter
         Lt         Title case letter
         Lu         Upper case letter
         L&         Ll, Lu, or Lt

M          Mark
         Mc         Spacing mark
         Me         Enclosing mark
         Mn         Non-spacing mark

N          Number
         Nd         Decimal number
         Nl         Letter number
         No         Other number

P          Punctuation
         Pc         Connector punctuation
         Pd         Dash punctuation
         Pe         Close punctuation
         Pf         Final punctuation
         Pi         Initial punctuation
         Po         Other punctuation
         Ps         Open punctuation

S          Symbol
         Sc         Currency symbol
         Sk         Modifier symbol
         Sm         Mathematical symbol
         So         Other symbol

Z          Separator
         Zl         Line separator
         Zp         Paragraph separator
         Zs         Space separator

SCRIPT NAMES FOR \p AND \P

CHARACTER CLASSES

[...]       positive character class
         [^...]      negative character class
         [x-y]       range (can be used for hex characters)
         [[:xxx:]]   positive POSIX named set
         [[:^xxx:]]  negative POSIX named set

alnum       alphanumeric
         alpha       alphabetic
         ascii       0-127
         blank       space or tab
         cntrl       control character
         digit       decimal digit
         graph       printing, excluding space
         lower       lower case letter
         print       printing, including space
         punct       printing, excluding alphanumeric
         space       whitespace
         upper       upper case letter
         word        same as \w
         xdigit      hexadecimal digit

In PCRE, POSIX character set names recognize only ASCII characters. You
       can use \Q...\E inside a character class.

QUANTIFIERS

?           0 or 1, greedy
         ?+          0 or 1, possessive
         ??          0 or 1, lazy
         *           0 or more, greedy
         *+          0 or more, possessive
         *?          0 or more, lazy
         +           1 or more, greedy
         ++          1 or more, possessive
         +?          1 or more, lazy
         {n}         exactly n
         {n,m}       at least n, no more than m, greedy
         {n,m}+      at least n, no more than m, possessive
         {n,m}?      at least n, no more than m, lazy
         {n,}        n or more, greedy
         {n,}+       n or more, possessive
         {n,}?       n or more, lazy

ANCHORS AND SIMPLE ASSERTIONS

\b          word boundary
         \B          not a word boundary
         ^           start of subject
                      also after internal newline in multiline mode
         \A          start of subject
         $           end of subject
                      also before newline at end of subject
                      also before internal newline in multiline mode
         \Z          end of subject
                      also before newline at end of subject
         \z          end of subject
         \G          first matching position in subject

MATCH POINT RESET

\K          reset start of match

ALTERNATION

expr|expr|expr...

CAPTURING

(...)          capturing group
         (?<name>...)   named capturing group (Perl)
         (?'name'...)   named capturing group (Perl)
         (?P<name>...)  named capturing group (Python)
         (?:...)        non-capturing group
         (?|...)        non-capturing group; reset group numbers for
                         capturing groups in each alternative

ATOMIC GROUPS

(?>...)        atomic, non-capturing group

COMMENT

(?#....)       comment (not nestable)

OPTION SETTING

(?i)           caseless
         (?J)           allow duplicate names
         (?m)           multiline
         (?s)           single line (dotall)
         (?U)           default ungreedy (lazy)
         (?x)           extended (ignore white space)
         (?-...)        unset option(s)

LOOKAHEAD AND LOOKBEHIND ASSERTIONS

(?=...)        positive look ahead
         (?!...)        negative look ahead
         (?<=...)       positive look behind
         (?<!...)       negative look behind

Each top-level branch of a look behind must be of a fixed length.

BACKREFERENCES

\n             reference by number (can be ambiguous)
         \gn            reference by number
         \g{n}          reference by number
         \g{-n}         relative reference by number
         \k<name>       reference by name (Perl)
         \k'name'       reference by name (Perl)
         \g{name}       reference by name (Perl)
         \k{name}       reference by name (.NET)
         (?P=name)      reference by name (Python)

SUBROUTINE REFERENCES (POSSIBLY RECURSIVE)

(?R)           recurse whole pattern
         (?n)           call subpattern by absolute number
         (?+n)          call subpattern by relative number
         (?-n)          call subpattern by relative number
         (?&name)       call subpattern by name (Perl)
         (?P>name)      call subpattern by name (Python)
         \g<name>       call subpattern by name (Oniguruma)
         \g'name'       call subpattern by name (Oniguruma)
         \g<n>          call subpattern by absolute number (Oniguruma)
         \g'n'          call subpattern by absolute number (Oniguruma)
         \g<+n>         call subpattern by relative number (PCRE extension)
         \g'+n'         call subpattern by relative number (PCRE extension)
         \g<-n>         call subpattern by relative number (PCRE extension)
         \g'-n'         call subpattern by relative number (PCRE extension)

CONDITIONAL PATTERNS

(?(condition)yes-pattern)
         (?(condition)yes-pattern|no-pattern)

(?(n)...       absolute reference condition
         (?(+n)...      relative reference condition
         (?(-n)...      relative reference condition
         (?(<name>)...  named reference condition (Perl)
         (?('name')...  named reference condition (Perl)
         (?(name)...    named reference condition (PCRE)
         (?(R)...       overall recursion condition
         (?(Rn)...      specific group recursion condition
         (?(R&name)...  specific recursion condition
         (?(DEFINE)...  define subpattern for reference
         (?(assert)...  assertion condition

BACKTRACKING CONTROL

The following act immediately they are reached:

(*ACCEPT)      force successful match
         (*FAIL)        force backtrack; synonym (*F)

The following act only when a subsequent match failure causes  a  back-
       track to reach them. They all force a match failure, but they differ in
       what happens afterwards. Those that advance the start-of-match point do
       so only if the pattern is not anchored.

(*COMMIT)      overall failure, no advance of starting point
         (*PRUNE)       advance to next starting character
         (*SKIP)        advance start to current matching position
         (*THEN)        local failure, backtrack to next alternation

NEWLINE CONVENTIONS

These  are  recognized only at the very start of the pattern or after a
       (*BSR_...) option.

(*CR)
         (*LF)
         (*CRLF)
         (*ANYCRLF)
         (*ANY)

WHAT \R MATCHES

These are recognized only at the very start of the pattern or  after  a
       (*...) option that sets the newline convention.

(*BSR_ANYCRLF)
         (*BSR_UNICODE)

CALLOUTS

(?C)      callout
         (?Cn)     callout with data n

SEE ALSO

pcrepattern(3), pcreapi(3), pcrecallout(3), pcrematching(3), pcre(3).

AUTHOR

Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.

