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Line 1	Line 1
.TH PCREPATTERN 3	.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 character strings. To use this,	there is now also support for UTF-8 strings in the original library, an
PCRE must be built to include UTF-8 support, and you must call	extra library that supports 16-bit and UTF-16 character strings, and a
\fBpcre_compile()\fP or \fBpcre_compile2()\fP with the PCRE_UTF8 option. There	third library that supports 32-bit and UTF-32 character strings. To use these
is also a special sequence that can be given at the start of a pattern:	features, PCRE must be built to include appropriate support. When using UTF
	strings you must either call the compiling function with the PCRE_UTF8,
	PCRE_UTF16, or PCRE_UTF32 option, or the pattern must start with one of
	these special sequences:
.sp	.sp
(*UTF8)	(*UTF8)
	(*UTF16)
	(*UTF32)
	(*UTF)
.sp	.sp
Starting a pattern with this sequence is equivalent to setting the PCRE_UTF8	(*UTF) is a generic sequence that can be used with any of the libraries.
option. This feature is not Perl-compatible. How setting UTF-8 mode affects	Starting a pattern with such a sequence is equivalent to setting the relevant
pattern matching is mentioned in several places below. There is also a summary	option. How setting a UTF mode affects pattern matching is mentioned in several
of UTF-8 features in the	places below. There is also a summary 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) 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 its main matching function, \fBpcre_exec()\fP, is used.
From release 6.0, PCRE offers a second matching function,
\fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not
Perl-compatible. Some of the features discussed below are not available when
\fBpcre_dfa_exec()\fP is used. The advantages and disadvantages of the
alternative function, and how it differs from the normal function, 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 94 string with one of the following five sequences:	Line 125 string with one of the following five sequences:
(*ANYCRLF) any of the three above	(*ANYCRLF) any of the three above
(*ANY) all Unicode newline sequences	(*ANY) all Unicode newline sequences
.sp	.sp
These override the default and the options given to \fBpcre_compile()\fP or	These override the default and the options given to the compiling function. For
\fBpcre_compile2()\fP. For example, on a Unix system where LF is the default	example, on a Unix system where LF is the default newline sequence, the pattern
newline sequence, the pattern
.sp	.sp
(*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 119 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 130 corresponding characters in the subject. As a trivial	Line 191 corresponding characters in the subject. As a trivial
.sp	.sp
matches a portion of a subject string that is identical to itself. When	matches a portion of a subject string that is identical to itself. When
caseless matching is specified (the PCRE_CASELESS option), letters are matched	caseless matching is specified (the PCRE_CASELESS option), letters are matched
independently of case. In UTF-8 mode, PCRE always understands the concept of	independently of case. In a UTF mode, PCRE always understands the concept of
case for characters whose values are less than 128, so caseless matching is	case for characters whose values are less than 128, so caseless matching is
always possible. For characters with higher values, the concept of case is	always possible. For characters with higher values, the concept of case is
supported if PCRE is compiled with Unicode property support, but not otherwise.	supported if PCRE is compiled with Unicode property support, but not otherwise.
If you want to use caseless matching for characters 128 and above, you must	If you want to use caseless matching for characters 128 and above, you must
ensure that PCRE is compiled with Unicode property support as well as with	ensure that PCRE is compiled with Unicode property support as well as with
UTF-8 support.	UTF support.
.P	.P
The power of regular expressions comes from the ability to include alternatives	The power of regular expressions comes from the ability to include alternatives
and repetitions in the pattern. These are encoded in the pattern by the use of	and repetitions in the pattern. These are encoded in the pattern by the use of
Line 192 otherwise be interpreted as a metacharacter, so it is	Line 253 otherwise be interpreted as a metacharacter, so it is
non-alphanumeric with backslash to specify that it stands for itself. In	non-alphanumeric with backslash to specify that it stands for itself. In
particular, if you want to match a backslash, you write \e\e.	particular, if you want to match a backslash, you write \e\e.
.P	.P
In UTF-8 mode, only ASCII numbers and letters have any special meaning after a	In a UTF mode, only ASCII numbers and letters have any special meaning after a
backslash. All other characters (in particular, those whose codepoints are	backslash. All other characters (in particular, those whose codepoints are
greater than 127) are treated as literals.	greater than 127) are treated as literals.
.P	.P
If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the	If a pattern is compiled with the PCRE_EXTENDED option, white space in the
pattern (other than in a character class) and characters between a # outside	pattern (other than in a character class) and characters between a # outside
a character class and the next newline are ignored. An escaping backslash can	a character class and the next newline are ignored. An escaping backslash can
be used to include a whitespace or # character as part of the pattern.	be used to include a white space or # character as part of the pattern.
