PCRE(3) Introduction to Library Functions 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 now at end
of life, and 8.45 is the final release. 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
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:

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 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.

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: 14 June 2021
Copyright (c) 1997-2021 University of Cambridge.

PCRE 8.45 14 June 2021 PCRE(3)