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PCRE(3)                              Library Functions Manual                             PCRE(3)

       PCRE - Perl-compatible regular expressions


       The  PCRE library is a set of functions that implement regular expression pattern matching
       using the same syntax and semantics as Perl, with just a few  differences.  Some  features
       that appeared in Python and PCRE before they appeared in Perl are also available using the
       Python syntax, there is some support for one or two .NET and Oniguruma syntax  items,  and
       there  is an option for requesting some minor changes that give better JavaScript compati-

       Starting with release 8.30, it is possible to compile two  separate  PCRE  libraries:  the
       original,  which  supports 8-bit character strings (including UTF-8 strings), and a second
       library that supports 16-bit character  strings  (including  UTF-16  strings).  The  build
       process  allows either one or both to be built. The majority of the work to make this pos-
       sible was done by Zoltan Herczeg.

       Starting with release 8.32 it is possible to compile a third separate PCRE library,  which
       supports 32-bit character strings (including UTF-32 strings). The build process allows any
       set of the 8-, 16- and 32-bit libraries. The work to make this possible was done by Chris-
       tian Persch.

       The  three  libraries  contain  identical  sets of functions, except that the names in the
       16-bit library start with pcre16_ instead of pcre_, and the names in  the  32-bit  library
       start  with pcre32_ instead of pcre_. To avoid over-complication and reduce the documenta-
       tion maintenance load, most of the documentation describes the  8-bit  library,  with  the
       differences  for  the  16-bit  and 32-bit libraries described separately in the pcre16 and
       pcre32 pages. References to functions or structures of the form pcre[16|32]_xxx should  be
       read  as  meaning "pcre_xxx when using the 8-bit library, pcre16_xxx when using the 16-bit
       library, or pcre32_xxx when using the 32-bit library".

       The current implementation of PCRE corresponds approximately  with  Perl  5.12,  including
       support  for UTF-8/16/32 encoded strings and Unicode general category properties. However,
       UTF-8/16/32 and Unicode support has to be explicitly enabled; it is not the  default.  The
       Unicode tables correspond to Unicode release 6.2.0.

       In  addition  to the Perl-compatible matching function, PCRE contains an alternative func-
       tion 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 algo-
       rithms, 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 for the 8-bit library. 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:


       Details of exactly which Perl regular expression features are and  are  not  supported  by
       PCRE are given in separate documents. See the pcrepattern 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 themselves are described  in  the  pcrebuild  page.  Documentation
       about building PCRE for various operating systems can be found in the README and NON-AUTO-
       TOOLS_BUILD files in the source distribution.

       The libraries 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_"  or  "_pcre16_"  or
       "_pcre32_", 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.


       If you are using PCRE in a non-UTF application that permits users to supply arbitrary pat-
       terns for compilation, you should be aware of a feature that allows users to turn  on  UTF
       support from within a pattern, provided that PCRE was built with UTF support. For example,
       an 8-bit pattern that begins with "(*UTF8)" or "(*UTF)" turns on UTF-8 mode, which  inter-
       prets  patterns  and  subjects  as strings of UTF-8 characters instead of individual 8-bit
       characters.  This causes both the pattern and any data against which it is matched  to  be
       checked for UTF-8 validity. If the data string is very long, such a check might use suffi-
       ciently many resources as to cause your application to lose performance.

       The best way of guarding against this possibility is to use the  pcre_fullinfo()  function
       to check the compiled pattern's options for UTF.

       If  your  application  is  one that supports UTF, be aware that validity checking can take
       time.  If  the  same  data  string  is  to  be  matched  many  times,  you  can  use   the
       PCRE_NO_UTF[8|16|32]_CHECK  option for the second and subsequent matches to save redundant

       Another way that performance can be hit is by running a pattern  that  has  a  very  large
       search  tree against a string that will never match. Nested unlimited repeats in a pattern
       are  a  common  example.  PCRE  provides   some   protection   against   this:   see   the
       PCRE_EXTRA_MATCH_LIMIT feature in the pcreapi page.


       The  user  documentation  for  PCRE comprises a number of different sections. 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, except the
       pcredemo section, are concatenated, for ease of searching. The sections are as follows:

         pcre              this document
         pcre16            details of the 16-bit library
         pcre32            details of the 32-bit library
         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 for the 8-bit library
         pcredemo          a demonstration C program that uses PCRE
         pcregrep          description of the pcregrep command (8-bit only)
         pcrejit           discussion of the just-in-time optimization support
         pcrelimits        details of size and other limits
         pcrematching      discussion of the two matching algorithms
         pcrepartial       details of the partial matching facility
         pcrepattern       syntax and semantics of supported
                             regular expressions
         pcreperform       discussion of performance issues
         pcreposix         the POSIX-compatible C API for the 8-bit library
         pcreprecompile    details of saving and re-using precompiled patterns
         pcresample        discussion of the pcredemo program
         pcrestack         discussion of stack usage
         pcresyntax        quick syntax reference
         pcretest          description of the pcretest testing command
         pcreunicode       discussion of Unicode and UTF-8/16/32 support

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


       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.


       Last updated: 11 November 2012
       Copyright (c) 1997-2012 University of Cambridge.

PCRE 8.32                                11 November 2012                                 PCRE(3)

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