.P	.P
If you want to remove the special meaning from a sequence of characters, you	If you want to remove the special meaning from a sequence of characters, you
can do so by putting them between \eQ and \eE. This is different from Perl in	can do so by putting them between \eQ and \eE. This is different from Perl in
Line 235 one of the following escape sequences than the binary	Line 296 one of the following escape sequences than the binary
\ea alarm, that is, the BEL character (hex 07)	\ea alarm, that is, the BEL character (hex 07)
\ecx "control-x", where x is any ASCII character	\ecx "control-x", where x is any ASCII character
\ee escape (hex 1B)	\ee escape (hex 1B)
\ef formfeed (hex 0C)	\ef form feed (hex 0C)
\en linefeed (hex 0A)	\en linefeed (hex 0A)
\er carriage return (hex 0D)	\er carriage return (hex 0D)
\et tab (hex 09)	\et tab (hex 09)
Line 244 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
both byte mode and UTF-8 mode. (When PCRE is compiled in EBCDIC mode, all byte	compile-time error occurs. This locks out non-ASCII characters in all modes.
values are valid. A 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 value of the character code must be less than 256	between \ex{ and }, but the character code is constrained as follows:
in non-UTF-8 mode, and less than 2**31 in UTF-8 mode. That is, the maximum	.sp
value in hexadecimal is 7FFFFFFF. Note that this is bigger than the largest	8-bit non-UTF mode less than 0x100
Unicode code point, which is 10FFFF.	8-bit UTF-8 mode less than 0x10ffff and a valid codepoint
	16-bit non-UTF mode less than 0x10000
	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
	Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-called
	"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 270 as just described only when it is followed by two hexa	Line 345 as just described only when it is followed by two hexa
Otherwise, it matches a literal "x" character. In JavaScript mode, support for	Otherwise, it matches a literal "x" character. In JavaScript mode, support for
code points greater than 256 is provided by \eu, which must be followed by	code points greater than 256 is provided by \eu, which must be followed by
four hexadecimal digits; otherwise it matches a literal "u" character.	four hexadecimal digits; otherwise it matches a literal "u" character.
	Character codes specified by \eu in JavaScript mode are constrained in the same
	was as those specified by \ex in non-JavaScript mode.
.P	.P
Characters whose value is less than 256 can be defined by either of the two	Characters whose value is less than 256 can be defined by either of the two
syntaxes for \ex (or by \eu in JavaScript mode). There is no difference in the	syntaxes for \ex (or by \eu in JavaScript mode). There is no difference in the
Line 300 parenthesized subpatterns.	Line 377 parenthesized subpatterns.
Inside a character class, or if the decimal number is greater than 9 and there	Inside a character class, or if the decimal number is greater than 9 and there
have not been that many capturing subpatterns, PCRE re-reads up to three octal	have not been that many capturing subpatterns, PCRE re-reads up to three octal
digits following the backslash, and uses them to generate a data character. Any	digits following the backslash, and uses them to generate a data character. Any
subsequent digits stand for themselves. In non-UTF-8 mode, the value of a	subsequent digits stand for themselves. The value of the character is
character specified in octal must be less than \e400. In UTF-8 mode, values up	constrained in the same way as characters specified in hexadecimal.
to \e777 are permitted. 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 319 to \e777 are permitted. For example:	Line 396 to \e777 are permitted. For example:
character with octal code 113	character with octal code 113
.\" JOIN	.\" JOIN
\e377 might be a back reference, otherwise	\e377 might be a back reference, otherwise
the byte consisting entirely of 1 bits	the value 255 (decimal)
.\" JOIN	.\" JOIN
\e81 is either a back reference, or a binary zero	\e81 is either a back reference, or a binary zero
followed by the two characters "8" and "1"	followed by the two characters "8" and "1"
Line 392 Another use of backslash is for specifying generic cha	Line 469 Another use of backslash is for specifying generic cha
.sp	.sp
\ed any decimal digit	\ed any decimal digit
\eD any character that is not a decimal digit	\eD any character that is not a decimal digit
\eh any horizontal whitespace character	\eh any horizontal white space character
\eH any character that is not a horizontal whitespace character	\eH any character that is not a horizontal white space character
\es any whitespace character	\es any white space character
\eS any character that is not a whitespace character	\eS any character that is not a white space character
\ev any vertical whitespace character	\ev any vertical white space character
\eV any character that is not a vertical whitespace character	\eV any character that is not a vertical white space character
\ew any "word" character	\ew any "word" character
\eW any "non-word" character	\eW any "non-word" character
.sp	.sp
Line 440 or "french" in Windows, some character codes greater t	Line 517 or "french" in Windows, some character codes greater t
accented letters, and these are then matched by \ew. The use of locales with	accented letters, and these are then matched by \ew. The use of locales with
Unicode is discouraged.	Unicode is discouraged.
.P	.P
By default, in UTF-8 mode, characters with values greater than 128 never match	By default, in a UTF mode, characters with values greater than 128 never match
\ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain	\ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain
their original meanings from before UTF-8 support was available, mainly for	their original meanings from before UTF support was available, mainly for
efficiency reasons. However, if PCRE is compiled with Unicode property support,	efficiency reasons. However, if PCRE is compiled with Unicode property support,
and the PCRE_UCP option is set, the behaviour is changed so that Unicode	and the PCRE_UCP option is set, the behaviour is changed so that Unicode
properties are used to determine character types, as follows:	properties are used to determine character types, as follows:
Line 459 is noticeably slower when PCRE_UCP is set.	Line 536 is noticeably slower when PCRE_UCP is set.
