PCRE2(3) PCRE2 10.38 PCRE2(3)
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NAME
PCRE2 - Perl-compatible regular expressions (revised API)
INTRODUCTION
PCRE2 is the name used for a revised API for the PCRE library, which
is a set of functions, written in C, that implement regular expression
pattern matching using the same syntax and semantics as Perl, with
just a few differences. After nearly two decades, the limitations of
the original API were making development increasingly difficult. The
new API is more extensible, and it was simplified by abolishing the
separate "study" optimizing function; in PCRE2, patterns are
automatically optimized where possible. Since forking from PCRE1, the
code has been extensively refactored and new features introduced. The
old library is now obsolete and is no longer maintained.
As well as Perl-style regular expression patterns, some features that
appeared in Python and the original PCRE before they appeared in Perl
are available using the Python syntax. There is also some support for
one or two .NET and Oniguruma syntax items, and there are options for
requesting some minor changes that give better ECMAScript (aka
JavaScript) compatibility.
The source code for PCRE2 can be compiled to support strings of 8-bit,
16-bit, or 32-bit code units, which means that up to three separate
libraries may be installed, one for each code unit size. The size of
code unit is not related to the bit size of the underlying hardware.
In a 64-bit environment that also supports 32-bit applications,
versions of PCRE2 that are compiled in both 64-bit and 32-bit modes
may be needed.
The original work to extend PCRE to 16-bit and 32-bit code units was
done by Zoltan Herczeg and Christian Persch, respectively. In all
three cases, strings can be interpreted either as one character per
code unit, or as UTF-encoded Unicode, with support for Unicode general
category properties. Unicode support is optional at build time (but is
the default). However, processing strings as UTF code units must be
enabled explicitly at run time. The version of Unicode in use can be
discovered by running
pcre2test -C
The three libraries contain identical sets of functions, with names
ending in _8, _16, or _32, respectively (for example,
pcre2_compile_8()). However, by defining PCRE2_CODE_UNIT_WIDTH to be
8, 16, or 32, a program that uses just one code unit width can be
written using generic names such as pcre2_compile(), and the
documentation is written assuming that this is the case.
In addition to the Perl-compatible matching function, PCRE2 contains
an alternative function that matches the same compiled patterns in a
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different way. In certain circumstances, the alternative function has
some advantages. For a discussion of the two matching algorithms, see
the pcre2matching page.
Details of exactly which Perl regular expression features are and are
not supported by PCRE2 are given in separate documents. See the
pcre2pattern and pcre2compat pages. There is a syntax summary in the
pcre2syntax page.
Some features of PCRE2 can be included, excluded, or changed when the
library is built. The pcre2_config() function makes it possible for a
client to discover which features are available. The features
themselves are described in the pcre2build page. Documentation about
building PCRE2 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 "_pcre2", 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 PCRE2 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. For example, an 8-bit pattern that begins with "(*UTF)" turns
on UTF-8 mode, which interprets patterns and subjects as strings of
UTF-8 code units 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
pcre2_pattern_info() function to check the compiled pattern's options
for PCRE2_UTF. Alternatively, you can set the PCRE2_NEVER_UTF option
when calling pcre2_compile(). This causes a compile time error if the
pattern contains a UTF-setting sequence.
The use of Unicode properties for character types such as \d can also
be enabled from within the pattern, by specifying "(*UCP)". This
feature can be disallowed by setting the PCRE2_NEVER_UCP option.
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 PCRE2_NO_UTF_CHECK option for the second and
subsequent matches to avoid running redundant checks.
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The use of the \C escape sequence in a UTF-8 or UTF-16 pattern can
lead to problems, because it may leave the current matching point in
the middle of a multi-code-unit character. The PCRE2_NEVER_BACKSLASH_C
option can be used by an application to lock out the use of \C,
causing a compile-time error if it is encountered. It is also possible
to build PCRE2 with the use of \C permanently disabled.
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. PCRE2
provides some protection against this: see the pcre2_set_match_limit()
function in the pcre2api page. There is a similar function called
pcre2_set_depth_limit() that can be used to restrict the amount of
memory that is used.
USER DOCUMENTATION
The user documentation for PCRE2 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 pcre2grep and
pcre2test programs are in files called pcre2grep.txt and
pcre2test.txt, respectively. The remaining sections, except for the
pcre2demo section (which is a program listing), and the short pages
for individual functions, are concatenated in pcre2.txt, for ease of
searching. The sections are as follows:
pcre2 this document
pcre2-config show PCRE2 installation configuration information
pcre2api details of PCRE2's native C API
pcre2build building PCRE2
pcre2callout details of the pattern callout feature
pcre2compat discussion of Perl compatibility
pcre2convert details of pattern conversion functions
pcre2demo a demonstration C program that uses PCRE2
pcre2grep description of the pcre2grep command (8-bit only)
pcre2jit discussion of just-in-time optimization support
pcre2limits details of size and other limits
pcre2matching discussion of the two matching algorithms
pcre2partial details of the partial matching facility
pcre2pattern syntax and semantics of supported regular
expression patterns
pcre2perform discussion of performance issues
pcre2posix the POSIX-compatible C API for the 8-bit library
pcre2sample discussion of the pcre2demo program
pcre2serialize details of pattern serialization
pcre2syntax quick syntax reference
pcre2test description of the pcre2test command
pcre2unicode discussion of Unicode and UTF support
In the "man" and HTML formats, there is also a short page for each C
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library function, listing its arguments and results.
AUTHOR
Philip Hazel
Retired from University Computing Service
Cambridge, England.
Putting an actual email address here is a spam magnet. If you want to
email me, use my two names separated by a dot at gmail.com.
REVISION
Last updated: 27 August 2021
Copyright (c) 1997-2021 University of Cambridge.
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