REVISION

Last updated: 09 April 2008
       Copyright (c) 1997-2008 University of Cambridge.
------------------------------------------------------------------------------

PCREPARTIAL(3)                                                  PCREPARTIAL(3)

NAME
       PCRE - Perl-compatible regular expressions

PARTIAL MATCHING IN PCRE

In  normal  use  of  PCRE,  if  the  subject  string  that is passed to
       pcre_exec() or pcre_dfa_exec() matches as far as it goes,  but  is  too
       short  to  match  the  entire  pattern, PCRE_ERROR_NOMATCH is returned.
       There are circumstances where it might be helpful to  distinguish  this
       case from other cases in which there is no match.

Consider, for example, an application where a human is required to type
       in data for a field with specific formatting requirements.  An  example
       might be a date in the form ddmmmyy, defined by this pattern:

^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$

If the application sees the user's keystrokes one by one, and can check
       that what has been typed so far is potentially valid,  it  is  able  to
       raise  an  error as soon as a mistake is made, possibly beeping and not
       reflecting the character that has been typed. This  immediate  feedback
       is  likely  to  be a better user interface than a check that is delayed
       until the entire string has been entered.

PCRE supports the concept of partial matching by means of the PCRE_PAR-
       TIAL   option,   which   can   be   set  when  calling  pcre_exec()  or
       pcre_dfa_exec(). When this flag is set for pcre_exec(), the return code
       PCRE_ERROR_NOMATCH  is converted into PCRE_ERROR_PARTIAL if at any time
       during the matching process the last part of the subject string matched
       part  of  the  pattern. Unfortunately, for non-anchored matching, it is
       not possible to obtain the position of the start of the partial  match.
       No captured data is set when PCRE_ERROR_PARTIAL is returned.

When   PCRE_PARTIAL   is  set  for  pcre_dfa_exec(),  the  return  code
       PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if the  end  of
       the  subject is reached, there have been no complete matches, but there
       is still at least one matching possibility. The portion of  the  string
       that provided the partial match is set as the first matching string.

Using PCRE_PARTIAL disables one of PCRE's optimizations. PCRE remembers
       the last literal byte in a pattern, and abandons  matching  immediately
       if  such a byte is not present in the subject string. This optimization
       cannot be used for a subject string that might match only partially.

RESTRICTED PATTERNS FOR PCRE_PARTIAL

Because of the way certain internal optimizations  are  implemented  in
       the  pcre_exec()  function, the PCRE_PARTIAL option cannot be used with
       all patterns. These restrictions do not apply when  pcre_dfa_exec()  is
       used.  For pcre_exec(), repeated single characters such as

a{2,4}

and repeated single metasequences such as

\d+

are  not permitted if the maximum number of occurrences is greater than
       one.  Optional items such as \d? (where the maximum is one) are permit-
       ted.   Quantifiers  with any values are permitted after parentheses, so
       the invalid examples above can be coded thus:

(a){2,4}
         (\d)+

These constructions run more slowly, but for the kinds  of  application
       that  are  envisaged  for this facility, this is not felt to be a major
       restriction.

If PCRE_PARTIAL is set for a pattern  that  does  not  conform  to  the
       restrictions,  pcre_exec() returns the error code PCRE_ERROR_BADPARTIAL
       (-13).  You can use the PCRE_INFO_OKPARTIAL call to pcre_fullinfo()  to
       find out if a compiled pattern can be used for partial matching.

EXAMPLE OF PARTIAL MATCHING USING PCRETEST

If  the  escape  sequence  \P  is  present in a pcretest data line, the
       PCRE_PARTIAL flag is used for the match. Here is a run of pcretest that
       uses the date example quoted above:

re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
         data> 25jun04\P
          0: 25jun04
          1: jun
         data> 25dec3\P
         Partial match
         data> 3ju\P
         Partial match
         data> 3juj\P
         No match
         data> j\P
         No match

The  first  data  string  is  matched completely, so pcretest shows the
       matched substrings. The remaining four strings do not  match  the  com-
       plete  pattern,  but  the first two are partial matches. The same test,
       using pcre_dfa_exec() matching (by means of the  \D  escape  sequence),
       produces the following output:

re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
         data> 25jun04\P\D
          0: 25jun04
         data> 23dec3\P\D
         Partial match: 23dec3
         data> 3ju\P\D
         Partial match: 3ju
         data> 3juj\P\D
         No match
         data> j\P\D
         No match

Notice  that in this case the portion of the string that was matched is
       made available.

MULTI-SEGMENT MATCHING WITH pcre_dfa_exec()

When a partial match has been found using pcre_dfa_exec(), it is possi-
       ble  to  continue  the  match  by providing additional subject data and
       calling pcre_dfa_exec() again with the same  compiled  regular  expres-
       sion, this time setting the PCRE_DFA_RESTART option. You must also pass
       the same working space as before, because this is where details of  the
       previous  partial  match are stored. Here is an example using pcretest,
       using the \R escape sequence to set the PCRE_DFA_RESTART option (\P and
       \D are as above):

re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
         data> 23ja\P\D
         Partial match: 23ja
         data> n05\R\D
          0: n05

The  first  call has "23ja" as the subject, and requests partial match-
       ing; the second call  has  "n05"  as  the  subject  for  the  continued
       (restarted)  match.   Notice  that when the match is complete, only the
       last part is shown; PCRE does  not  retain  the  previously  partially-
       matched  string. It is up to the calling program to do that if it needs
       to.

You can set PCRE_PARTIAL  with  PCRE_DFA_RESTART  to  continue  partial
       matching over multiple segments. This facility can be used to pass very
       long subject strings to pcre_dfa_exec(). However, some care  is  needed
       for certain types of pattern.

1.  If  the  pattern contains tests for the beginning or end of a line,
       you need to pass the PCRE_NOTBOL or PCRE_NOTEOL options,  as  appropri-
       ate,  when  the subject string for any call does not contain the begin-
       ning or end of a line.

2. If the pattern contains backward assertions (including  \b  or  \B),
       you  need  to  arrange for some overlap in the subject strings to allow
       for this. For example, you could pass the subject in  chunks  that  are
       500  bytes long, but in a buffer of 700 bytes, with the starting offset
       set to 200 and the previous 200 bytes at the start of the buffer.

3. Matching a subject string that is split into multiple segments  does
       not  always produce exactly the same result as matching over one single
       long string.  The difference arises when there  are  multiple  matching
       possibilities,  because a partial match result is given only when there
       are no completed matches in a call to pcre_dfa_exec(). This means  that
       as  soon  as  the  shortest match has been found, continuation to a new
       subject segment is no longer possible.  Consider this pcretest example:

re> /dog(sbody)?/
         data> do\P\D
         Partial match: do
         data> gsb\R\P\D
          0: g
         data> dogsbody\D
          0: dogsbody
          1: dog

The  pattern matches the words "dog" or "dogsbody". When the subject is
       presented in several parts ("do" and "gsb" being  the  first  two)  the
       match  stops  when "dog" has been found, and it is not possible to con-
       tinue. On the other hand,  if  "dogsbody"  is  presented  as  a  single
       string, both matches are found.

