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version 1.1.1.3, 2012/10/09 09:19:17	version 1.1.1.4, 2013/07/22 08:25:57
Line 1	Line 1
.TH PCREPATTERN 3 "04 May 2012" "PCRE 8.31"	.TH PCREPATTERN 3 "26 April 2013" "PCRE 8.33"
.SH NAME	.SH NAME
PCRE - Perl-compatible regular expressions	PCRE - Perl-compatible regular expressions
.SH "PCRE REGULAR EXPRESSION DETAILS"	.SH "PCRE REGULAR EXPRESSION DETAILS"
Line 20 have copious examples. Jeffrey Friedl's "Mastering Reg	Line 20 have copious examples. Jeffrey Friedl's "Mastering Reg
published by O'Reilly, covers regular expressions in great detail. This	published by O'Reilly, covers regular expressions in great detail. This
description of PCRE's regular expressions is intended as reference material.	description of PCRE's regular expressions is intended as reference material.
.P	.P
	This document discusses the patterns that are supported by PCRE when one its
	main matching functions, \fBpcre_exec()\fP (8-bit) or \fBpcre[16\|32]_exec()\fP
	(16- or 32-bit), is used. PCRE also has alternative matching functions,
	\fBpcre_dfa_exec()\fP and \fBpcre[16\|32_dfa_exec()\fP, which match using a
	different algorithm that is not Perl-compatible. Some of the features discussed
	below are not available when DFA matching is used. The advantages and
	disadvantages of the alternative functions, and how they differ from the normal
	functions, are discussed in the
	.\" HREF
	\fBpcrematching\fP
	.\"
	page.
	.
	.
	.SH "SPECIAL START-OF-PATTERN ITEMS"
	.rs
	.sp
	A number of options that can be passed to \fBpcre_compile()\fP can also be set
	by special items at the start of a pattern. These are not Perl-compatible, but
	are provided to make these options accessible to pattern writers who are not
	able to change the program that processes the pattern. Any number of these
	items may appear, but they must all be together right at the start of the
	pattern string, and the letters must be in upper case.
	.
	.
	.SS "UTF support"
	.rs
	.sp
The original operation of PCRE was on strings of one-byte characters. However,	The original operation of PCRE was on strings of one-byte characters. However,
there is now also support for UTF-8 strings in the original library, and a	there is now also support for UTF-8 strings in the original library, an
second library that supports 16-bit and UTF-16 character strings. To use these	extra library that supports 16-bit and UTF-16 character strings, and a
	third library that supports 32-bit and UTF-32 character strings. To use these
features, PCRE must be built to include appropriate support. When using UTF	features, PCRE must be built to include appropriate support. When using UTF
strings you must either call the compiling function with the PCRE_UTF8 or	strings you must either call the compiling function with the PCRE_UTF8,
PCRE_UTF16 option, or the pattern must start with one of these special	PCRE_UTF16, or PCRE_UTF32 option, or the pattern must start with one of
sequences:	these special sequences:
.sp	.sp
(*UTF8)	(*UTF8)
(*UTF16)	(*UTF16)
	(*UTF32)
	(*UTF)
.sp	.sp
	(*UTF) is a generic sequence that can be used with any of the libraries.
Starting a pattern with such a sequence is equivalent to setting the relevant	Starting a pattern with such a sequence is equivalent to setting the relevant
option. This feature is not Perl-compatible. How setting a UTF mode affects	option. How setting a UTF mode affects pattern matching is mentioned in several
pattern matching is mentioned in several places below. There is also a summary	places below. There is also a summary of features in the
of features in the
.\" HREF	.\" HREF
\fBpcreunicode\fP	\fBpcreunicode\fP
.\"	.\"
page.	page.
.P	.P
Another special sequence that may appear at the start of a pattern or in	Some applications that allow their users to supply patterns may wish to
combination with (UTF8) or (UTF16) is:	restrict them to non-UTF data for security reasons. If the PCRE_NEVER_UTF
	option is set at compile time, (*UTF) etc. are not allowed, and their
	appearance causes an error.
	.
	.
	.SS "Unicode property support"
	.rs
.sp	.sp
	Another special sequence that may appear at the start of a pattern is
	.sp
(*UCP)	(*UCP)
.sp	.sp
This has the same effect as setting the PCRE_UCP option: it causes sequences	This has the same effect as setting the PCRE_UCP option: it causes sequences
such as \ed and \ew to use Unicode properties to determine character types,	such as \ed and \ew to use Unicode properties to determine character types,
instead of recognizing only characters with codes less than 128 via a lookup	instead of recognizing only characters with codes less than 128 via a lookup
table.	table.
.P	.
	.
	.SS "Disabling start-up optimizations"
	.rs
	.sp
If a pattern starts with (*NO_START_OPT), it has the same effect as setting the	If a pattern starts with (*NO_START_OPT), it has the same effect as setting the
PCRE_NO_START_OPTIMIZE option either at compile or matching time. There are	PCRE_NO_START_OPTIMIZE option either at compile or matching time.
also some more of these special sequences that are concerned with the handling
of newlines; they are described below.
.P
The remainder of this document discusses the patterns that are supported by
PCRE when one its main matching functions, \fBpcre_exec()\fP (8-bit) or
\fBpcre16_exec()\fP (16-bit), is used. PCRE also has alternative matching
functions, \fBpcre_dfa_exec()\fP and \fBpcre16_dfa_exec()\fP, which match using
a different algorithm that is not Perl-compatible. Some of the features
discussed below are not available when DFA matching is used. The advantages and
disadvantages of the alternative functions, and how they differ from the normal
functions, are discussed in the
.\" HREF
\fBpcrematching\fP
.\"
page.
.	.
.	.
.\" HTML <a name="newlines"></a>	.\" HTML <a name="newlines"></a>
.SH "NEWLINE CONVENTIONS"	.SS "Newline conventions"
.rs	.rs
.sp	.sp
PCRE supports five different conventions for indicating line breaks in	PCRE supports five different conventions for indicating line breaks in
Line 103 example, on a Unix system where LF is the default newl	Line 131 example, on a Unix system where LF is the default newl
(*CR)a.b	(*CR)a.b
.sp	.sp
changes the convention to CR. That pattern matches "a\enb" because LF is no	changes the convention to CR. That pattern matches "a\enb" because LF is no
longer a newline. Note that these special settings, which are not	longer a newline. If more than one of these settings is present, the last one
Perl-compatible, are recognized only at the very start of a pattern, and that
they must be in upper case. If more than one of them is present, the last one
is used.	is used.
