196 lines
7.9 KiB
Groff
196 lines
7.9 KiB
Groff
.TH PCRE2 3 "23 March 2017" "PCRE2 10.30"
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.SH NAME
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PCRE2 - Perl-compatible regular expressions (revised API)
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.SH INTRODUCTION
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.rs
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.sp
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PCRE2 is the name used for a revised API for the PCRE library, which is a set
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of functions, written in C, that implement regular expression pattern matching
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using the same syntax and semantics as Perl, with just a few differences. Some
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features that appeared in Python and the original PCRE before they appeared in
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Perl are also available using the Python syntax. There is also some support for
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one or two .NET and Oniguruma syntax items, and there are options for
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requesting some minor changes that give better ECMAScript (aka JavaScript)
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compatibility.
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.P
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The source code for PCRE2 can be compiled to support 8-bit, 16-bit, or 32-bit
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code units, which means that up to three separate libraries may be installed.
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The original work to extend PCRE to 16-bit and 32-bit code units was done by
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Zoltan Herczeg and Christian Persch, respectively. In all three cases, strings
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can be interpreted either as one character per code unit, or as UTF-encoded
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Unicode, with support for Unicode general category properties. Unicode support
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is optional at build time (but is the default). However, processing strings as
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UTF code units must be enabled explicitly at run time. The version of Unicode
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in use can be discovered by running
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.sp
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pcre2test -C
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.P
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The three libraries contain identical sets of functions, with names ending in
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_8, _16, or _32, respectively (for example, \fBpcre2_compile_8()\fP). However,
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by defining PCRE2_CODE_UNIT_WIDTH to be 8, 16, or 32, a program that uses just
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one code unit width can be written using generic names such as
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\fBpcre2_compile()\fP, and the documentation is written assuming that this is
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the case.
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.P
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In addition to the Perl-compatible matching function, PCRE2 contains an
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alternative function that matches the same compiled patterns in a different
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way. In certain circumstances, the alternative function has some advantages.
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For a discussion of the two matching algorithms, see the
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.\" HREF
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\fBpcre2matching\fP
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.\"
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page.
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.P
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Details of exactly which Perl regular expression features are and are not
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supported by PCRE2 are given in separate documents. See the
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.\" HREF
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\fBpcre2pattern\fP
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.\"
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and
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.\" HREF
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\fBpcre2compat\fP
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.\"
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pages. There is a syntax summary in the
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.\" HREF
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\fBpcre2syntax\fP
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.\"
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page.
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.P
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Some features of PCRE2 can be included, excluded, or changed when the library
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is built. The
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.\" HREF
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\fBpcre2_config()\fP
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.\"
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function makes it possible for a client to discover which features are
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available. The features themselves are described in the
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.\" HREF
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\fBpcre2build\fP
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.\"
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page. Documentation about building PCRE2 for various operating systems can be
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found in the
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.\" HTML <a href="README.txt">
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.\" </a>
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\fBREADME\fP
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.\"
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and
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.\" HTML <a href="NON-AUTOTOOLS-BUILD.txt">
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.\" </a>
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\fBNON-AUTOTOOLS_BUILD\fP
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.\"
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files in the source distribution.
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.P
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The libraries contains a number of undocumented internal functions and data
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tables that are used by more than one of the exported external functions, but
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which are not intended for use by external callers. Their names all begin with
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"_pcre2", which hopefully will not provoke any name clashes. In some
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environments, it is possible to control which external symbols are exported
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when a shared library is built, and in these cases the undocumented symbols are
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not exported.
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.
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.
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.SH "SECURITY CONSIDERATIONS"
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.rs
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.sp
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If you are using PCRE2 in a non-UTF application that permits users to supply
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arbitrary patterns for compilation, you should be aware of a feature that
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allows users to turn on UTF support from within a pattern. For example, an
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8-bit pattern that begins with "(*UTF)" turns on UTF-8 mode, which interprets
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patterns and subjects as strings of UTF-8 code units instead of individual
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8-bit characters. This causes both the pattern and any data against which it is
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matched to be checked for UTF-8 validity. If the data string is very long, such
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a check might use sufficiently many resources as to cause your application to
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lose performance.