Because  of  this  phenomenon,  it does not usually make sense to end a
       pattern that is going to be matched in this way with a variable repeat.

4. Patterns that contain alternatives at the top level which do not all
       start with the same pattern item may not work as expected. For example,
       consider this pattern:

1234|3789

If  the  first  part of the subject is "ABC123", a partial match of the
       first alternative is found at offset 3. There is no partial  match  for
       the second alternative, because such a match does not start at the same
       point in the subject string. Attempting to  continue  with  the  string
       "789" does not yield a match because only those alternatives that match
       at one point in the subject are remembered. The problem arises  because
       the  start  of the second alternative matches within the first alterna-
       tive. There is no problem with anchored patterns or patterns such as:

1234|ABCD

where no string can be a partial match for both alternatives.

AUTHOR

Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.

REVISION

Last updated: 04 June 2007
       Copyright (c) 1997-2007 University of Cambridge.
------------------------------------------------------------------------------

PCREPRECOMPILE(3)                                            PCREPRECOMPILE(3)

NAME
       PCRE - Perl-compatible regular expressions

SAVING AND RE-USING PRECOMPILED PCRE PATTERNS

If  you  are running an application that uses a large number of regular
       expression patterns, it may be useful to store them  in  a  precompiled
       form  instead  of  having to compile them every time the application is
       run.  If you are not  using  any  private  character  tables  (see  the
       pcre_maketables()  documentation),  this is relatively straightforward.
       If you are using private tables, it is a little bit more complicated.

If you save compiled patterns to a file, you can copy them to a differ-
       ent  host  and  run them there. This works even if the new host has the
       opposite endianness to the one on which  the  patterns  were  compiled.
       There  may  be a small performance penalty, but it should be insignifi-
       cant. However, compiling regular expressions with one version  of  PCRE
       for  use  with  a  different  version is not guaranteed to work and may
       cause crashes.

SAVING A COMPILED PATTERN
       The value returned by pcre_compile() points to a single block of memory
       that  holds  the compiled pattern and associated data. You can find the
       length of this block in bytes by calling pcre_fullinfo() with an  argu-
       ment  of  PCRE_INFO_SIZE. You can then save the data in any appropriate
       manner. Here is sample code that compiles a pattern and writes it to  a
       file. It assumes that the variable fd refers to a file that is open for
       output:

int erroroffset, rc, size;
         char *error;
         pcre *re;

re = pcre_compile("my pattern", 0, &error, &erroroffset, NULL);
         if (re == NULL) { ... handle errors ... }
         rc = pcre_fullinfo(re, NULL, PCRE_INFO_SIZE, &size);
         if (rc < 0) { ... handle errors ... }
         rc = fwrite(re, 1, size, fd);
         if (rc != size) { ... handle errors ... }

In this example, the bytes  that  comprise  the  compiled  pattern  are
       copied  exactly.  Note that this is binary data that may contain any of
       the 256 possible byte  values.  On  systems  that  make  a  distinction
       between binary and non-binary data, be sure that the file is opened for
       binary output.

If you want to write more than one pattern to a file, you will have  to
       devise  a  way of separating them. For binary data, preceding each pat-
       tern with its length is probably  the  most  straightforward  approach.
       Another  possibility is to write out the data in hexadecimal instead of
       binary, one pattern to a line.

Saving compiled patterns in a file is only one possible way of  storing
       them  for later use. They could equally well be saved in a database, or
       in the memory of some daemon process that passes them  via  sockets  to
       the processes that want them.

If  the pattern has been studied, it is also possible to save the study
       data in a similar way to the compiled  pattern  itself.  When  studying
       generates  additional  information, pcre_study() returns a pointer to a
       pcre_extra data block. Its format is defined in the section on matching
       a  pattern in the pcreapi documentation. The study_data field points to
       the binary study data,  and  this  is  what  you  must  save  (not  the
       pcre_extra  block itself). The length of the study data can be obtained
       by calling pcre_fullinfo() with  an  argument  of  PCRE_INFO_STUDYSIZE.
       Remember  to check that pcre_study() did return a non-NULL value before
       trying to save the study data.

RE-USING A PRECOMPILED PATTERN

Re-using a precompiled pattern is straightforward. Having  reloaded  it
       into   main   memory,   you   pass   its   pointer  to  pcre_exec()  or
       pcre_dfa_exec() in the usual way. This  should  work  even  on  another
       host,  and  even  if  that  host has the opposite endianness to the one
       where the pattern was compiled.

However, if you passed a pointer to custom character  tables  when  the
       pattern  was  compiled  (the  tableptr argument of pcre_compile()), you
       must now pass a similar  pointer  to  pcre_exec()  or  pcre_dfa_exec(),
       because  the  value  saved  with the compiled pattern will obviously be
       nonsense. A field in a pcre_extra() block is used to pass this data, as
       described  in the section on matching a pattern in the pcreapi documen-
       tation.

If you did not provide custom character tables  when  the  pattern  was
       compiled,  the  pointer  in  the compiled pattern is NULL, which causes
       pcre_exec() to use PCRE's internal tables. Thus, you  do  not  need  to
       take any special action at run time in this case.

If  you  saved study data with the compiled pattern, you need to create
       your own pcre_extra data block and set the study_data field to point to
       the  reloaded  study  data. You must also set the PCRE_EXTRA_STUDY_DATA
       bit in the flags field to indicate that study  data  is  present.  Then
       pass  the  pcre_extra  block  to  pcre_exec() or pcre_dfa_exec() in the
       usual way.

COMPATIBILITY WITH DIFFERENT PCRE RELEASES

In general, it is safest to  recompile  all  saved  patterns  when  you
       update  to  a new PCRE release, though not all updates actually require
       this. Recompiling is definitely needed for release 7.2.

AUTHOR

Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.

REVISION

Last updated: 13 June 2007
       Copyright (c) 1997-2007 University of Cambridge.
------------------------------------------------------------------------------

PCREPERFORM(3)                                                  PCREPERFORM(3)

NAME
       PCRE - Perl-compatible regular expressions

PCRE PERFORMANCE

Two  aspects  of performance are discussed below: memory usage and pro-
       cessing time. The way you express your pattern as a regular  expression
       can affect both of them.

MEMORY USAGE

Patterns are compiled by PCRE into a reasonably efficient byte code, so
       that most simple patterns do not use much memory. However, there is one
       case where memory usage can be unexpectedly large. When a parenthesized
       subpattern has a quantifier with a minimum greater than 1 and/or a lim-
       ited  maximum,  the  whole subpattern is repeated in the compiled code.
       For example, the pattern

(abc|def){2,4}

is compiled as if it were

(abc|def)(abc|def)((abc|def)(abc|def)?)?

