doc/pcre2.3 - platform/external/pcre - Gitiles

 .TH PCRE2 3 "27 August 2021" "PCRE2 10.38"
 .SH NAME
 PCRE2 - Perl-compatible regular expressions (revised API)
 .SH INTRODUCTION
 .rs
 .sp
 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 obsolete and is no longer maintained.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"
 .rs
 .sp
 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"
 .rs
 .sp
 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
 .rs
 .sp
 .nf
 Philip Hazel
 Retired from University Computing Service
 Cambridge, England.
 .fi
 .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.
 .
 .
 .SH REVISION
 .rs
 .sp
 .nf
 Last updated: 27 August 2021
 Copyright (c) 1997-2021 University of Cambridge.
 .fi
	.TH PCRE2 3 "27 August 2021" "PCRE2 10.38"
	.SH NAME
	PCRE2 - Perl-compatible regular expressions (revised API)
	.SH INTRODUCTION
	.rs
	.sp
	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"
	.rs
	.sp
	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"
	.rs
	.sp
	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
	.rs
	.sp
	.nf
	Philip Hazel
	Retired from University Computing Service
	Cambridge, England.
	.fi
	.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.
	.
	.
	.SH REVISION
	.rs
	.sp
	.nf
	Last updated: 27 August 2021
	Copyright (c) 1997-2021 University of Cambridge.
	.fi