22

✏️ Editing: pcre2.html

<html>
<head>
<title>pcre2 specification</title>
</head>
<body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
<h1>pcre2 man page</h1>

Return to the <a href="index.html">PCRE2 index page</a>.


This page is part of the PCRE2 HTML documentation. It was generated
automatically from the original man page. If there is any nonsense in it,
please consult the man page, in case the conversion went wrong.
 
<ul>
<li><a name="TOC1" href="#SEC1">INTRODUCTION</a>
<li><a name="TOC2" href="#SEC2">SECURITY CONSIDERATIONS</a>
<li><a name="TOC3" href="#SEC3">USER DOCUMENTATION</a>
<li><a name="TOC4" href="#SEC4">AUTHOR</a>
<li><a name="TOC5" href="#SEC5">REVISION</a>
</ul>
 <a name="SEC1" href="#TOC1">INTRODUCTION</a> 

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.


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 8-bit, 16-bit, or 32-bit
code units, which means that up to three separate libraries may be installed.
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
<pre>
 pcre2test -C
</PRE>


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
<a href="pcre2matching.html">pcre2matching</a>
page.


Details of exactly which Perl regular expression features are and are not
supported by PCRE2 are given in separate documents. See the
<a href="pcre2pattern.html">pcre2pattern</a>
and
<a href="pcre2compat.html">pcre2compat</a>
pages. There is a syntax summary in the
<a href="pcre2syntax.html">pcre2syntax</a>
page.


Some features of PCRE2 can be included, excluded, or changed when the library
is built. The
<a href="pcre2_config.html">pcre2_config()</a>
function makes it possible for a client to discover which features are
available. The features themselves are described in the
<a href="pcre2build.html">pcre2build</a>
page. Documentation about building PCRE2 for various operating systems can be
found in the
<a href="README.txt">README</a>
and
<a href="NON-AUTOTOOLS-BUILD.txt">NON-AUTOTOOLS_BUILD</a>
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.

 <a name="SEC2" href="#TOC1">SECURITY CONSIDERATIONS</a> 

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
<a href="pcre2api.html">pcre2api</a>
page. There is a similar function called pcre2_set_depth_limit() that can
be used to restrict the amount of memory that is used.

 <a name="SEC3" href="#TOC1">USER DOCUMENTATION</a> 

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:
<pre>
 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 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
</pre>
In the "man" and HTML formats, there is also a short page for each C library
function, listing its arguments and results.

 <a name="SEC4" href="#TOC1">AUTHOR</a> 

Philip Hazel
 
University Computing Service
 
Cambridge, England.
 


Putting an actual email address here is a spam magnet. If you want to email me,
use my two initials, followed by the two digits 10, at the domain cam.ac.uk.

 <a name="SEC5" href="#TOC1">REVISION</a> 

Last updated: 11 July 2018
 
Copyright &copy; 1997-2018 University of Cambridge.
 

Return to the <a href="index.html">PCRE2 index page</a>.

MR SKB