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.P
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One way of guarding against this possibility is to use the
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\fBpcre2_pattern_info()\fP function to check the compiled pattern's options for
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PCRE2_UTF. Alternatively, you can set the PCRE2_NEVER_UTF option when calling
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\fBpcre2_compile()\fP. This causes a compile time error if the pattern contains
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a UTF-setting sequence.
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.P
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The use of Unicode properties for character types such as \ed can also be
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enabled from within the pattern, by specifying "(*UCP)". This feature can be
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disallowed by setting the PCRE2_NEVER_UCP option.
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.P
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If your application is one that supports UTF, be aware that validity checking
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can take time. If the same data string is to be matched many times, you can use
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the PCRE2_NO_UTF_CHECK option for the second and subsequent matches to avoid
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running redundant checks.
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.P
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The use of the \eC escape sequence in a UTF-8 or UTF-16 pattern can lead to
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problems, because it may leave the current matching point in the middle of a
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multi-code-unit character. The PCRE2_NEVER_BACKSLASH_C option can be used by an
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application to lock out the use of \eC, causing a compile-time error if it is
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encountered. It is also possible to build PCRE2 with the use of \eC permanently
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disabled.
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.P
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Another way that performance can be hit is by running a pattern that has a very
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large search tree against a string that will never match. Nested unlimited
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repeats in a pattern are a common example. PCRE2 provides some protection
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against this: see the \fBpcre2_set_match_limit()\fP function in the
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.\" HREF
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\fBpcre2api\fP
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.\"
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page. There is a similar function called \fBpcre2_set_depth_limit()\fP that can
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be used to restrict the amount of memory that is used.
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.
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.
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.SH "USER DOCUMENTATION"
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.rs
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.sp
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The user documentation for PCRE2 comprises a number of different sections. In
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the "man" format, each of these is a separate "man page". In the HTML format,
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each is a separate page, linked from the index page. In the plain text format,
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the descriptions of the \fBpcre2grep\fP and \fBpcre2test\fP programs are in
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files called \fBpcre2grep.txt\fP and \fBpcre2test.txt\fP, respectively. The
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remaining sections, except for the \fBpcre2demo\fP section (which is a program
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listing), and the short pages for individual functions, are concatenated in
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\fBpcre2.txt\fP, for ease of searching. The sections are as follows:
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.sp
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pcre2 this document
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pcre2-config show PCRE2 installation configuration information
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pcre2api details of PCRE2's native C API
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pcre2build building PCRE2
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pcre2callout details of the callout feature
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pcre2compat discussion of Perl compatibility
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pcre2demo a demonstration C program that uses PCRE2
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pcre2grep description of the \fBpcre2grep\fP command (8-bit only)
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pcre2jit discussion of just-in-time optimization support
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pcre2limits details of size and other limits
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pcre2matching discussion of the two matching algorithms
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pcre2partial details of the partial matching facility
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.\" JOIN
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pcre2pattern syntax and semantics of supported regular
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expression patterns
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pcre2perform discussion of performance issues
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pcre2posix the POSIX-compatible C API for the 8-bit library
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pcre2sample discussion of the pcre2demo program
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pcre2stack discussion of stack and memory usage
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pcre2syntax quick syntax reference
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pcre2test description of the \fBpcre2test\fP command
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pcre2unicode discussion of Unicode and UTF support
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.sp
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In the "man" and HTML formats, there is also a short page for each C library
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function, listing its arguments and results.
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.
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.
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.SH AUTHOR
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.rs
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.sp
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.nf
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Philip Hazel
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University Computing Service
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Cambridge, England.
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.fi
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.P
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Putting an actual email address here is a spam magnet. If you want to email me,
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use my two initials, followed by the two digits 10, at the domain cam.ac.uk.
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.
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.
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.SH REVISION
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.rs
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.sp
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.nf
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Last updated: 27 March 2017
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Copyright (c) 1997-2017 University of Cambridge.
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.fi
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