PCRE(3) 						     Library Functions Manual							   PCRE(3)

NAME

       PCRE - Perl-compatible regular expressions (original API)

PLEASE TAKE NOTE


       This  document  relates	to PCRE releases that use the original API, with library names libpcre, libpcre16, and libpcre32. January 2015 saw
       the first release of a new API, known as PCRE2, with release numbers starting at 10.00  and  library  names  libpcre2-8,  libpcre2-16,  and
       libpcre2-32.  The  old  libraries  (now	called	PCRE1) are still being maintained for bug fixes, but there will be no new development. New
       projects are advised to use the new PCRE2 libraries.

INTRODUCTION


       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 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  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  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
       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 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:

       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
       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-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.

SECURITY CONSIDERATIONS


       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  char-
       acters  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 sufficiently many resources as to cause your  application  to  lose
       performance.

       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 provides some protection against this: see the PCRE_EXTRA_MATCH_LIMIT fea-
       ture in the pcreapi page.

USER DOCUMENTATION


       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, the descriptions of the pcregrep and
       pcretest programs are in files called pcregrep.txt and pcretest.txt, respectively. The remaining sections, except for the pcredemo  section
       (which is a program listing), are concatenated in pcre.txt, for ease of searching. The sections are as follows:

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

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: 10 February 2015
       Copyright (c) 1997-2015 University of Cambridge.

PCRE 8.37							 10 February 2015							   PCRE(3)