.P	.P
The newline convention affects the interpretation of the dot metacharacter when	The newline convention affects where the circumflex and dollar assertions are
PCRE_DOTALL is not set, and also the behaviour of \eN. However, it does not	true. It also affects the interpretation of the dot metacharacter when
affect what the \eR escape sequence matches. By default, this is any Unicode	PCRE_DOTALL is not set, and the behaviour of \eN. However, it does not affect
newline sequence, for Perl compatibility. However, this can be changed; see the	what the \eR escape sequence matches. By default, this is any Unicode newline
	sequence, for Perl compatibility. However, this can be changed; see the
description of \eR in the section entitled	description of \eR in the section entitled
.\" HTML <a href="#newlineseq">	.\" HTML <a href="#newlineseq">
.\" </a>	.\" </a>
Line 121 below. A change of \eR setting can be combined with a	Line 148 below. A change of \eR setting can be combined with a
convention.	convention.
.	.
.	.
	.SS "Setting match and recursion limits"
	.rs
	.sp
	The caller of \fBpcre_exec()\fP can set a limit on the number of times the
	internal \fBmatch()\fP function is called and on the maximum depth of
	recursive calls. These facilities are provided to catch runaway matches that
	are provoked by patterns with huge matching trees (a typical example is a
	pattern with nested unlimited repeats) and to avoid running out of system stack
	by too much recursion. When one of these limits is reached, \fBpcre_exec()\fP
	gives an error return. The limits can also be set by items at the start of the
	pattern of the form
	.sp
	(*LIMIT_MATCH=d)
	(*LIMIT_RECURSION=d)
	.sp
	where d is any number of decimal digits. However, the value of the setting must
	be less than the value set by the caller of \fBpcre_exec()\fP for it to have
	any effect. In other words, the pattern writer can lower the limit set by the
	programmer, but not raise it. If there is more than one setting of one of these
	limits, the lower value is used.
	.
	.
	.SH "EBCDIC CHARACTER CODES"
	.rs
	.sp
	PCRE can be compiled to run in an environment that uses EBCDIC as its character
	code rather than ASCII or Unicode (typically a mainframe system). In the
	sections below, character code values are ASCII or Unicode; in an EBCDIC
	environment these characters may have different code values, and there are no
	code points greater than 255.
	.
	.
.SH "CHARACTERS AND METACHARACTERS"	.SH "CHARACTERS AND METACHARACTERS"
.rs	.rs
.sp	.sp
Line 246 one of the following escape sequences than the binary	Line 305 one of the following escape sequences than the binary
\ex{hhh..} character with hex code hhh.. (non-JavaScript mode)	\ex{hhh..} character with hex code hhh.. (non-JavaScript mode)
\euhhhh character with hex code hhhh (JavaScript mode only)	\euhhhh character with hex code hhhh (JavaScript mode only)
.sp	.sp
The precise effect of \ecx is as follows: if x is a lower case letter, it	The precise effect of \ecx on ASCII characters is as follows: if x is a lower
is converted to upper case. Then bit 6 of the character (hex 40) is inverted.	case letter, it is converted to upper case. Then bit 6 of the character (hex
Thus \ecz becomes hex 1A (z is 7A), but \ec{ becomes hex 3B ({ is 7B), while	40) is inverted. Thus \ecA to \ecZ become hex 01 to hex 1A (A is 41, Z is 5A),
\ec; becomes hex 7B (; is 3B). If the byte following \ec has a value greater	but \ec{ becomes hex 3B ({ is 7B), and \ec; becomes hex 7B (; is 3B). If the
than 127, a compile-time error occurs. This locks out non-ASCII characters in	data item (byte or 16-bit value) following \ec has a value greater than 127, a
all modes. (When PCRE is compiled in EBCDIC mode, all byte values are valid. A	compile-time error occurs. This locks out non-ASCII characters in all modes.
lower case letter is converted to upper case, and then the 0xc0 bits are
flipped.)
.P	.P
	The \ec facility was designed for use with ASCII characters, but with the
	extension to Unicode it is even less useful than it once was. It is, however,
	recognized when PCRE is compiled in EBCDIC mode, where data items are always
	bytes. In this mode, all values are valid after \ec. If the next character is a
	lower case letter, it is converted to upper case. Then the 0xc0 bits of the
	byte are inverted. Thus \ecA becomes hex 01, as in ASCII (A is C1), but because
	the EBCDIC letters are disjoint, \ecZ becomes hex 29 (Z is E9), and other
	characters also generate different values.
	.P
By default, after \ex, from zero to two hexadecimal digits are read (letters	By default, after \ex, from zero to two hexadecimal digits are read (letters
can be in upper or lower case). Any number of hexadecimal digits may appear	can be in upper or lower case). Any number of hexadecimal digits may appear
between \ex{ and }, but the character code is constrained as follows:	between \ex{ and }, but the character code is constrained as follows:
Line 263 between \ex{ and }, but the character code is constrai	Line 329 between \ex{ and }, but the character code is constrai
8-bit UTF-8 mode less than 0x10ffff and a valid codepoint	8-bit UTF-8 mode less than 0x10ffff and a valid codepoint
16-bit non-UTF mode less than 0x10000	16-bit non-UTF mode less than 0x10000
16-bit UTF-16 mode less than 0x10ffff and a valid codepoint	16-bit UTF-16 mode less than 0x10ffff and a valid codepoint
	32-bit non-UTF mode less than 0x80000000
	32-bit UTF-32 mode less than 0x10ffff and a valid codepoint
.sp	.sp
Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-called	Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-called
"surrogate" codepoints).	"surrogate" codepoints), and 0xffef.
.P	.P
If characters other than hexadecimal digits appear between \ex{ and }, or if	If characters other than hexadecimal digits appear between \ex{ and }, or if
there is no terminating }, this form of escape is not recognized. Instead, the	there is no terminating }, this form of escape is not recognized. Instead, the
Line 313 subsequent digits stand for themselves. The value of t	Line 381 subsequent digits stand for themselves. The value of t
constrained in the same way as characters specified in hexadecimal.	constrained in the same way as characters specified in hexadecimal.