.P	.P
The sequences \eh, \eH, \ev, and \eV are features that were added to Perl at	The sequences \eh, \eH, \ev, and \eV are features that were added to Perl at
release 5.10. In contrast to the other sequences, which match only ASCII	release 5.10. In contrast to the other sequences, which match only ASCII
characters by default, these always match certain high-valued codepoints in	characters by default, these always match certain high-valued codepoints,
UTF-8 mode, whether or not PCRE_UCP is set. The horizontal space characters	whether or not PCRE_UCP is set. The horizontal space characters are:
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 485 are:	Line 561 are:
.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 Formfeed	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
	In 8-bit, non-UTF-8 mode, only the characters with codepoints less than 256 are
	relevant.
.	.
.	.
.\" HTML <a name="newlineseq"></a>	.\" HTML <a name="newlineseq"></a>
Line 499 The vertical space characters are:	Line 578 The vertical space characters are:
.rs	.rs
.sp	.sp
Outside a character class, by default, the escape sequence \eR matches any	Outside a character class, by default, the escape sequence \eR matches any
Unicode newline sequence. In non-UTF-8 mode \eR is equivalent to the following:	Unicode newline sequence. In 8-bit non-UTF-8 mode \eR is equivalent to the
	following:
.sp	.sp
(?>\er\en\|\en\|\ex0b\|\ef\|\er\|\ex85)	(?>\er\en\|\en\|\ex0b\|\ef\|\er\|\ex85)
.sp	.sp
Line 510 below.	Line 590 below.
.\"	.\"
This particular group matches either the two-character sequence CR followed by	This particular group matches either the two-character sequence CR followed by
LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,	LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next	U+000B), FF (form feed, U+000C), CR (carriage return, U+000D), or NEL (next
line, U+0085). The two-character sequence is treated as a single unit that	line, U+0085). The two-character sequence is treated as a single unit that
cannot be split.	cannot be split.
.P	.P
In UTF-8 mode, two additional characters whose codepoints are greater than 255	In other modes, two additional characters whose codepoints are greater than 255
are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).	are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
Unicode character property support is not needed for these characters to be	Unicode character property support is not needed for these characters to be
recognized.	recognized.
Line 530 one of the following sequences:	Line 610 one of the following sequences:
(*BSR_ANYCRLF) CR, LF, or CRLF only	(*BSR_ANYCRLF) CR, LF, or CRLF only
(*BSR_UNICODE) any Unicode newline sequence	(*BSR_UNICODE) any Unicode newline sequence
.sp	.sp
These override the default and the options given to \fBpcre_compile()\fP or	These override the default and the options given to the compiling function, but
\fBpcre_compile2()\fP, but they can be overridden by options given to	they can themselves be overridden by options given to a matching function. Note
\fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. Note that these special settings,	that these special settings, which are not Perl-compatible, are recognized only
which are not Perl-compatible, are recognized only at the very start of a	at the very start of a pattern, and that they must be in upper case. If more
pattern, and that they must be in upper case. If more than one of them is	than one of them is present, the last one is used. They can be combined with a
present, the last one is used. They can be combined with a change of newline	change of newline convention; for example, a pattern can start with:
convention; for example, a pattern can start with:
.sp	.sp
(ANY)(BSR_ANYCRLF)	(ANY)(BSR_ANYCRLF)
.sp	.sp
They can also be combined with the (UTF8) or (UCP) special sequences. Inside	They can also be combined with the (UTF8), (UTF16), (UTF32), (UTF) or
a character class, \eR is treated as an unrecognized escape sequence, and so	(*UCP) special sequences. Inside a character class, \eR is treated as an
matches the letter "R" by default, but causes an error if PCRE_EXTRA is set.	unrecognized escape sequence, and so matches the letter "R" by default, but
	causes an error if PCRE_EXTRA is set.
.	.
.	.
.\" HTML <a name="uniextseq"></a>	.\" HTML <a name="uniextseq"></a>
Line 551 matches the letter "R" by default, but causes an error	Line 631 matches the letter "R" by default, but causes an error
.sp	.sp
When PCRE is built with Unicode character property support, three additional	When PCRE is built with Unicode character property support, three additional
escape sequences that match characters with specific properties are available.	escape sequences that match characters with specific properties are available.
When not in UTF-8 mode, these sequences are of course limited to testing	When in 8-bit non-UTF-8 mode, these sequences are of course limited to testing
characters whose codepoints are less than 256, but they do work in this mode.	characters whose codepoints are less than 256, but they do work in this mode.
