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PCRE(3)								       PCRE(3)

       PCRE - Perl-compatible regular expressions


       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. Some features that appeared in Python and PCRE
       before they appeared in Perl are also available using the  Python  syn‐
       tax,  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 compatibility.

       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
       possible was done by Zoltan Herczeg.

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

       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 documentation mainte‐
       nance load, most of the documentation describes the 8-bit library, with
       the  differences	 for  the  16-bit and 32-bit libraries described sepa‐
       rately 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 6.3.0.

       In  addition to the Perl-compatible matching function, PCRE contains an
       alternative function that matches the same compiled patterns in a  dif‐
       ferent 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 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 contribu‐
       tions 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 pcrepat‐
       tern  and pcrecompat pages. There is a syntax summary in the pcresyntax

       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  and
       NON-AUTOTOOLS_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  patterns  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  interprets  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 valid‐
       ity. If the data string is very long, such a  check  might  use	suffi‐
       ciently	many  resources	 as  to cause your application to lose perfor‐

       One  way	 of  guarding  against	this  possibility  is	to   use   the
       pcre_fullinfo()	function  to  check the compiled pattern's options for
       UTF.  Alternatively, from release 8.33, you can set the	PCRE_NEVER_UTF
       option  at compile time. This causes an compile time error if a pattern
       contains a UTF-setting sequence.

       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 checks.

       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 pro‐
       vides some protection against this: see the PCRE_EXTRA_MATCH_LIMIT fea‐
       ture in the pcreapi page.


       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, except  the	pcredemo  sec‐
       tion, are concatenated, for ease of searching. The sections are as fol‐

	 pcre		   this document
	 pcre-config	   show PCRE installation configuration information
	 pcre16		   details of the 16-bit library
	 pcre32		   details of the 32-bit library
	 pcreapi	   details of PCRE's native C API
	 pcrebuild	   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 function, 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: 13 May 2013
       Copyright (c) 1997-2013 University of Cambridge.

PCRE 8.33			  01 Oct 2013			       PCRE(3)

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