For example:	For example:
.sp	.sp
\e040 is another way of writing a space	\e040 is another way of writing an ASCII space
.\" JOIN	.\" JOIN
\e40 is the same, provided there are fewer than 40	\e40 is the same, provided there are fewer than 40
previous capturing subpatterns	previous capturing subpatterns
Line 471 release 5.10. In contrast to the other sequences, whic	Line 539 release 5.10. In contrast to the other sequences, whic
characters by default, these always match certain high-valued codepoints,	characters by default, these always match certain high-valued codepoints,
whether or not PCRE_UCP is set. The horizontal space characters are:	whether or not PCRE_UCP is set. The horizontal space characters are:
.sp	.sp
U+0009 Horizontal tab	U+0009 Horizontal tab (HT)
U+0020 Space	U+0020 Space
U+00A0 Non-break space	U+00A0 Non-break space
U+1680 Ogham space mark	U+1680 Ogham space mark
Line 493 whether or not PCRE_UCP is set. The horizontal space c	Line 561 whether or not PCRE_UCP is set. The horizontal space c
.sp	.sp
The vertical space characters are:	The vertical space characters are:
.sp	.sp
U+000A Linefeed	U+000A Linefeed (LF)
U+000B Vertical tab	U+000B Vertical tab (VT)
U+000C Form feed	U+000C Form feed (FF)
U+000D Carriage return	U+000D Carriage return (CR)
U+0085 Next line	U+0085 Next line (NEL)
U+2028 Line separator	U+2028 Line separator
U+2029 Paragraph separator	U+2029 Paragraph separator
.sp	.sp
Line 551 change of newline convention; for example, a pattern c	Line 619 change of newline convention; for example, a pattern c
.sp	.sp
(ANY)(BSR_ANYCRLF)	(ANY)(BSR_ANYCRLF)
.sp	.sp
They can also be combined with the (UTF8), (UTF16), or (*UCP) special	They can also be combined with the (UTF8), (UTF16), (UTF32), (UTF) or
sequences. Inside a character class, \eR is treated as an unrecognized escape	(*UCP) special sequences. Inside a character class, \eR is treated as an
sequence, and so matches the letter "R" by default, but causes an error if	unrecognized escape sequence, and so matches the letter "R" by default, but
PCRE_EXTRA is set.	causes an error if PCRE_EXTRA is set.
.	.
.	.
.\" HTML <a name="uniextseq"></a>	.\" HTML <a name="uniextseq"></a>
Line 569 The extra escape sequences are:	Line 637 The extra escape sequences are:
.sp	.sp
\ep{\fIxx\fP} a character with the \fIxx\fP property	\ep{\fIxx\fP} a character with the \fIxx\fP property
\eP{\fIxx\fP} a character without the \fIxx\fP property	\eP{\fIxx\fP} a character without the \fIxx\fP property
\eX an extended Unicode sequence	\eX a Unicode extended grapheme cluster
.sp	.sp
The property names represented by \fIxx\fP above are limited to the Unicode	The property names represented by \fIxx\fP above are limited to the Unicode
script names, the general category properties, "Any", which matches any	script names, the general category properties, "Any", which matches any
Line 762 a modifier or "other".	Line 830 a modifier or "other".
The Cs (Surrogate) property applies only to characters in the range U+D800 to	The Cs (Surrogate) property applies only to characters in the range U+D800 to
U+DFFF. Such characters are not valid in Unicode strings and so	U+DFFF. Such characters are not valid in Unicode strings and so
cannot be tested by PCRE, unless UTF validity checking has been turned off	cannot be tested by PCRE, unless UTF validity checking has been turned off
(see the discussion of PCRE_NO_UTF8_CHECK and PCRE_NO_UTF16_CHECK in the	(see the discussion of PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK and
	PCRE_NO_UTF32_CHECK in the
.\" HREF	.\" HREF
\fBpcreapi\fP	\fBpcreapi\fP
.\"	.\"
Line 777 Instead, this property is assumed for any code point t	Line 846 Instead, this property is assumed for any code point t
Unicode table.	Unicode table.
.P	.P
Specifying caseless matching does not affect these escape sequences. For	Specifying caseless matching does not affect these escape sequences. For
example, \ep{Lu} always matches only upper case letters.	example, \ep{Lu} always matches only upper case letters. This is different from
	the behaviour of current versions of Perl.
.P	.P
The \eX escape matches any number of Unicode characters that form an extended	Matching characters by Unicode property is not fast, because PCRE has to do a
Unicode sequence. \eX is equivalent to	multistage table lookup in order to find a character's property. That is why
	the traditional escape sequences such as \ed and \ew do not use Unicode
	properties in PCRE by default, though you can make them do so by setting the
	PCRE_UCP option or by starting the pattern with (*UCP).
	.
	.
	.SS Extended grapheme clusters
	.rs
.sp	.sp
(?>\ePM\epM*)	The \eX escape matches any number of Unicode characters that form an "extended
.sp	grapheme cluster", and treats the sequence as an atomic group
That is, it matches a character without the "mark" property, followed by zero
or more characters with the "mark" property, and treats the sequence as an
atomic group
.\" HTML <a href="#atomicgroup">	.\" HTML <a href="#atomicgroup">
.\" </a>	.\" </a>
(see below).	(see below).
.\"	.\"
Characters with the "mark" property are typically accents that affect the	Up to and including release 8.31, PCRE matched an earlier, simpler definition
preceding character. None of them have codepoints less than 256, so in	that was equivalent to
8-bit non-UTF-8 mode \eX matches any one character.	.sp
	(?>\ePM\epM*)
	.sp
	That is, it matched a character without the "mark" property, followed by zero
	or more characters with the "mark" property. Characters with the "mark"
	property are typically non-spacing accents that affect the preceding character.
.P	.P
Note that recent versions of Perl have changed \eX to match what Unicode calls	This simple definition was extended in Unicode to include more complicated
an "extended grapheme cluster", which has a more complicated definition.	kinds of composite character by giving each character a grapheme breaking
	property, and creating rules that use these properties to define the boundaries
	of extended grapheme clusters. In releases of PCRE later than 8.31, \eX matches
	one of these clusters.
.P	.P
Matching characters by Unicode property is not fast, because PCRE has to search	\eX always matches at least one character. Then it decides whether to add
a structure that contains data for over fifteen thousand characters. That is	additional characters according to the following rules for ending a cluster:
why the traditional escape sequences such as \ed and \ew do not use Unicode	.P
properties in PCRE by default, though you can make them do so by setting the	1. End at the end of the subject string.
PCRE_UCP option or by starting the pattern with (*UCP).	.P
	2. Do not end between CR and LF; otherwise end after any control character.
	.P
	3. Do not break Hangul (a Korean script) syllable sequences. Hangul characters
	are of five types: L, V, T, LV, and LVT. An L character may be followed by an
	L, V, LV, or LVT character; an LV or V character may be followed by a V or T
	character; an LVT or T character may be follwed only by a T character.
	.P
	4. Do not end before extending characters or spacing marks. Characters with
	the "mark" property always have the "extend" grapheme breaking property.
	.P
	5. Do not end after prepend characters.
	.P
	6. Otherwise, end the cluster.
.	.
.	.