(Technical aside: It is done this way so that backtrack  points  within
       each of the repetitions can be independently maintained.)

For  regular expressions whose quantifiers use only small numbers, this
       is not usually a problem. However, if the numbers are large,  and  par-
       ticularly  if  such repetitions are nested, the memory usage can become
       an embarrassment. For example, the very simple pattern

((ab){1,1000}c){1,3}

uses 51K bytes when compiled. When PCRE is compiled  with  its  default
       internal  pointer  size of two bytes, the size limit on a compiled pat-
       tern is 64K, and this is reached with the above pattern  if  the  outer
       repetition is increased from 3 to 4. PCRE can be compiled to use larger
       internal pointers and thus handle larger compiled patterns, but  it  is
       better to try to rewrite your pattern to use less memory if you can.

One  way  of reducing the memory usage for such patterns is to make use
       of PCRE's "subroutine" facility. Re-writing the above pattern as

((ab)(?2){0,999}c)(?1){0,2}

reduces the memory requirements to 18K, and indeed it remains under 20K
       even  with the outer repetition increased to 100. However, this pattern
       is not exactly equivalent, because the "subroutine" calls  are  treated
       as  atomic groups into which there can be no backtracking if there is a
       subsequent matching failure. Therefore, PCRE cannot  do  this  kind  of
       rewriting  automatically.   Furthermore,  there is a noticeable loss of
       speed when executing the modified pattern. Nevertheless, if the  atomic
       grouping  is  not  a  problem and the loss of speed is acceptable, this
       kind of rewriting will allow you to process patterns that  PCRE  cannot
       otherwise handle.

PROCESSING TIME

Certain  items  in regular expression patterns are processed more effi-
       ciently than others. It is more efficient to use a character class like
       [aeiou]   than   a   set   of  single-character  alternatives  such  as
       (a|e|i|o|u). In general, the simplest construction  that  provides  the
       required behaviour is usually the most efficient. Jeffrey Friedl's book
       contains a lot of useful general discussion  about  optimizing  regular
       expressions  for  efficient  performance.  This document contains a few
       observations about PCRE.

Using Unicode character properties (the \p,  \P,  and  \X  escapes)  is
       slow,  because PCRE has to scan a structure that contains data for over
       fifteen thousand characters whenever it needs a  character's  property.
       If  you  can  find  an  alternative pattern that does not use character
       properties, it will probably be faster.

When a pattern begins with .* not in  parentheses,  or  in  parentheses
       that are not the subject of a backreference, and the PCRE_DOTALL option
       is set, the pattern is implicitly anchored by PCRE, since it can  match
       only  at  the start of a subject string. However, if PCRE_DOTALL is not
       set, PCRE cannot make this optimization, because  the  .  metacharacter
       does  not then match a newline, and if the subject string contains new-
       lines, the pattern may match from the character  immediately  following
       one of them instead of from the very start. For example, the pattern

.*second

matches  the subject "first\nand second" (where \n stands for a newline
       character), with the match starting at the seventh character. In  order
       to do this, PCRE has to retry the match starting after every newline in
       the subject.

If you are using such a pattern with subject strings that do  not  con-
       tain newlines, the best performance is obtained by setting PCRE_DOTALL,
       or starting the pattern with ^.* or ^.*? to indicate  explicit  anchor-
       ing.  That saves PCRE from having to scan along the subject looking for
       a newline to restart at.

Beware of patterns that contain nested indefinite  repeats.  These  can
       take  a  long time to run when applied to a string that does not match.
       Consider the pattern fragment

^(a+)*

This can match "aaaa" in 16 different ways, and this  number  increases
       very  rapidly  as the string gets longer. (The * repeat can match 0, 1,
       2, 3, or 4 times, and for each of those cases other than 0 or 4, the  +
       repeats  can  match  different numbers of times.) When the remainder of
       the pattern is such that the entire match is going to fail, PCRE has in
       principle  to  try  every  possible  variation,  and  this  can take an
       extremely long time, even for relatively short strings.

An optimization catches some of the more simple cases such as

(a+)*b

where a literal character follows. Before  embarking  on  the  standard
       matching  procedure,  PCRE checks that there is a "b" later in the sub-
       ject string, and if there is not, it fails the match immediately.  How-
       ever,  when  there  is no following literal this optimization cannot be
       used. You can see the difference by comparing the behaviour of

(a+)*\d

with the pattern above. The former gives  a  failure  almost  instantly
       when  applied  to  a  whole  line of "a" characters, whereas the latter
       takes an appreciable time with strings longer than about 20 characters.

In many cases, the solution to this kind of performance issue is to use
       an atomic group or a possessive quantifier.

AUTHOR

Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.

REVISION

Last updated: 06 March 2007
       Copyright (c) 1997-2007 University of Cambridge.
------------------------------------------------------------------------------

PCREPOSIX(3)                                                      PCREPOSIX(3)

NAME
       PCRE - Perl-compatible regular expressions.

SYNOPSIS OF POSIX API

#include <pcreposix.h>

int regcomp(regex_t *preg, const char *pattern,
            int cflags);

int regexec(regex_t *preg, const char *string,
            size_t nmatch, regmatch_t pmatch[], int eflags);

size_t regerror(int errcode, const regex_t *preg,
            char *errbuf, size_t errbuf_size);

void regfree(regex_t *preg);

DESCRIPTION

This  set  of  functions provides a POSIX-style API to the PCRE regular
       expression package. See the pcreapi documentation for a description  of
       PCRE's native API, which contains much additional functionality.

The functions described here are just wrapper functions that ultimately
       call  the  PCRE  native  API.  Their  prototypes  are  defined  in  the
       pcreposix.h  header  file,  and  on  Unix systems the library itself is
       called pcreposix.a, so can be accessed by  adding  -lpcreposix  to  the
       command  for  linking  an application that uses them. Because the POSIX
       functions call the native ones, it is also necessary to add -lpcre.

I have implemented only those option bits that can be reasonably mapped
       to PCRE native options. In addition, the option REG_EXTENDED is defined
       with the value zero. This has no effect, but since  programs  that  are
       written  to  the  POSIX interface often use it, this makes it easier to
       slot in PCRE as a replacement library. Other POSIX options are not even
       defined.

When  PCRE  is  called  via these functions, it is only the API that is
       POSIX-like in style. The syntax and semantics of  the  regular  expres-
       sions  themselves  are  still  those of Perl, subject to the setting of
       various PCRE options, as described below. "POSIX-like in  style"  means
       that  the  API  approximates  to  the POSIX definition; it is not fully
       POSIX-compatible, and in multi-byte encoding  domains  it  is  probably
       even less compatible.

