PCRE - Perl-compatible regular expressions (original API)
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.
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 6.3.0.
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:
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.
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 a 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.
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-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
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.
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