.\" HTML <a name="extraprops"></a>	.\" HTML <a name="extraprops"></a>
.SS PCRE's additional properties	.SS PCRE's additional properties
.rs	.rs
.sp	.sp
As well as the standard Unicode properties described in the previous	As well as the standard Unicode properties described above, PCRE supports four
section, PCRE supports four more that make it possible to convert traditional	more that make it possible to convert traditional escape sequences such as \ew
escape sequences such as \ew and \es and POSIX character classes to use Unicode	and \es and POSIX character classes to use Unicode properties. PCRE uses these
properties. PCRE uses these non-standard, non-Perl properties internally when	non-standard, non-Perl properties internally when PCRE_UCP is set. However,
PCRE_UCP is set. They are:	they may also be used explicitly. These properties are:
.sp	.sp
Xan Any alphanumeric character	Xan Any alphanumeric character
Xps Any POSIX space character	Xps Any POSIX space character
Line 825 property. Xps matches the characters tab, linefeed, ve	Line 920 property. Xps matches the characters tab, linefeed, ve
carriage return, and any other character that has the Z (separator) property.	carriage return, and any other character that has the Z (separator) property.
Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the	Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
same characters as Xan, plus underscore.	same characters as Xan, plus underscore.
	.P
	There is another non-standard property, Xuc, which matches any character that
	can be represented by a Universal Character Name in C++ and other programming
	languages. These are the characters $, @, ` (grave accent), and all characters
	with Unicode code points greater than or equal to U+00A0, except for the
	surrogates U+D800 to U+DFFF. Note that most base (ASCII) characters are
	excluded. (Universal Character Names are of the form \euHHHH or \eUHHHHHHHH
	where H is a hexadecimal digit. Note that the Xuc property does not match these
	sequences but the characters that they represent.)
.	.
.	.
.\" HTML <a name="resetmatchstart"></a>	.\" HTML <a name="resetmatchstart"></a>
Line 930 regular expression.	Line 1034 regular expression.
.SH "CIRCUMFLEX AND DOLLAR"	.SH "CIRCUMFLEX AND DOLLAR"
.rs	.rs
.sp	.sp
	The circumflex and dollar metacharacters are zero-width assertions. That is,
	they test for a particular condition being true without consuming any
	characters from the subject string.
	.P
Outside a character class, in the default matching mode, the circumflex	Outside a character class, in the default matching mode, the circumflex
character is an assertion that is true only if the current matching point is	character is an assertion that is true only if the current matching point is at
at the start of the subject string. If the \fIstartoffset\fP argument of	the start of the subject string. If the \fIstartoffset\fP argument of
\fBpcre_exec()\fP is non-zero, circumflex can never match if the PCRE_MULTILINE	\fBpcre_exec()\fP is non-zero, circumflex can never match if the PCRE_MULTILINE
option is unset. Inside a character class, circumflex has an entirely different	option is unset. Inside a character class, circumflex has an entirely different
meaning	meaning
Line 949 constrained to match only at the start of the subject,	Line 1057 constrained to match only at the start of the subject,
"anchored" pattern. (There are also other constructs that can cause a pattern	"anchored" pattern. (There are also other constructs that can cause a pattern
to be anchored.)	to be anchored.)
.P	.P
A dollar character is an assertion that is true only if the current matching	The dollar character is an assertion that is true only if the current matching
point is at the end of the subject string, or immediately before a newline	point is at the end of the subject string, or immediately before a newline at
at the end of the string (by default). Dollar need not be the last character of	the end of the string (by default). Note, however, that it does not actually
the pattern if a number of alternatives are involved, but it should be the last	match the newline. Dollar need not be the last character of the pattern if a
item in any branch in which it appears. Dollar has no special meaning in a	number of alternatives are involved, but it should be the last item in any
character class.	branch in which it appears. Dollar has no special meaning in a character class.
.P	.P
The meaning of dollar can be changed so that it matches only at the very end of	The meaning of dollar can be changed so that it matches only at the very end of
the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This	the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This
Line 1015 name; PCRE does not support this.	Line 1123 name; PCRE does not support this.
.sp	.sp
Outside a character class, the escape sequence \eC matches any one data unit,	Outside a character class, the escape sequence \eC matches any one data unit,
whether or not a UTF mode is set. In the 8-bit library, one data unit is one	whether or not a UTF mode is set. In the 8-bit library, one data unit is one
byte; in the 16-bit library it is a 16-bit unit. Unlike a dot, \eC always	byte; in the 16-bit library it is a 16-bit unit; in the 32-bit library it is
	a 32-bit unit. Unlike a dot, \eC always
matches line-ending characters. The feature is provided in Perl in order to	matches line-ending characters. The feature is provided in Perl in order to
match individual bytes in UTF-8 mode, but it is unclear how it can usefully be	match individual bytes in UTF-8 mode, but it is unclear how it can usefully be
used. Because \eC breaks up characters into individual data units, matching one	used. Because \eC breaks up characters into individual data units, matching one
unit with \eC in a UTF mode means that the rest of the string may start with a	unit with \eC in a UTF mode means that the rest of the string may start with a
malformed UTF character. This has undefined results, because PCRE assumes that	malformed UTF character. This has undefined results, because PCRE assumes that
it is dealing with valid UTF strings (and by default it checks this at the	it is dealing with valid UTF strings (and by default it checks this at the
start of processing unless the PCRE_NO_UTF8_CHECK or PCRE_NO_UTF16_CHECK option	start of processing unless the PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK or
is used).	PCRE_NO_UTF32_CHECK option is used).
.P	.P
PCRE does not allow \eC to appear in lookbehind assertions	PCRE does not allow \eC to appear in lookbehind assertions
.\" HTML <a href="#lookbehind">	.\" HTML <a href="#lookbehind">
Line 1082 circumflex is not an assertion; it still consumes a ch	Line 1191 circumflex is not an assertion; it still consumes a ch
string, and therefore it fails if the current pointer is at the end of the	string, and therefore it fails if the current pointer is at the end of the
string.	string.
.P	.P
In UTF-8 (UTF-16) mode, characters with values greater than 255 (0xffff) can be	In UTF-8 (UTF-16, UTF-32) mode, characters with values greater than 255 (0xffff)
included in a class as a literal string of data units, or by using the \ex{	can be included in a class as a literal string of data units, or by using the
escaping mechanism.	\ex{ escaping mechanism.