The extra escape sequences are:	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 586 Armenian,	Line 666 Armenian,
Avestan,	Avestan,
Balinese,	Balinese,
Bamum,	Bamum,
	Batak,
Bengali,	Bengali,
Bopomofo,	Bopomofo,
	Brahmi,
Braille,	Braille,
Buginese,	Buginese,
Buhid,	Buhid,
Canadian_Aboriginal,	Canadian_Aboriginal,
Carian,	Carian,
	Chakma,
Cham,	Cham,
Cherokee,	Cherokee,
Common,	Common,
Line 635 Lisu,	Line 718 Lisu,
Lycian,	Lycian,
Lydian,	Lydian,
Malayalam,	Malayalam,
	Mandaic,
Meetei_Mayek,	Meetei_Mayek,
	Meroitic_Cursive,
	Meroitic_Hieroglyphs,
	Miao,
Mongolian,	Mongolian,
Myanmar,	Myanmar,
New_Tai_Lue,	New_Tai_Lue,
Line 654 Rejang,	Line 741 Rejang,
Runic,	Runic,
Samaritan,	Samaritan,
Saurashtra,	Saurashtra,
	Sharada,
Shavian,	Shavian,
Sinhala,	Sinhala,
	Sora_Sompeng,
Sundanese,	Sundanese,
Syloti_Nagri,	Syloti_Nagri,
Syriac,	Syriac,
Line 664 Tagbanwa,	Line 753 Tagbanwa,
Tai_Le,	Tai_Le,
Tai_Tham,	Tai_Tham,
Tai_Viet,	Tai_Viet,
	Takri,
Tamil,	Tamil,
Telugu,	Telugu,
Thaana,	Thaana,
Line 738 the Lu, Ll, or Lt property, in other words, a letter t	Line 828 the Lu, Ll, or Lt property, in other words, a letter t
a modifier or "other".	a modifier or "other".
.P	.P
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 UTF-8 strings (see RFC 3629) and so	U+DFFF. Such characters are not valid in Unicode strings and so
cannot be tested by PCRE, unless UTF-8 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 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 755 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
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 for \fBpcre_compile()\fP or by starting the pattern with	.P
(*UCP).	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 800 PCRE_UCP is set. They are:	Line 916 PCRE_UCP is set. They are:
Xwd Any Perl "word" character	Xwd Any Perl "word" character
.sp	.sp
Xan matches characters that have either the L (letter) or the N (number)	Xan matches characters that have either the L (letter) or the N (number)
property. Xps matches the characters tab, linefeed, vertical tab, formfeed, or	property. Xps matches the characters tab, linefeed, vertical tab, form feed, or
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 870 escape sequence" error is generated instead.	Line 995 escape sequence" error is generated instead.
A word boundary is a position in the subject string where the current character	A word boundary is a position in the subject string where the current character
and the previous character do not both match \ew or \eW (i.e. one matches	and the previous character do not both match \ew or \eW (i.e. one matches
\ew and the other matches \eW), or the start or end of the string if the	\ew and the other matches \eW), or the start or end of the string if the
first or last character matches \ew, respectively. In UTF-8 mode, the meanings	first or last character matches \ew, respectively. In a UTF mode, the meanings
of \ew and \eW can be changed by setting the PCRE_UCP option. When this is	of \ew and \eW can be changed by setting the PCRE_UCP option. When this is
done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start	done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start
of word" or "end of word" metasequence. However, whatever follows \eb normally	of word" or "end of word" metasequence. However, whatever follows \eb normally
Line 909 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 928 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 965 end of the subject in both modes, and if all branches	Line 1094 end of the subject in both modes, and if all branches
.sp	.sp
Outside a character class, a dot in the pattern matches any one character in	Outside a character class, a dot in the pattern matches any one character in
the subject string except (by default) a character that signifies the end of a	the subject string except (by default) a character that signifies the end of a
line. In UTF-8 mode, the matched character may be more than one byte long.	line.
.P	.P
When a line ending is defined as a single character, dot never matches that	When a line ending is defined as a single character, dot never matches that
character; when the two-character sequence CRLF is used, dot does not match CR	character; when the two-character sequence CRLF is used, dot does not match CR
Line 989 that signifies the end of a line. Perl also uses \eN t	Line 1118 that signifies the end of a line. Perl also uses \eN t
name; PCRE does not support this.	name; PCRE does not support this.
.	.
.	.
.SH "MATCHING A SINGLE BYTE"	.SH "MATCHING A SINGLE DATA UNIT"
.rs	.rs
.sp	.sp
Outside a character class, the escape sequence \eC matches any one byte, both	Outside a character class, the escape sequence \eC matches any one data unit,
in and out of UTF-8 mode. Unlike a dot, it always matches line-ending	whether or not a UTF mode is set. In the 8-bit library, one data unit is one
characters. The feature is provided in Perl in order to match individual bytes	byte; in the 16-bit library it is a 16-bit unit; in the 32-bit library it is
in UTF-8 mode, but it is unclear how it can usefully be used. Because \eC	a 32-bit unit. Unlike a dot, \eC always
breaks up characters into individual bytes, matching one byte with \eC in UTF-8	matches line-ending characters. The feature is provided in Perl in order to
mode means that the rest of the string may start with a malformed UTF-8	match individual bytes in UTF-8 mode, but it is unclear how it can usefully be
character. This has undefined results, because PCRE assumes that it is dealing	used. Because \eC breaks up characters into individual data units, matching one
with valid UTF-8 strings (and by default it checks this at the start of	unit with \eC in a UTF mode means that the rest of the string may start with a
processing unless the PCRE_NO_UTF8_CHECK option is used).	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
	start of processing unless the PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK or
	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">
.\" </a>	.\" </a>
(described below)	(described below)
.\"	.\"
in UTF-8 mode, because this would make it impossible to calculate the length of	in a UTF mode, because this would make it impossible to calculate the length of
the lookbehind.	the lookbehind.
