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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, 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 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.
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.
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 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.
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 supportIn 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: 11 November 2012
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