.P	.P
When caseless matching is set, any letters in a class represent both their	When caseless matching is set, any letters in a class represent both their
upper case and lower case versions, so for example, a caseless [aeiou] matches	upper case and lower case versions, so for example, a caseless [aeiou] matches
Line 1297 the section entitled	Line 1406 the section entitled
.\" </a>	.\" </a>
"Newline sequences"	"Newline sequences"
.\"	.\"
above. There are also the (UTF8), (UTF16), and (*UCP) leading sequences that	above. There are also the (UTF8), (UTF16),(UTF32), and (UCP) leading
can be used to set UTF and Unicode property modes; they are equivalent to	sequences that can be used to set UTF and Unicode property modes; they are
setting the PCRE_UTF8, PCRE_UTF16, and the PCRE_UCP options, respectively.	equivalent to setting the PCRE_UTF8, PCRE_UTF16, PCRE_UTF32 and the PCRE_UCP
	options, respectively. The (*UTF) sequence is a generic version that can be
	used with any of the libraries. However, the application can set the
	PCRE_NEVER_UTF option, which locks out the use of the (*UTF) sequences.
.	.
.	.
.\" HTML <a name="subpattern"></a>	.\" HTML <a name="subpattern"></a>
Line 1534 quantifier, but a literal string of four characters.	Line 1646 quantifier, but a literal string of four characters.
In UTF modes, quantifiers apply to characters rather than to individual data	In UTF modes, quantifiers apply to characters rather than to individual data
units. Thus, for example, \ex{100}{2} matches two characters, each of	units. Thus, for example, \ex{100}{2} matches two characters, each of
which is represented by a two-byte sequence in a UTF-8 string. Similarly,	which is represented by a two-byte sequence in a UTF-8 string. Similarly,
\eX{3} matches three Unicode extended sequences, each of which may be several	\eX{3} matches three Unicode extended grapheme clusters, each of which may be
data units long (and they may be of different lengths).	several data units long (and they may be of different lengths).
.P	.P
The quantifier {0} is permitted, causing the expression to behave as if the	The quantifier {0} is permitted, causing the expression to behave as if the
previous item and the quantifier were not present. This may be useful for	previous item and the quantifier were not present. This may be useful for
Line 1621 In cases where it is known that the subject string con	Line 1733 In cases where it is known that the subject string con
worth setting PCRE_DOTALL in order to obtain this optimization, or	worth setting PCRE_DOTALL in order to obtain this optimization, or
alternatively using ^ to indicate anchoring explicitly.	alternatively using ^ to indicate anchoring explicitly.
.P	.P
However, there is one situation where the optimization cannot be used. When .*	However, there are some cases where the optimization cannot be used. When .*
is inside capturing parentheses that are the subject of a back reference	is inside capturing parentheses that are the subject of a back reference
elsewhere in the pattern, a match at the start may fail where a later one	elsewhere in the pattern, a match at the start may fail where a later one
succeeds. Consider, for example:	succeeds. Consider, for example:
Line 1631 succeeds. Consider, for example:	Line 1743 succeeds. Consider, for example:
If the subject is "xyz123abc123" the match point is the fourth character. For	If the subject is "xyz123abc123" the match point is the fourth character. For
this reason, such a pattern is not implicitly anchored.	this reason, such a pattern is not implicitly anchored.
.P	.P
	Another case where implicit anchoring is not applied is when the leading .* is
	inside an atomic group. Once again, a match at the start may fail where a later
	one succeeds. Consider this pattern:
	.sp
	(?>.*?a)b
	.sp
	It matches "ab" in the subject "aab". The use of the backtracking control verbs
	(PRUNE) and (SKIP) also disable this optimization.
	.P
When a capturing subpattern is repeated, the value captured is the substring	When a capturing subpattern is repeated, the value captured is the substring
that matched the final iteration. For example, after	that matched the final iteration. For example, after
.sp	.sp
Line 1899 except that it does not cause the current matching pos	Line 2020 except that it does not cause the current matching pos
Assertion subpatterns are not capturing subpatterns. If such an assertion	Assertion subpatterns are not capturing subpatterns. If such an assertion
contains capturing subpatterns within it, these are counted for the purposes of	contains capturing subpatterns within it, these are counted for the purposes of
numbering the capturing subpatterns in the whole pattern. However, substring	numbering the capturing subpatterns in the whole pattern. However, substring
capturing is carried out only for positive assertions, because it does not make	capturing is carried out only for positive assertions. (Perl sometimes, but not
sense for negative assertions.	always, does do capturing in negative assertions.)
.P	.P
For compatibility with Perl, assertion subpatterns may be repeated; though	For compatibility with Perl, assertion subpatterns may be repeated; though
it makes no sense to assert the same thing several times, the side effect of	it makes no sense to assert the same thing several times, the side effect of
Line 2552 same pair of parentheses when there is a repetition.	Line 2673 same pair of parentheses when there is a repetition.
PCRE provides a similar feature, but of course it cannot obey arbitrary Perl	PCRE provides a similar feature, but of course it cannot obey arbitrary Perl
code. The feature is called "callout". The caller of PCRE provides an external	code. The feature is called "callout". The caller of PCRE provides an external
function by putting its entry point in the global variable \fIpcre_callout\fP	function by putting its entry point in the global variable \fIpcre_callout\fP
(8-bit library) or \fIpcre16_callout\fP (16-bit library). By default, this	(8-bit library) or \fIpcre[16\|32]_callout\fP (16-bit or 32-bit library).
variable contains NULL, which disables all calling out.	By default, this variable contains NULL, which disables all calling out.
.P	.P
Within a regular expression, (?C) indicates the points at which the external	Within a regular expression, (?C) indicates the points at which the external
function is to be called. If you want to identify different callout points, you	function is to be called. If you want to identify different callout points, you
Line 2564 For example, this pattern has two callout points:	Line 2685 For example, this pattern has two callout points:
.sp	.sp
If the PCRE_AUTO_CALLOUT flag is passed to a compiling function, callouts are	If the PCRE_AUTO_CALLOUT flag is passed to a compiling function, callouts are
automatically installed before each item in the pattern. They are all numbered	automatically installed before each item in the pattern. They are all numbered
255.	255. If there is a conditional group in the pattern whose condition is an
	assertion, an additional callout is inserted just before the condition. An
	explicit callout may also be set at this position, as in this example:
	.sp
	(?(?C9)(?=a)abc\|def)
	.sp
	Note that this applies only to assertion conditions, not to other types of
	condition.
.P	.P
During matching, when PCRE reaches a callout point, the external function is	During matching, when PCRE reaches a callout point, the external function is
called. It is provided with the number of the callout, the position in the	called. It is provided with the number of the callout, the position in the
Line 2583 documentation.	Line 2711 documentation.
.rs	.rs
.sp	.sp
Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which	Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
are described in the Perl documentation as "experimental and subject to change	are still described in the Perl documentation as "experimental and subject to
or removal in a future version of Perl". It goes on to say: "Their usage in	change or removal in a future version of Perl". It goes on to say: "Their usage
production code should be noted to avoid problems during upgrades." The same	in production code should be noted to avoid problems during upgrades." The same
remarks apply to the PCRE features described in this section.	remarks apply to the PCRE features described in this section.