.P	.P
In general, the \eC escape sequence is best avoided in UTF-8 mode. However, one	In general, the \eC escape sequence is best avoided. However, one
way of using it that avoids the problem of malformed UTF-8 characters is to	way of using it that avoids the problem of malformed UTF characters is to use a
use a lookahead to check the length of the next character, as in this pattern	lookahead to check the length of the next character, as in this pattern, which
(ignore white space and line breaks):	could be used with a UTF-8 string (ignore white space and line breaks):
.sp	.sp
(?\| (?=[\ex00-\ex7f])(\eC) \|	(?\| (?=[\ex00-\ex7f])(\eC) \|
(?=[\ex80-\ex{7ff}])(\eC)(\eC) \|	(?=[\ex80-\ex{7ff}])(\eC)(\eC) \|
Line 1043 bracket causes a compile-time error. If a closing squa	Line 1175 bracket causes a compile-time error. If a closing squa
a member of the class, it should be the first data character in the class	a member of the class, it should be the first data character in the class
(after an initial circumflex, if present) or escaped with a backslash.	(after an initial circumflex, if present) or escaped with a backslash.
.P	.P
A character class matches a single character in the subject. In UTF-8 mode, the	A character class matches a single character in the subject. In a UTF mode, the
character may be more than one byte long. A matched character must be in the	character may be more than one data unit long. A matched character must be in
set of characters defined by the class, unless the first character in the class	the set of characters defined by the class, unless the first character in the
definition is a circumflex, in which case the subject character must not be in	class definition is a circumflex, in which case the subject character must not
the set defined by the class. If a circumflex is actually required as a member	be in the set defined by the class. If a circumflex is actually required as a
of the class, ensure it is not the first character, or escape it with a	member of the class, ensure it is not the first character, or escape it with a
backslash.	backslash.
.P	.P
For example, the character class [aeiou] matches any lower case vowel, while	For example, the character class [aeiou] matches any lower case vowel, while
Line 1059 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 mode, characters with values greater than 255 can be included in a	In UTF-8 (UTF-16, UTF-32) mode, characters with values greater than 255 (0xffff)
class as a literal string of bytes, or by using the \ex{ escaping mechanism.	can be included in a class as a literal string of data units, or by using the
	\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
"A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a	"A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
caseful version would. In UTF-8 mode, PCRE always understands the concept of	caseful version would. In a UTF mode, PCRE always understands the concept of
case for characters whose values are less than 128, so caseless matching is	case for characters whose values are less than 128, so caseless matching is
always possible. For characters with higher values, the concept of case is	always possible. For characters with higher values, the concept of case is
supported if PCRE is compiled with Unicode property support, but not otherwise.	supported if PCRE is compiled with Unicode property support, but not otherwise.
If you want to use caseless matching in UTF8-mode for characters 128 and above,	If you want to use caseless matching in a UTF mode for characters 128 and
you must ensure that PCRE is compiled with Unicode property support as well as	above, you must ensure that PCRE is compiled with Unicode property support as
with UTF-8 support.	well as with UTF support.
.P	.P
Characters that might indicate line breaks are never treated in any special way	Characters that might indicate line breaks are never treated in any special way
when matching character classes, whatever line-ending sequence is in use, and	when matching character classes, whatever line-ending sequence is in use, and
Line 1093 followed by two other characters. The octal or hexadec	Line 1226 followed by two other characters. The octal or hexadec
"]" can also be used to end a range.	"]" can also be used to end a range.
.P	.P
Ranges operate in the collating sequence of character values. They can also be	Ranges operate in the collating sequence of character values. They can also be
used for characters specified numerically, for example [\e000-\e037]. In UTF-8	used for characters specified numerically, for example [\e000-\e037]. Ranges
mode, ranges can include characters whose values are greater than 255, for	can include any characters that are valid for the current mode.
example [\ex{100}-\ex{2ff}].
.P	.P
If a range that includes letters is used when caseless matching is set, it	If a range that includes letters is used when caseless matching is set, it
matches the letters in either case. For example, [W-c] is equivalent to	matches the letters in either case. For example, [W-c] is equivalent to
[][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character	[][\e\e^_`wxyzabc], matched caselessly, and in a non-UTF mode, if character
tables for a French locale are in use, [\exc8-\excb] matches accented E	tables for a French locale are in use, [\exc8-\excb] matches accented E
characters in both cases. In UTF-8 mode, PCRE supports the concept of case for	characters in both cases. In UTF modes, PCRE supports the concept of case for
characters with values greater than 128 only when it is compiled with Unicode	characters with values greater than 128 only when it is compiled with Unicode
property support.	property support.
