PCRE(3)                   OpenBSD Programmer's Manual                  PCRE(3)



NAME
       PCRE - Perl-compatible regular expressions

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 Christian
       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
       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 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 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
       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
       feature 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, all the sections, except the pcredemo
       section, are concatenated, 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 addition, in the "man" and HTML formats, there is 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: 13 May 2013
       Copyright (c) 1997-2013 University of Cambridge.



PCRE 8.33                         13 May 2013                          PCRE(3)