.P	.P
	The new verbs make use of what was previously invalid syntax: an opening
	parenthesis followed by an asterisk. They are generally of the form
	(VERB) or (VERB:NAME). Some may take either form, possibly behaving
	differently depending on whether or not a name is present. A name is any
	sequence of characters that does not include a closing parenthesis. The maximum
	length of name is 255 in the 8-bit library and 65535 in the 16-bit and 32-bit
	libraries. If the name is empty, that is, if the closing parenthesis
	immediately follows the colon, the effect is as if the colon were not there.
	Any number of these verbs may occur in a pattern.
	.P
Since these verbs are specifically related to backtracking, most of them can be	Since these verbs are specifically related to backtracking, most of them can be
used only when the pattern is to be matched using one of the traditional	used only when the pattern is to be matched using one of the traditional
matching functions, which use a backtracking algorithm. With the exception of	matching functions, because these use a backtracking algorithm. With the
(*FAIL), which behaves like a failing negative assertion, they cause an error	exception of (*FAIL), which behaves like a failing negative assertion, the
if encountered by a DFA matching function.	backtracking control verbs cause an error if encountered by a DFA matching
	function.
.P	.P
If any of these verbs are used in an assertion or in a subpattern that is	The behaviour of these verbs in
called as a subroutine (whether or not recursively), their effect is confined	.\" HTML <a href="#btrepeat">
to that subpattern; it does not extend to the surrounding pattern, with one	.\" </a>
exception: the name from a (MARK), (PRUNE), or (*THEN) that is encountered in	repeated groups,
a successful positive assertion \fIis\fP passed back when a match succeeds	.\"
(compare capturing parentheses in assertions). Note that such subpatterns are	.\" HTML <a href="#btassert">
processed as anchored at the point where they are tested. Note also that Perl's	.\" </a>
treatment of subroutines and assertions is different in some cases.	assertions,
.P	.\"
The new verbs make use of what was previously invalid syntax: an opening	and in
parenthesis followed by an asterisk. They are generally of the form	.\" HTML <a href="#btsub">
(VERB) or (VERB:NAME). Some may take either form, with differing behaviour,	.\" </a>
depending on whether or not an argument is present. A name is any sequence of	subpatterns called as subroutines
characters that does not include a closing parenthesis. The maximum length of	.\"
name is 255 in the 8-bit library and 65535 in the 16-bit library. If the name	(whether or not recursively) is documented below.
is empty, that is, if the closing parenthesis immediately follows the colon,
the effect is as if the colon were not there. Any number of these verbs may
occur in a pattern.
.	.
.	.
.\" HTML <a name="nooptimize"></a>	.\" HTML <a name="nooptimize"></a>
Line 2621 occur in a pattern.	Line 2757 occur in a pattern.
PCRE contains some optimizations that are used to speed up matching by running	PCRE contains some optimizations that are used to speed up matching by running
some checks at the start of each match attempt. For example, it may know the	some checks at the start of each match attempt. For example, it may know the
minimum length of matching subject, or that a particular character must be	minimum length of matching subject, or that a particular character must be
present. When one of these optimizations suppresses the running of a match, any	present. When one of these optimizations bypasses the running of a match, any
included backtracking verbs will not, of course, be processed. You can suppress	included backtracking verbs will not, of course, be processed. You can suppress
the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option	the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the	when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the
Line 2652 followed by a name.	Line 2788 followed by a name.
This verb causes the match to end successfully, skipping the remainder of the	This verb causes the match to end successfully, skipping the remainder of the
pattern. However, when it is inside a subpattern that is called as a	pattern. However, when it is inside a subpattern that is called as a
subroutine, only that subpattern is ended successfully. Matching then continues	subroutine, only that subpattern is ended successfully. Matching then continues
at the outer level. If (*ACCEPT) is inside capturing parentheses, the data so	at the outer level. If (*ACCEPT) in triggered in a positive assertion, the
far is captured. For example:	assertion succeeds; in a negative assertion, the assertion fails.
	.P
	If (*ACCEPT) is inside capturing parentheses, the data so far is captured. For
	example:
.sp	.sp
A((?:A\|B(*ACCEPT)\|C)D)	A((?:A\|B(*ACCEPT)\|C)D)
.sp	.sp
Line 2686 starting point (see (*SKIP) below).	Line 2825 starting point (see (*SKIP) below).
A name is always required with this verb. There may be as many instances of	A name is always required with this verb. There may be as many instances of
(*MARK) as you like in a pattern, and their names do not have to be unique.	(*MARK) as you like in a pattern, and their names do not have to be unique.
.P	.P
When a match succeeds, the name of the last-encountered (*MARK) on the matching	When a match succeeds, the name of the last-encountered (*MARK:NAME),
path is passed back to the caller as described in the section entitled	(PRUNE:NAME), or (THEN:NAME) on the matching path is passed back to the
	caller as described in the section entitled
.\" HTML <a href="pcreapi.html#extradata">	.\" HTML <a href="pcreapi.html#extradata">
.\" </a>	.\" </a>
"Extra data for \fBpcre_exec()\fP"	"Extra data for \fBpcre_exec()\fP"
Line 2712 indicates which of the two alternatives matched. This	Line 2852 indicates which of the two alternatives matched. This
of obtaining this information than putting each alternative in its own	of obtaining this information than putting each alternative in its own
capturing parentheses.	capturing parentheses.
.P	.P
If (*MARK) is encountered in a positive assertion, its name is recorded and	If a verb with a name is encountered in a positive assertion that is true, the
passed back if it is the last-encountered. This does not happen for negative	name is recorded and passed back if it is the last-encountered. This does not
assertions.	happen for negative assertions or failing positive assertions.
.P	.P
After a partial match or a failed match, the name of the last encountered	After a partial match or a failed match, the last encountered name in the
(*MARK) in the entire match process is returned. For example:	entire match process is returned. For example:
.sp	.sp
re> /X(MARK:A)Y\|X(MARK:B)Z/K	re> /X(MARK:A)Y\|X(MARK:B)Z/K
data> XP	data> XP
Line 2743 to ensure that the match is always attempted.	Line 2883 to ensure that the match is always attempted.
The following verbs do nothing when they are encountered. Matching continues	The following verbs do nothing when they are encountered. Matching continues
with what follows, but if there is no subsequent match, causing a backtrack to	with what follows, but if there is no subsequent match, causing a backtrack to
the verb, a failure is forced. That is, backtracking cannot pass to the left of	the verb, a failure is forced. That is, backtracking cannot pass to the left of
the verb. However, when one of these verbs appears inside an atomic group, its	the verb. However, when one of these verbs appears inside an atomic group or an
effect is confined to that group, because once the group has been matched,	assertion that is true, its effect is confined to that group, because once the
there is never any backtracking into it. In this situation, backtracking can	group has been matched, there is never any backtracking into it. In this
"jump back" to the left of the entire atomic group. (Remember also, as stated	situation, backtracking can "jump back" to the left of the entire atomic group
above, that this localization also applies in subroutine calls and assertions.)	or assertion. (Remember also, as stated above, that this localization also
	applies in subroutine calls.)