.P	.P
The character escape sequences \ed, \eD, \eh, \eH, \ep, \eP, \es, \eS, \ev,	The character escape sequences \ed, \eD, \eh, \eH, \ep, \eP, \es, \eS, \ev,
\eV, \ew, and \eW may appear in a character class, and add the characters that	\eV, \ew, and \eW may appear in a character class, and add the characters that
they match to the class. For example, [\edABCDEF] matches any hexadecimal	they match to the class. For example, [\edABCDEF] matches any hexadecimal
digit. In UTF-8 mode, the PCRE_UCP option affects the meanings of \ed, \es, \ew	digit. In UTF modes, the PCRE_UCP option affects the meanings of \ed, \es, \ew
and their upper case partners, just as it does when they appear outside a	and their upper case partners, just as it does when they appear outside a
character class, as described in the section entitled	character class, as described in the section entitled
.\" HTML <a href="#genericchartypes">	.\" HTML <a href="#genericchartypes">
Line 1178 matches "1", "2", or any non-digit. PCRE (and Perl) al	Line 1310 matches "1", "2", or any non-digit. PCRE (and Perl) al
syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not	syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
supported, and an error is given if they are encountered.	supported, and an error is given if they are encountered.
.P	.P
By default, in UTF-8 mode, characters with values greater than 128 do not match	By default, in UTF modes, characters with values greater than 128 do not match
any of the POSIX character classes. However, if the PCRE_UCP option is passed	any of the POSIX character classes. However, if the PCRE_UCP option is passed
to \fBpcre_compile()\fP, some of the classes are changed so that Unicode	to \fBpcre_compile()\fP, some of the classes are changed so that Unicode
character properties are used. This is achieved by replacing the POSIX classes	character properties are used. This is achieved by replacing the POSIX classes
Line 1266 option settings happen at compile time. There would be	Line 1398 option settings happen at compile time. There would be
behaviour otherwise.	behaviour otherwise.
.P	.P
\fBNote:\fP There are other PCRE-specific options that can be set by the	\fBNote:\fP There are other PCRE-specific options that can be set by the
application when the compile or match functions are called. In some cases the	application when the compiling or matching functions are called. In some cases
pattern can contain special leading sequences such as (*CRLF) to override what	the pattern can contain special leading sequences such as (*CRLF) to override
the application has set or what has been defaulted. Details are given in the	what the application has set or what has been defaulted. Details are given in
section entitled	the section entitled
.\" HTML <a href="#newlineseq">	.\" HTML <a href="#newlineseq">
.\" </a>	.\" </a>
"Newline sequences"	"Newline sequences"
.\"	.\"
above. There are also the (UTF8) and (UCP) leading sequences that can be used	above. There are also the (UTF8), (UTF16),(UTF32), and (UCP) leading
to set UTF-8 and Unicode property modes; they are equivalent to setting the	sequences that can be used to set UTF and Unicode property modes; they are
PCRE_UTF8 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 1295 match "cataract", "erpillar" or an empty string.	Line 1430 match "cataract", "erpillar" or an empty string.
.sp	.sp
2. It sets up the subpattern as a capturing subpattern. This means that, when	2. It sets up the subpattern as a capturing subpattern. This means that, when
the whole pattern matches, that portion of the subject string that matched the	the whole pattern matches, that portion of the subject string that matched the
subpattern is passed back to the caller via the \fIovector\fP argument of	subpattern is passed back to the caller via the \fIovector\fP argument of the
\fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting	matching function. (This applies only to the traditional matching functions;
from 1) to obtain numbers for the capturing subpatterns. For example, if the	the DFA matching functions do not support capturing.)
string "the red king" is matched against the pattern	.P
	Opening parentheses are counted from left to right (starting from 1) to obtain
	numbers for the capturing subpatterns. For example, if the string "the red
	king" is matched against the pattern
.sp	.sp
the ((red\|white) (king\|queen))	the ((red\|white) (king\|queen))
.sp	.sp
Line 1474 items:	Line 1612 items:
a literal data character	a literal data character
the dot metacharacter	the dot metacharacter
the \eC escape sequence	the \eC escape sequence
the \eX escape sequence (in UTF-8 mode with Unicode properties)	the \eX escape sequence
the \eR escape sequence	the \eR escape sequence
an escape such as \ed or \epL that matches a single character	an escape such as \ed or \epL that matches a single character
a character class	a character class
Line 1505 where a quantifier is not allowed, or one that does no	Line 1643 where a quantifier is not allowed, or one that does no
quantifier, is taken as a literal character. For example, {,6} is not a	quantifier, is taken as a literal character. For example, {,6} is not a
quantifier, but a literal string of four characters.	quantifier, but a literal string of four characters.