The  header for these functions is supplied as pcreposix.h to avoid any
       potential clash with other POSIX  libraries.  It  can,  of  course,  be
       renamed or aliased as regex.h, which is the "correct" name. It provides
       two structure types, regex_t for  compiled  internal  forms,  and  reg-
       match_t  for  returning  captured substrings. It also defines some con-
       stants whose names start  with  "REG_";  these  are  used  for  setting
       options and identifying error codes.

COMPILING A PATTERN

The  function regcomp() is called to compile a pattern into an internal
       form. The pattern is a C string terminated by a  binary  zero,  and  is
       passed  in  the  argument  pattern. The preg argument is a pointer to a
       regex_t structure that is used as a base for storing information  about
       the compiled regular expression.

The argument cflags is either zero, or contains one or more of the bits
       defined by the following macros:

REG_DOTALL

The PCRE_DOTALL option is set when the regular expression is passed for
       compilation to the native function. Note that REG_DOTALL is not part of
       the POSIX standard.

REG_ICASE

The PCRE_CASELESS option is set when the regular expression  is  passed
       for compilation to the native function.

REG_NEWLINE

The  PCRE_MULTILINE option is set when the regular expression is passed
       for compilation to the native function. Note that this does  not  mimic
       the  defined  POSIX  behaviour  for REG_NEWLINE (see the following sec-
       tion).

REG_NOSUB

The PCRE_NO_AUTO_CAPTURE option is set when the regular  expression  is
       passed for compilation to the native function. In addition, when a pat-
       tern that is compiled with this flag is passed to regexec() for  match-
       ing,  the  nmatch  and  pmatch  arguments  are ignored, and no captured
       strings are returned.

REG_UTF8

The PCRE_UTF8 option is set when the regular expression is  passed  for
       compilation  to the native function. This causes the pattern itself and
       all data strings used for matching it to be treated as  UTF-8  strings.
       Note that REG_UTF8 is not part of the POSIX standard.

In  the  absence  of  these  flags, no options are passed to the native
       function.  This means the the  regex  is  compiled  with  PCRE  default
       semantics.  In particular, the way it handles newline characters in the
       subject string is the Perl way, not the POSIX way.  Note  that  setting
       PCRE_MULTILINE  has only some of the effects specified for REG_NEWLINE.
       It does not affect the way newlines are matched by . (they  aren't)  or
       by a negative class such as [^a] (they are).

The  yield of regcomp() is zero on success, and non-zero otherwise. The
       preg structure is filled in on success, and one member of the structure
       is  public: re_nsub contains the number of capturing subpatterns in the
       regular expression. Various error codes are defined in the header file.

MATCHING NEWLINE CHARACTERS

This area is not simple, because POSIX and Perl take different views of
       things.  It is not possible to get PCRE to obey  POSIX  semantics,  but
       then  PCRE was never intended to be a POSIX engine. The following table
       lists the different possibilities for matching  newline  characters  in
       PCRE:

Default   Change with

. matches newline          no     PCRE_DOTALL
         newline matches [^a]       yes    not changeable
         $ matches \n at end        yes    PCRE_DOLLARENDONLY
         $ matches \n in middle     no     PCRE_MULTILINE
         ^ matches \n in middle     no     PCRE_MULTILINE

This is the equivalent table for POSIX:

Default   Change with

. matches newline          yes    REG_NEWLINE
         newline matches [^a]       yes    REG_NEWLINE
         $ matches \n at end        no     REG_NEWLINE
         $ matches \n in middle     no     REG_NEWLINE
         ^ matches \n in middle     no     REG_NEWLINE

PCRE's behaviour is the same as Perl's, except that there is no equiva-
       lent for PCRE_DOLLAR_ENDONLY in Perl. In both PCRE and Perl,  there  is
       no way to stop newline from matching [^a].

The   default  POSIX  newline  handling  can  be  obtained  by  setting
       PCRE_DOTALL and PCRE_DOLLAR_ENDONLY, but there is no way to  make  PCRE
       behave exactly as for the REG_NEWLINE action.

MATCHING A PATTERN

The  function  regexec()  is  called  to  match a compiled pattern preg
       against a given string, which is by default terminated by a  zero  byte
       (but  see  REG_STARTEND below), subject to the options in eflags. These
       can be:

REG_NOTBOL

The PCRE_NOTBOL option is set when calling the underlying PCRE matching
       function.

REG_NOTEOL

The PCRE_NOTEOL option is set when calling the underlying PCRE matching
       function.

REG_STARTEND

The string is considered to start at string +  pmatch[0].rm_so  and  to
       have  a terminating NUL located at string + pmatch[0].rm_eo (there need
       not actually be a NUL at that location), regardless  of  the  value  of
       nmatch.  This  is a BSD extension, compatible with but not specified by
       IEEE Standard 1003.2 (POSIX.2), and should  be  used  with  caution  in
       software intended to be portable to other systems. Note that a non-zero
       rm_so does not imply REG_NOTBOL; REG_STARTEND affects only the location
       of the string, not how it is matched.

If  the pattern was compiled with the REG_NOSUB flag, no data about any
       matched strings  is  returned.  The  nmatch  and  pmatch  arguments  of
       regexec() are ignored.

Otherwise,the portion of the string that was matched, and also any cap-
       tured substrings, are returned via the pmatch argument, which points to
       an  array  of nmatch structures of type regmatch_t, containing the mem-
       bers rm_so and rm_eo. These contain the offset to the  first  character
       of  each  substring and the offset to the first character after the end
       of each substring, respectively. The 0th element of the vector  relates
       to  the  entire portion of string that was matched; subsequent elements
       relate to the capturing subpatterns of the regular  expression.  Unused
       entries in the array have both structure members set to -1.

A  successful  match  yields  a  zero  return;  various error codes are
       defined in the header file, of  which  REG_NOMATCH  is  the  "expected"
       failure code.

ERROR MESSAGES

The regerror() function maps a non-zero errorcode from either regcomp()
       or regexec() to a printable message. If preg is  not  NULL,  the  error
       should have arisen from the use of that structure. A message terminated
       by a binary zero is placed  in  errbuf.  The  length  of  the  message,
       including  the  zero, is limited to errbuf_size. The yield of the func-
       tion is the size of buffer needed to hold the whole message.

MEMORY USAGE

Compiling a regular expression causes memory to be allocated and  asso-
       ciated  with  the preg structure. The function regfree() frees all such
       memory, after which preg may no longer be used as  a  compiled  expres-
       sion.

AUTHOR

Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.

