PCRE2(3) Introduction to Library Functions PCRE2(3)

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 The old library is now obsolete and is no longer
maintained.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
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.

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
library function, listing its arguments and results.

AUTHORS

The current maintainers of PCRE2 are Nicholas Wilson and Zoltan
Herczeg.

PCRE2 was written by Philip Hazel, of the University Computing
Service, Cambridge, England. Many others have also contributed.

To contact the maintainers, please use the GitHub issues tracker or
PCRE2 mailing list, as described at the project page:
https://github.com/PCRE2Project/pcre2

REVISION

Last updated: 18 December 2024
Copyright (c) 1997-2021 University of Cambridge.

PCRE2 10.45 18 December 2024 PCRE2(3)