.P	.P
These verbs differ in exactly what kind of failure occurs when backtracking	These verbs differ in exactly what kind of failure occurs when backtracking
reaches them.	reaches them. The behaviour described below is what happens when the verb is
	not in a subroutine or an assertion. Subsequent sections cover these special
	cases.
.sp	.sp
(*COMMIT)	(*COMMIT)
.sp	.sp
This verb, which may not be followed by a name, causes the whole match to fail	This verb, which may not be followed by a name, causes the whole match to fail
outright if the rest of the pattern does not match. Even if the pattern is	outright if there is a later matching failure that causes backtracking to reach
unanchored, no further attempts to find a match by advancing the starting point	it. Even if the pattern is unanchored, no further attempts to find a match by
take place. Once (*COMMIT) has been passed, \fBpcre_exec()\fP is committed to	advancing the starting point take place. If (*COMMIT) is the only backtracking
finding a match at the current starting point, or not at all. For example:	verb that is encountered, once it has been passed \fBpcre_exec()\fP is
	committed to finding a match at the current starting point, or not at all. For
	example:
.sp	.sp
a+(*COMMIT)b	a+(*COMMIT)b
.sp	.sp
Line 2767 dynamic anchor, or "I've started, so I must finish." T	Line 2912 dynamic anchor, or "I've started, so I must finish." T
recently passed (MARK) in the path is passed back when (COMMIT) forces a	recently passed (MARK) in the path is passed back when (COMMIT) forces a
match failure.	match failure.
.P	.P
	If there is more than one backtracking verb in a pattern, a different one that
	follows (COMMIT) may be triggered first, so merely passing (COMMIT) during a
	match does not always guarantee that a match must be at this starting point.
	.P
Note that (*COMMIT) at the start of a pattern is not the same as an anchor,	Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
unless PCRE's start-of-match optimizations are turned off, as shown in this	unless PCRE's start-of-match optimizations are turned off, as shown in this
\fBpcretest\fP example:	\fBpcretest\fP example:
Line 2786 starting points.	Line 2935 starting points.
(PRUNE) or (PRUNE:NAME)	(PRUNE) or (PRUNE:NAME)
.sp	.sp
This verb causes the match to fail at the current starting position in the	This verb causes the match to fail at the current starting position in the
subject if the rest of the pattern does not match. If the pattern is	subject if there is a later matching failure that causes backtracking to reach
unanchored, the normal "bumpalong" advance to the next starting character then	it. If the pattern is unanchored, the normal "bumpalong" advance to the next
happens. Backtracking can occur as usual to the left of (*PRUNE), before it is	starting character then happens. Backtracking can occur as usual to the left of
reached, or when matching to the right of (*PRUNE), but if there is no match to	(PRUNE), before it is reached, or when matching to the right of (PRUNE), but
the right, backtracking cannot cross (*PRUNE). In simple cases, the use of	if there is no match to the right, backtracking cannot cross (*PRUNE). In
(*PRUNE) is just an alternative to an atomic group or possessive quantifier,	simple cases, the use of (*PRUNE) is just an alternative to an atomic group or
but there are some uses of (*PRUNE) that cannot be expressed in any other way.	possessive quantifier, but there are some uses of (*PRUNE) that cannot be
The behaviour of (PRUNE:NAME) is the same as (MARK:NAME)(*PRUNE). In an	expressed in any other way. In an anchored pattern (*PRUNE) has the same effect
anchored pattern (PRUNE) has the same effect as (COMMIT).	as (*COMMIT).
	.P
	The behaviour of (PRUNE:NAME) is the not the same as (MARK:NAME)(*PRUNE).
	It is like (*MARK:NAME) in that the name is remembered for passing back to the
	caller. However, (SKIP:NAME) searches only for names set with (MARK).
.sp	.sp
(*SKIP)	(*SKIP)
.sp	.sp
Line 2815 instead of skipping on to "c".	Line 2968 instead of skipping on to "c".
.sp	.sp
(*SKIP:NAME)	(*SKIP:NAME)
.sp	.sp
When (*SKIP) has an associated name, its behaviour is modified. If the	When (*SKIP) has an associated name, its behaviour is modified. When it is
following pattern fails to match, the previous path through the pattern is	triggered, the previous path through the pattern is searched for the most
searched for the most recent (*MARK) that has the same name. If one is found,	recent (*MARK) that has the same name. If one is found, the "bumpalong" advance
the "bumpalong" advance is to the subject position that corresponds to that	is to the subject position that corresponds to that (*MARK) instead of to where
(MARK) instead of to where (SKIP) was encountered. If no (*MARK) with a	(SKIP) was encountered. If no (MARK) with a matching name is found, the
matching name is found, the (*SKIP) is ignored.	(*SKIP) is ignored.
	.P
	Note that (SKIP:NAME) searches only for names set by (MARK:NAME). It ignores
	names that are set by (PRUNE:NAME) or (THEN:NAME).
.sp	.sp
(THEN) or (THEN:NAME)	(THEN) or (THEN:NAME)
.sp	.sp
This verb causes a skip to the next innermost alternative if the rest of the	This verb causes a skip to the next innermost alternative when backtracking
pattern does not match. That is, it cancels pending backtracking, but only	reaches it. That is, it cancels any further backtracking within the current
within the current alternative. Its name comes from the observation that it can	alternative. Its name comes from the observation that it can be used for a
be used for a pattern-based if-then-else block:	pattern-based if-then-else block:
.sp	.sp
( COND1 (THEN) FOO \| COND2 (THEN) BAR \| COND3 (*THEN) BAZ ) ...	( COND1 (THEN) FOO \| COND2 (THEN) BAR \| COND3 (*THEN) BAZ ) ...
.sp	.sp
If the COND1 pattern matches, FOO is tried (and possibly further items after	If the COND1 pattern matches, FOO is tried (and possibly further items after
the end of the group if FOO succeeds); on failure, the matcher skips to the	the end of the group if FOO succeeds); on failure, the matcher skips to the
second alternative and tries COND2, without backtracking into COND1. The	second alternative and tries COND2, without backtracking into COND1. If that
behaviour of (THEN:NAME) is exactly the same as (MARK:NAME)(*THEN).	succeeds and BAR fails, COND3 is tried. If subsequently BAZ fails, there are no
If (THEN) is not inside an alternation, it acts like (PRUNE).	more alternatives, so there is a backtrack to whatever came before the entire
	group. If (THEN) is not inside an alternation, it acts like (PRUNE).