REVISION

Last updated: 05 April 2008
       Copyright (c) 1997-2008 University of Cambridge.
------------------------------------------------------------------------------

PCRECPP(3)                                                          PCRECPP(3)

NAME
       PCRE - Perl-compatible regular expressions.

SYNOPSIS OF C++ WRAPPER

#include <pcrecpp.h>

DESCRIPTION

The  C++  wrapper  for PCRE was provided by Google Inc. Some additional
       functionality was added by Giuseppe Maxia. This brief man page was con-
       structed  from  the  notes  in the pcrecpp.h file, which should be con-
       sulted for further details.

MATCHING INTERFACE

The "FullMatch" operation checks that supplied text matches a  supplied
       pattern  exactly.  If pointer arguments are supplied, it copies matched
       sub-strings that match sub-patterns into them.

Example: successful match
            pcrecpp::RE re("h.*o");
            re.FullMatch("hello");

Example: unsuccessful match (requires full match):
            pcrecpp::RE re("e");
            !re.FullMatch("hello");

Example: creating a temporary RE object:
            pcrecpp::RE("h.*o").FullMatch("hello");

You can pass in a "const char*" or a "string" for "text". The  examples
       below  tend to use a const char*. You can, as in the different examples
       above, store the RE object explicitly in a variable or use a  temporary
       RE  object.  The  examples below use one mode or the other arbitrarily.
       Either could correctly be used for any of these examples.

You must supply extra pointer arguments to extract matched subpieces.

Example: extracts "ruby" into "s" and 1234 into "i"
            int i;
            string s;
            pcrecpp::RE re("(\\w+):(\\d+)");
            re.FullMatch("ruby:1234", &s, &i);

Example: does not try to extract any extra sub-patterns
            re.FullMatch("ruby:1234", &s);

Example: does not try to extract into NULL
            re.FullMatch("ruby:1234", NULL, &i);

Example: integer overflow causes failure
            !re.FullMatch("ruby:1234567891234", NULL, &i);

Example: fails because there aren't enough sub-patterns:
            !pcrecpp::RE("\\w+:\\d+").FullMatch("ruby:1234", &s);

Example: fails because string cannot be stored in integer
            !pcrecpp::RE("(.*)").FullMatch("ruby", &i);

The provided pointer arguments can be pointers to  any  scalar  numeric
       type, or one of:

string        (matched piece is copied to string)
          StringPiece   (StringPiece is mutated to point to matched piece)
          T             (where "bool T::ParseFrom(const char*, int)" exists)
          NULL          (the corresponding matched sub-pattern is not copied)

The  function returns true iff all of the following conditions are sat-
       isfied:

a. "text" matches "pattern" exactly;

b. The number of matched sub-patterns is >= number of supplied
            pointers;

c. The "i"th argument has a suitable type for holding the
            string captured as the "i"th sub-pattern. If you pass in
            void * NULL for the "i"th argument, or a non-void * NULL
            of the correct type, or pass fewer arguments than the
            number of sub-patterns, "i"th captured sub-pattern is
            ignored.

CAVEAT: An optional sub-pattern that does  not  exist  in  the  matched
       string  is  assigned  the  empty  string. Therefore, the following will
       return false (because the empty string is not a valid number):

int number;
          pcrecpp::RE::FullMatch("abc", "[a-z]+(\\d+)?", &number);

The matching interface supports at most 16 arguments per call.  If  you
       need    more,    consider    using    the    more   general   interface
       pcrecpp::RE::DoMatch. See pcrecpp.h for the signature for DoMatch.

QUOTING METACHARACTERS

You can use the "QuoteMeta" operation to insert backslashes before  all
       potentially  meaningful  characters  in  a string. The returned string,
       used as a regular expression, will exactly match the original string.

Example:
            string quoted = RE::QuoteMeta(unquoted);

Note that it's legal to escape a character even if it  has  no  special
       meaning  in  a  regular expression -- so this function does that. (This
       also makes it identical to the perl function  of  the  same  name;  see
       "perldoc    -f    quotemeta".)    For   example,   "1.5-2.0?"   becomes
       "1\.5\-2\.0\?".

PARTIAL MATCHES

You can use the "PartialMatch" operation when you want the  pattern  to
       match any substring of the text.

Example: simple search for a string:
            pcrecpp::RE("ell").PartialMatch("hello");

Example: find first number in a string:
            int number;
            pcrecpp::RE re("(\\d+)");
            re.PartialMatch("x*100 + 20", &number);
            assert(number == 100);

UTF-8 AND THE MATCHING INTERFACE

By  default,  pattern  and text are plain text, one byte per character.
       The UTF8 flag, passed to  the  constructor,  causes  both  pattern  and
       string to be treated as UTF-8 text, still a byte stream but potentially
       multiple bytes per character. In practice, the text is likelier  to  be
       UTF-8  than  the pattern, but the match returned may depend on the UTF8
       flag, so always use it when matching UTF8 text. For example,  "."  will
       match  one  byte normally but with UTF8 set may match up to three bytes
       of a multi-byte character.

Example:
            pcrecpp::RE_Options options;
            options.set_utf8();
            pcrecpp::RE re(utf8_pattern, options);
            re.FullMatch(utf8_string);

Example: using the convenience function UTF8():
            pcrecpp::RE re(utf8_pattern, pcrecpp::UTF8());
            re.FullMatch(utf8_string);

NOTE: The UTF8 flag is ignored if pcre was not configured with the
             --enable-utf8 flag.

PASSING MODIFIERS TO THE REGULAR EXPRESSION ENGINE

PCRE defines some modifiers to  change  the  behavior  of  the  regular
       expression   engine.  The  C++  wrapper  defines  an  auxiliary  class,
       RE_Options, as a vehicle to pass such modifiers to  a  RE  class.  Cur-
       rently, the following modifiers are supported:

modifier              description               Perl corresponding

PCRE_CASELESS         case insensitive match      /i
          PCRE_MULTILINE        multiple lines match        /m
          PCRE_DOTALL           dot matches newlines        /s
          PCRE_DOLLAR_ENDONLY   $ matches only at end       N/A
          PCRE_EXTRA            strict escape parsing       N/A
          PCRE_EXTENDED         ignore whitespaces          /x
          PCRE_UTF8             handles UTF8 chars          built-in
          PCRE_UNGREEDY         reverses * and *?           N/A
          PCRE_NO_AUTO_CAPTURE  disables capturing parens   N/A (*)

(*)  Both Perl and PCRE allow non capturing parentheses by means of the
       "?:" modifier within the pattern itself. e.g. (?:ab|cd) does  not  cap-
       ture, while (ab|cd) does.

For  a  full  account on how each modifier works, please check the PCRE
       API reference page.