.P	.P
In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual	In UTF modes, quantifiers apply to characters rather than to individual data
bytes. Thus, for example, \ex{100}{2} matches two UTF-8 characters, each of	units. Thus, for example, \ex{100}{2} matches two characters, each of
which is represented by a two-byte sequence. Similarly, when Unicode property	which is represented by a two-byte sequence in a UTF-8 string. Similarly,
support is available, \eX{3} matches three Unicode extended sequences, each of	\eX{3} matches three Unicode extended grapheme clusters, each of which may be
which may be several bytes 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 1595 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 1605 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 1819 Because there may be many capturing parentheses in a p	Line 1966 Because there may be many capturing parentheses in a p
following a backslash are taken as part of a potential back reference number.	following a backslash are taken as part of a potential back reference number.
If the pattern continues with a digit character, some delimiter must be used to	If the pattern continues with a digit character, some delimiter must be used to
terminate the back reference. If the PCRE_EXTENDED option is set, this can be	terminate the back reference. If the PCRE_EXTENDED option is set, this can be
whitespace. Otherwise, the \eg{ syntax or an empty comment (see	white space. Otherwise, the \eg{ syntax or an empty comment (see
.\" HTML <a href="#comments">	.\" HTML <a href="#comments">
.\" </a>	.\" </a>
"Comments"	"Comments"
Line 1873 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 1972 temporarily move the current position back by the fixe	Line 2119 temporarily move the current position back by the fixe
match. If there are insufficient characters before the current position, the	match. If there are insufficient characters before the current position, the
assertion fails.	assertion fails.
.P	.P
In UTF-8 mode, PCRE does not allow the \eC escape (which matches a single byte,	In a UTF mode, PCRE does not allow the \eC escape (which matches a single data
even in UTF-8 mode) to appear in lookbehind assertions, because it makes it	unit even in a UTF mode) to appear in lookbehind assertions, because it makes
impossible to calculate the length of the lookbehind. The \eX and \eR escapes,	it impossible to calculate the length of the lookbehind. The \eX and \eR
which can match different numbers of bytes, are also not permitted.	escapes, which can match different numbers of data units, are also not
	permitted.
.P	.P
.\" HTML <a href="#subpatternsassubroutines">	.\" HTML <a href="#subpatternsassubroutines">
.\" </a>	.\" </a>
Line 2175 subroutines that can be referenced from elsewhere. (Th	Line 2323 subroutines that can be referenced from elsewhere. (Th
subroutines	subroutines
.\"	.\"
is described below.) For example, a pattern to match an IPv4 address such as	is described below.) For example, a pattern to match an IPv4 address such as
"192.168.23.245" could be written like this (ignore whitespace and line	"192.168.23.245" could be written like this (ignore white space and line
breaks):	breaks):
.sp	.sp
(?(DEFINE) (?<byte> 2[0-4]\ed \| 25[0-5] \| 1\ed\ed \| [1-9]?\ed) )	(?(DEFINE) (?<byte> 2[0-4]\ed \| 25[0-5] \| 1\ed\ed \| [1-9]?\ed) )
Line 2222 closing parenthesis. Nested parentheses are not permit	Line 2370 closing parenthesis. Nested parentheses are not permit
option is set, an unescaped # character also introduces a comment, which in	option is set, an unescaped # character also introduces a comment, which in
this case continues to immediately after the next newline character or	this case continues to immediately after the next newline character or
character sequence in the pattern. Which characters are interpreted as newlines	character sequence in the pattern. Which characters are interpreted as newlines
is controlled by the options passed to \fBpcre_compile()\fP or by a special	is controlled by the options passed to a compiling function or by a special
sequence at the start of the pattern, as described in the section entitled	sequence at the start of the pattern, as described in the section entitled
.\" HTML <a href="#newlines">	.\" HTML <a href="#newlines">
.\" </a>	.\" </a>
Line 2524 same pair of parentheses when there is a repetition.	Line 2672 same pair of parentheses when there is a repetition.
.P	.P
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 \fIpcre[16\|32]_callout\fP (16-bit or 32-bit library).
By default, this 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
Line 2534 For example, this pattern has two callout points:	Line 2683 For example, this pattern has two callout points:
.sp	.sp
(?C1)abc(?C2)def	(?C1)abc(?C2)def
.sp	.sp
If the PCRE_AUTO_CALLOUT flag is passed to \fBpcre_compile()\fP, 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 (and \fIpcre_callout\fP is	During matching, when PCRE reaches a callout point, the external function is
set), the external function is called. It is provided with the number of the	called. It is provided with the number of the callout, the position in the
callout, the position in the pattern, and, optionally, one item of data	pattern, and, optionally, one item of data originally supplied by the caller of
originally supplied by the caller of \fBpcre_exec()\fP. The callout function	the matching function. The callout function may cause matching to proceed, to
may cause matching to proceed, to backtrack, or to fail altogether. A complete	backtrack, or to fail altogether. A complete description of the interface to
description of the interface to the callout function is given in the	the callout function is given in the
.\" HREF	.\" HREF
\fBpcrecallout\fP	\fBpcrecallout\fP
.\"	.\"
Line 2555 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
Since these verbs are specifically related to backtracking, most of them can be
used only when the pattern is to be matched using \fBpcre_exec()\fP, which uses
a backtracking algorithm. With the exception of (*FAIL), which behaves like a
failing negative assertion, they cause an error if encountered by
\fBpcre_dfa_exec()\fP.