.P	.P
Note that a subpattern that does not contain a \| character is just a part of	The behaviour of (THEN:NAME) is the not the same as (MARK:NAME)(*THEN).
the enclosing alternative; it is not a nested alternation with only one	It is like (*MARK:NAME) in that the name is remembered for passing back to the
	caller. However, (SKIP:NAME) searches only for names set with (MARK).
	.P
	A subpattern that does not contain a \| character is just a part of the
	enclosing alternative; it is not a nested alternation with only one
alternative. The effect of (*THEN) extends beyond such a subpattern to the	alternative. The effect of (*THEN) extends beyond such a subpattern to the
enclosing alternative. Consider this pattern, where A, B, etc. are complex	enclosing alternative. Consider this pattern, where A, B, etc. are complex
pattern fragments that do not contain any \| characters at this level:	pattern fragments that do not contain any \| characters at this level:
Line 2857 in C, matching moves to (*FAIL), which causes the whol	Line 3018 in C, matching moves to (*FAIL), which causes the whol
because there are no more alternatives to try. In this case, matching does now	because there are no more alternatives to try. In this case, matching does now
backtrack into A.	backtrack into A.
.P	.P
Note also that a conditional subpattern is not considered as having two	Note that a conditional subpattern is not considered as having two
alternatives, because only one is ever used. In other words, the \| character in	alternatives, because only one is ever used. In other words, the \| character in
a conditional subpattern has a different meaning. Ignoring white space,	a conditional subpattern has a different meaning. Ignoring white space,
consider:	consider:
Line 2879 starting position, but allowing an advance to the next	Line 3040 starting position, but allowing an advance to the next
unanchored pattern). (*SKIP) is similar, except that the advance may be more	unanchored pattern). (*SKIP) is similar, except that the advance may be more
than one character. (*COMMIT) is the strongest, causing the entire match to	than one character. (*COMMIT) is the strongest, causing the entire match to
fail.	fail.
.P	.
If more than one such verb is present in a pattern, the "strongest" one wins.	.
For example, consider this pattern, where A, B, etc. are complex pattern	.SS "More than one backtracking verb"
fragments:	.rs
.sp	.sp
(A(COMMIT)B(THEN)C\|D)	If more than one backtracking verb is present in a pattern, the one that is
	backtracked onto first acts. For example, consider this pattern, where A, B,
	etc. are complex pattern fragments:
.sp	.sp
Once A has matched, PCRE is committed to this match, at the current starting	(A(COMMIT)B(THEN)C\|ABD)
position. If subsequently B matches, but C does not, the normal (*THEN) action	.sp
of trying the next alternative (that is, D) does not happen because (*COMMIT)	If A matches but B fails, the backtrack to (*COMMIT) causes the entire match to
overrides.	fail. However, if A and B match, but C fails, the backtrack to (*THEN) causes
	the next alternative (ABD) to be tried. This behaviour is consistent, but is
	not always the same as Perl's. It means that if two or more backtracking verbs
	appear in succession, all the the last of them has no effect. Consider this
	example:
	.sp
	...(COMMIT)(PRUNE)...
	.sp
	If there is a matching failure to the right, backtracking onto (*PRUNE) cases
	it to be triggered, and its action is taken. There can never be a backtrack
	onto (*COMMIT).
.	.
.	.
	.\" HTML <a name="btrepeat"></a>
	.SS "Backtracking verbs in repeated groups"
	.rs
	.sp
	PCRE differs from Perl in its handling of backtracking verbs in repeated
	groups. For example, consider:
	.sp
	/(a(*COMMIT)b)+ac/
	.sp
	If the subject is "abac", Perl matches, but PCRE fails because the (*COMMIT) in
	the second repeat of the group acts.
	.
	.
	.\" HTML <a name="btassert"></a>
	.SS "Backtracking verbs in assertions"
	.rs
	.sp
	(*FAIL) in an assertion has its normal effect: it forces an immediate backtrack.
	.P
	(*ACCEPT) in a positive assertion causes the assertion to succeed without any
	further processing. In a negative assertion, (*ACCEPT) causes the assertion to
	fail without any further processing.
	.P
	The other backtracking verbs are not treated specially if they appear in a
	positive assertion. In particular, (*THEN) skips to the next alternative in the
	innermost enclosing group that has alternations, whether or not this is within
	the assertion.
	.P
	Negative assertions are, however, different, in order to ensure that changing a
	positive assertion into a negative assertion changes its result. Backtracking
	into (COMMIT), (SKIP), or (*PRUNE) causes a negative assertion to be true,
	without considering any further alternative branches in the assertion.
	Backtracking into (*THEN) causes it to skip to the next enclosing alternative
	within the assertion (the normal behaviour), but if the assertion does not have
	such an alternative, (THEN) behaves like (PRUNE).
	.
	.
	.\" HTML <a name="btsub"></a>
	.SS "Backtracking verbs in subroutines"
	.rs
	.sp
	These behaviours occur whether or not the subpattern is called recursively.
	Perl's treatment of subroutines is different in some cases.
	.P
	(*FAIL) in a subpattern called as a subroutine has its normal effect: it forces
	an immediate backtrack.
	.P
	(*ACCEPT) in a subpattern called as a subroutine causes the subroutine match to
	succeed without any further processing. Matching then continues after the
	subroutine call.
	.P
	(COMMIT), (SKIP), and (*PRUNE) in a subpattern called as a subroutine cause
	the subroutine match to fail.
	.P
	(*THEN) skips to the next alternative in the innermost enclosing group within
	the subpattern that has alternatives. If there is no such group within the
	subpattern, (*THEN) causes the subroutine match to fail.
	.
	.
.SH "SEE ALSO"	.SH "SEE ALSO"
.rs	.rs
.sp	.sp
\fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),	\fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
\fBpcresyntax\fP(3), \fBpcre\fP(3), \fBpcre16(3)\fP.	\fBpcresyntax\fP(3), \fBpcre\fP(3), \fBpcre16(3)\fP, \fBpcre32(3)\fP.
.	.
.	.
.SH AUTHOR	.SH AUTHOR
Line 2913 Cambridge CB2 3QH, England.	Line 3145 Cambridge CB2 3QH, England.
.rs	.rs
.sp	.sp
.nf	.nf
Last updated: 17 June 2012	Last updated: 26 April 2013
Copyright (c) 1997-2012 University of Cambridge.	Copyright (c) 1997-2013 University of Cambridge.
.fi	.fi

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