For each modifier, there are two member functions whose  name  is  made
       out  of  the  modifier  in  lowercase,  without the "PCRE_" prefix. For
       instance, PCRE_CASELESS is handled by

bool caseless()

which returns true if the modifier is set, and

RE_Options & set_caseless(bool)

which sets or unsets the modifier. Moreover, PCRE_EXTRA_MATCH_LIMIT can
       be  accessed  through  the  set_match_limit()  and match_limit() member
       functions. Setting match_limit to a non-zero value will limit the  exe-
       cution  of pcre to keep it from doing bad things like blowing the stack
       or taking an eternity to return a result.  A  value  of  5000  is  good
       enough  to stop stack blowup in a 2MB thread stack. Setting match_limit
       to  zero  disables  match  limiting.  Alternatively,   you   can   call
       match_limit_recursion()  which uses PCRE_EXTRA_MATCH_LIMIT_RECURSION to
       limit how much  PCRE  recurses.  match_limit()  limits  the  number  of
       matches PCRE does; match_limit_recursion() limits the depth of internal
       recursion, and therefore the amount of stack that is used.

Normally, to pass one or more modifiers to a RE class,  you  declare  a
       RE_Options object, set the appropriate options, and pass this object to
       a RE constructor. Example:

RE_options opt;
          opt.set_caseless(true);
          if (RE("HELLO", opt).PartialMatch("hello world")) ...

RE_options has two constructors. The default constructor takes no argu-
       ments  and creates a set of flags that are off by default. The optional
       parameter option_flags is to facilitate transfer of legacy code from  C
       programs.  This lets you do

RE(pattern,
            RE_Options(PCRE_CASELESS|PCRE_MULTILINE)).PartialMatch(str);

However, new code is better off doing

RE(pattern,
            RE_Options().set_caseless(true).set_multiline(true))
              .PartialMatch(str);

If you are going to pass one of the most used modifiers, there are some
       convenience functions that return a RE_Options class with the appropri-
       ate  modifier  already  set: CASELESS(), UTF8(), MULTILINE(), DOTALL(),
       and EXTENDED().

If you need to set several options at once, and you don't  want  to  go
       through  the pains of declaring a RE_Options object and setting several
       options, there is a parallel method that give you such ability  on  the
       fly.  You  can  concatenate several set_xxxxx() member functions, since
       each of them returns a reference to its class object. For  example,  to
       pass  PCRE_CASELESS, PCRE_EXTENDED, and PCRE_MULTILINE to a RE with one
       statement, you may write:

RE(" ^ xyz \\s+ .* blah$",
            RE_Options()
              .set_caseless(true)
              .set_extended(true)
              .set_multiline(true)).PartialMatch(sometext);

SCANNING TEXT INCREMENTALLY

The "Consume" operation may be useful if you want to  repeatedly  match
       regular expressions at the front of a string and skip over them as they
       match. This requires use of the "StringPiece" type, which represents  a
       sub-range  of  a  real  string.  Like RE, StringPiece is defined in the
       pcrecpp namespace.

Example: read lines of the form "var = value" from a string.
            string contents = ...;                 // Fill string somehow
            pcrecpp::StringPiece input(contents);  // Wrap in a StringPiece

string var;
            int value;
            pcrecpp::RE re("(\\w+) = (\\d+)\n");
            while (re.Consume(&input, &var, &value)) {
              ...;
            }

Each successful call  to  "Consume"  will  set  "var/value",  and  also
       advance "input" so it points past the matched text.

The  "FindAndConsume"  operation  is  similar to "Consume" but does not
       anchor your match at the beginning of  the  string.  For  example,  you
       could extract all words from a string by repeatedly calling

pcrecpp::RE("(\\w+)").FindAndConsume(&input, &word)

PARSING HEX/OCTAL/C-RADIX NUMBERS

By default, if you pass a pointer to a numeric value, the corresponding
       text is interpreted as a base-10  number.  You  can  instead  wrap  the
       pointer with a call to one of the operators Hex(), Octal(), or CRadix()
       to interpret the text in another base. The CRadix  operator  interprets
       C-style  "0"  (base-8)  and  "0x"  (base-16)  prefixes, but defaults to
       base-10.

Example:
           int a, b, c, d;
           pcrecpp::RE re("(.*) (.*) (.*) (.*)");
           re.FullMatch("100 40 0100 0x40",
                        pcrecpp::Octal(&a), pcrecpp::Hex(&b),
                        pcrecpp::CRadix(&c), pcrecpp::CRadix(&d));

will leave 64 in a, b, c, and d.

REPLACING PARTS OF STRINGS

You can replace the first match of "pattern" in "str"  with  "rewrite".
       Within  "rewrite",  backslash-escaped  digits (\1 to \9) can be used to
       insert text matching corresponding parenthesized group  from  the  pat-
       tern. \0 in "rewrite" refers to the entire matching text. For example:

string s = "yabba dabba doo";
         pcrecpp::RE("b+").Replace("d", &s);

will  leave  "s" containing "yada dabba doo". The result is true if the
       pattern matches and a replacement occurs, false otherwise.

GlobalReplace is like Replace except that it replaces  all  occurrences
       of  the  pattern  in  the string with the rewrite. Replacements are not
       subject to re-matching. For example:

string s = "yabba dabba doo";
         pcrecpp::RE("b+").GlobalReplace("d", &s);

will leave "s" containing "yada dada doo". It  returns  the  number  of
       replacements made.

Extract  is like Replace, except that if the pattern matches, "rewrite"
       is copied into "out" (an additional argument) with substitutions.   The
       non-matching  portions  of "text" are ignored. Returns true iff a match
       occurred and the extraction happened successfully;  if no match occurs,
       the string is left unaffected.

AUTHOR

The C++ wrapper was contributed by Google Inc.
       Copyright (c) 2007 Google Inc.

REVISION

Last updated: 12 November 2007
------------------------------------------------------------------------------

PCRESAMPLE(3)                                                    PCRESAMPLE(3)

NAME
       PCRE - Perl-compatible regular expressions

PCRE SAMPLE PROGRAM

A simple, complete demonstration program, to get you started with using
       PCRE, is supplied in the file pcredemo.c in the PCRE distribution.

The program compiles the regular expression that is its first argument,
       and  matches  it  against the subject string in its second argument. No
       PCRE options are set, and default character tables are used. If  match-
       ing  succeeds,  the  program  outputs  the  portion of the subject that
       matched, together with the contents of any captured substrings.

If the -g option is given on the command line, the program then goes on
       to check for further matches of the same regular expression in the same
       subject string. The logic is a little bit tricky because of the  possi-
       bility  of  matching an empty string. Comments in the code explain what
       is going on.

