pcre2/doc/pcre2.3

.TH PCRE2 3 "27 August 2021" "PCRE2 10.38"
.SH NAME
PCRE2 - Perl-compatible regular expressions (revised API)
.SH INTRODUCTION
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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obsolete and is no longer maintained.
.P
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.
.P
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.
.P
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
.sp
  pcre2test -C
.P
The three libraries contain identical sets of functions, with names ending in
_8, _16, or _32, respectively (for example, \fBpcre2_compile_8()\fP). 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
\fBpcre2_compile()\fP, and the documentation is written assuming that this is
the case.
.P
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
.\" HREF
\fBpcre2matching\fP
.\"
page.
.P
Details of exactly which Perl regular expression features are and are not
supported by PCRE2 are given in separate documents. See the
.\" HREF
\fBpcre2pattern\fP
.\"
and
.\" HREF
\fBpcre2compat\fP
.\"
pages. There is a syntax summary in the
.\" HREF
\fBpcre2syntax\fP
.\"
page.
.P
Some features of PCRE2 can be included, excluded, or changed when the library
is built. The
.\" HREF
\fBpcre2_config()\fP
.\"
function makes it possible for a client to discover which features are
available. The features themselves are described in the
.\" HREF
\fBpcre2build\fP
.\"
page. Documentation about building PCRE2 for various operating systems can be
found in the
.\" HTML <a href="README.txt">
.\" </a>
\fBREADME\fP
.\"
and
.\" HTML <a href="NON-AUTOTOOLS-BUILD.txt">
.\" </a>
\fBNON-AUTOTOOLS_BUILD\fP
.\"
files in the source distribution.
.P
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.
.
.
.SH "SECURITY CONSIDERATIONS"
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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.
.P
One way of guarding against this possibility is to use the
\fBpcre2_pattern_info()\fP function to check the compiled pattern's options for
PCRE2_UTF. Alternatively, you can set the PCRE2_NEVER_UTF option when calling
\fBpcre2_compile()\fP. This causes a compile time error if the pattern contains
a UTF-setting sequence.
.P
The use of Unicode properties for character types such as \ed can also be
enabled from within the pattern, by specifying "(*UCP)". This feature can be
disallowed by setting the PCRE2_NEVER_UCP option.
.P
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.
.P
The use of the \eC 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 \eC, causing a compile-time error if it is
encountered. It is also possible to build PCRE2 with the use of \eC permanently
disabled.
.P
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 \fBpcre2_set_match_limit()\fP function in the
.\" HREF
\fBpcre2api\fP
.\"
page. There is a similar function called \fBpcre2_set_depth_limit()\fP that can
be used to restrict the amount of memory that is used.
.
.
.SH "USER DOCUMENTATION"
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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 \fBpcre2grep\fP and \fBpcre2test\fP programs are in
files called \fBpcre2grep.txt\fP and \fBpcre2test.txt\fP, respectively. The
remaining sections, except for the \fBpcre2demo\fP section (which is a program
listing), and the short pages for individual functions, are concatenated in
\fBpcre2.txt\fP, for ease of searching. The sections are as follows:
.sp
  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 \fBpcre2grep\fP 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
.\" JOIN
  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 \fBpcre2test\fP command
  pcre2unicode       discussion of Unicode and UTF support
.sp
In the "man" and HTML formats, there is also a short page for each C library
function, listing its arguments and results.
.
.
.SH AUTHOR
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.nf
Philip Hazel
Retired from University Computing Service
Cambridge, England.
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.P
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.
.
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.SH REVISION
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.nf
Last updated: 27 August 2021
Copyright (c) 1997-2021 University of Cambridge.
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