.P
If any of these verbs are used in an assertion or in a subpattern that is
called as a subroutine (whether or not recursively), their effect is confined
to that subpattern; it does not extend to the surrounding pattern, with one
exception: the name from a (MARK), (PRUNE), or (*THEN) that is encountered in
a successful positive assertion \fIis\fP passed back when a match succeeds
(compare capturing parentheses in assertions). Note that such subpatterns are
processed as anchored at the point where they are tested. Note also that Perl's
treatment of subroutines is different in some cases.
.P
The new verbs make use of what was previously invalid syntax: an opening	The new verbs make use of what was previously invalid syntax: an opening
parenthesis followed by an asterisk. They are generally of the form	parenthesis followed by an asterisk. They are generally of the form
(VERB) or (VERB:NAME). Some may take either form, with differing behaviour,	(VERB) or (VERB:NAME). Some may take either form, possibly behaving
depending on whether or not an argument is present. A name is any sequence of	differently depending on whether or not a name is present. A name is any
characters that does not include a closing parenthesis. If the name is empty,	sequence of characters that does not include a closing parenthesis. The maximum
that is, if the closing parenthesis immediately follows the colon, the effect	length of name is 255 in the 8-bit library and 65535 in the 16-bit and 32-bit
is as if the colon were not there. Any number of these verbs may occur in a	libraries. If the name is empty, that is, if the closing parenthesis
pattern.	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	.P
	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
	matching functions, because these use a backtracking algorithm. With the
	exception of (*FAIL), which behaves like a failing negative assertion, the
	backtracking control verbs cause an error if encountered by a DFA matching
	function.
	.P
	The behaviour of these verbs in
	.\" HTML <a href="#btrepeat">
	.\" </a>
	repeated groups,
	.\"
	.\" HTML <a href="#btassert">
	.\" </a>
	assertions,
	.\"
	and in
	.\" HTML <a href="#btsub">
	.\" </a>
	subpatterns called as subroutines
	.\"
	(whether or not recursively) is documented below.
	.
	.
	.\" HTML <a name="nooptimize"></a>
	.SS "Optimizations that affect backtracking verbs"
	.rs
	.sp
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
pattern with (*NO_START_OPT).	pattern with (*NO_START_OPT). There is more discussion of this option in the
	section entitled
	.\" HTML <a href="pcreapi.html#execoptions">
	.\" </a>
	"Option bits for \fBpcre_exec()\fP"
	.\"
	in the
	.\" HREF
	\fBpcreapi\fP
	.\"
	documentation.
.P	.P
Experiments with Perl suggest that it too has similar optimizations, sometimes	Experiments with Perl suggest that it too has similar optimizations, sometimes
leading to anomalous results.	leading to anomalous results.
Line 2608 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 2642 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 via the \fIpcre_extra\fP data structure, as	(PRUNE:NAME), or (THEN:NAME) on the matching path is passed back to the
described in the	caller as described in the section entitled
.\" HTML <a href="pcreapi.html#extradata">	.\" HTML <a href="pcreapi.html#extradata">
.\" </a>	.\" </a>
section on \fIpcre_extra\fP	"Extra data for \fBpcre_exec()\fP"
.\"	.\"
in the	in the
.\" HREF	.\" HREF
Line 2669 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
No match, mark = B	No match, mark = B
.sp	.sp
Note that in this unanchored example the mark is retained from the match	Note that in this unanchored example the mark is retained from the match
attempt that started at the letter "X". Subsequent match attempts starting at	attempt that started at the letter "X" in the subject. Subsequent match
"P" and then with an empty string do not get as far as the (*MARK) item, but	attempts starting at "P" and then with an empty string do not get as far as the
nevertheless do not reset it.	(*MARK) item, but nevertheless do not reset it.
	.P
	If you are interested in (*MARK) values after failed matches, you should
	probably set the PCRE_NO_START_OPTIMIZE option
	.\" HTML <a href="#nooptimize">
	.\" </a>
	(see above)
	.\"
	to ensure that the match is always attempted.
.	.
.	.
.SS "Verbs that act after backtracking"	.SS "Verbs that act after backtracking"
Line 2692 nevertheless do not reset it.	Line 2883 nevertheless do not reset it.
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 2716 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 2735 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 2764 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 2806 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 2828 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).	\fBpcresyntax\fP(3), \fBpcre\fP(3), \fBpcre16(3)\fP, \fBpcre32(3)\fP.
.	.
.	.
.SH AUTHOR	.SH AUTHOR
Line 2862 Cambridge CB2 3QH, England.	Line 3145 Cambridge CB2 3QH, England.
.rs	.rs
.sp	.sp
.nf	.nf
Last updated: 29 November 2011	Last updated: 26 April 2013
Copyright (c) 1997-2011 University of Cambridge.	Copyright (c) 1997-2013 University of Cambridge.
.fi	.fi

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