If PCRE is installed in the standard include  and  library  directories
       for  your  system, you should be able to compile the demonstration pro-
       gram using this command:

gcc -o pcredemo pcredemo.c -lpcre

If PCRE is installed elsewhere, you may need to add additional  options
       to  the  command line. For example, on a Unix-like system that has PCRE
       installed in /usr/local, you  can  compile  the  demonstration  program
       using a command like this:

gcc -o pcredemo -I/usr/local/include pcredemo.c \
             -L/usr/local/lib -lpcre

Once  you  have  compiled the demonstration program, you can run simple
       tests like this:

./pcredemo 'cat|dog' 'the cat sat on the mat'
         ./pcredemo -g 'cat|dog' 'the dog sat on the cat'

Note that there is a  much  more  comprehensive  test  program,  called
       pcretest,  which  supports  many  more  facilities  for testing regular
       expressions and the PCRE library. The pcredemo program is provided as a
       simple coding example.

On some operating systems (e.g. Solaris), when PCRE is not installed in
       the standard library directory, you may get an error like this when you
       try to run pcredemo:

ld.so.1:  a.out:  fatal:  libpcre.so.0:  open failed: No such file or
       directory

This is caused by the way shared library support works  on  those  sys-
       tems. You need to add

-R/usr/local/lib

(for example) to the compile command to get round this problem.

AUTHOR

Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.

REVISION

Last updated: 23 January 2008
       Copyright (c) 1997-2008 University of Cambridge.
------------------------------------------------------------------------------
PCRESTACK(3)                                                      PCRESTACK(3)

NAME
       PCRE - Perl-compatible regular expressions

PCRE DISCUSSION OF STACK USAGE

When  you call pcre_exec(), it makes use of an internal function called
       match(). This calls itself recursively at branch points in the pattern,
       in  order to remember the state of the match so that it can back up and
       try a different alternative if the first one fails.  As  matching  pro-
       ceeds  deeper  and deeper into the tree of possibilities, the recursion
       depth increases.

Not all calls of match() increase the recursion depth; for an item such
       as  a* it may be called several times at the same level, after matching
       different numbers of a's. Furthermore, in a number of cases  where  the
       result  of  the  recursive call would immediately be passed back as the
       result of the current call (a "tail recursion"), the function  is  just
       restarted instead.

The pcre_dfa_exec() function operates in an entirely different way, and
       hardly uses recursion at all. The limit on its complexity is the amount
       of  workspace  it  is  given.  The comments that follow do NOT apply to
       pcre_dfa_exec(); they are relevant only for pcre_exec().

You can set limits on the number of times that match() is called,  both
       in  total  and  recursively. If the limit is exceeded, an error occurs.
       For details, see the section on  extra  data  for  pcre_exec()  in  the
       pcreapi documentation.

Each  time  that match() is actually called recursively, it uses memory
       from the process stack. For certain kinds of  pattern  and  data,  very
       large  amounts of stack may be needed, despite the recognition of "tail
       recursion".  You can often reduce the amount of recursion,  and  there-
       fore  the  amount of stack used, by modifying the pattern that is being
       matched. Consider, for example, this pattern:

([^<]|<(?!inet))+

It matches from wherever it starts until it encounters "<inet"  or  the
       end  of  the  data,  and is the kind of pattern that might be used when
       processing an XML file. Each iteration of the outer parentheses matches
       either  one  character that is not "<" or a "<" that is not followed by
       "inet". However, each time a  parenthesis  is  processed,  a  recursion
       occurs, so this formulation uses a stack frame for each matched charac-
       ter. For a long string, a lot of stack is required. Consider  now  this
       rewritten pattern, which matches exactly the same strings:

([^<]++|<(?!inet))+

This  uses very much less stack, because runs of characters that do not
       contain "<" are "swallowed" in one item inside the parentheses.  Recur-
       sion  happens  only when a "<" character that is not followed by "inet"
       is encountered (and we assume this is relatively  rare).  A  possessive
       quantifier  is  used  to stop any backtracking into the runs of non-"<"
       characters, but that is not related to stack usage.

This example shows that one way of avoiding stack problems when  match-
       ing long subject strings is to write repeated parenthesized subpatterns
       to match more than one character whenever possible.

Compiling PCRE to use heap instead of stack

In environments where stack memory is constrained, you  might  want  to
       compile  PCRE to use heap memory instead of stack for remembering back-
       up points. This makes it run a lot more slowly, however. Details of how
       to do this are given in the pcrebuild documentation. When built in this
       way, instead of using the stack, PCRE obtains and frees memory by call-
       ing  the  functions  that  are  pointed to by the pcre_stack_malloc and
       pcre_stack_free variables. By default,  these  point  to  malloc()  and
       free(),  but you can replace the pointers to cause PCRE to use your own
       functions. Since the block sizes are always the same,  and  are  always
       freed in reverse order, it may be possible to implement customized mem-
       ory handlers that are more efficient than the standard functions.

Limiting PCRE's stack usage

PCRE has an internal counter that can be used to  limit  the  depth  of
       recursion,  and  thus cause pcre_exec() to give an error code before it
       runs out of stack. By default, the limit is very  large,  and  unlikely
       ever  to operate. It can be changed when PCRE is built, and it can also
       be set when pcre_exec() is called. For details of these interfaces, see
       the pcrebuild and pcreapi documentation.

As a very rough rule of thumb, you should reckon on about 500 bytes per
       recursion. Thus, if you want to limit your  stack  usage  to  8Mb,  you
       should  set  the  limit at 16000 recursions. A 64Mb stack, on the other
       hand, can support around 128000 recursions. The pcretest  test  program
       has a command line option (-S) that can be used to increase the size of
       its stack.

Changing stack size in Unix-like systems

In Unix-like environments, there is not often a problem with the  stack
       unless  very  long  strings  are  involved, though the default limit on
       stack size varies from system to system. Values from 8Mb  to  64Mb  are
       common. You can find your default limit by running the command:

ulimit -s

Unfortunately,  the  effect  of  running out of stack is often SIGSEGV,
       though sometimes a more explicit error message is given. You  can  nor-
       mally increase the limit on stack size by code such as this:

struct rlimit rlim;
         getrlimit(RLIMIT_STACK, &rlim);
         rlim.rlim_cur = 100*1024*1024;
         setrlimit(RLIMIT_STACK, &rlim);

This  reads  the current limits (soft and hard) using getrlimit(), then
       attempts to increase the soft limit to  100Mb  using  setrlimit().  You
       must do this before calling pcre_exec().

Changing stack size in Mac OS X

Using setrlimit(), as described above, should also work on Mac OS X. It
       is also possible to set a stack size when linking a program. There is a
       discussion   about   stack  sizes  in  Mac  OS  X  at  this  web  site:
       http://developer.apple.com/qa/qa2005/qa1419.html.

AUTHOR

Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.

REVISION

Last updated: 09 July 2008
       Copyright (c) 1997-2008 University of Cambridge.
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