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PCREPATTERN(3)		   Library Functions Manual		PCREPATTERN(3)

NAME
       PCRE - Perl-compatible regular expressions

PCRE REGULAR EXPRESSION	DETAILS
       The  syntax and semantics of the	regular	expressions that are supported
       by PCRE are described in	detail below. There is a quick-reference  syn-
       tax summary in the pcresyntax page. PCRE	tries to match Perl syntax and
       semantics as closely as it can. PCRE  also  supports  some  alternative
       regular	expression  syntax (which does not conflict with the Perl syn-
       tax) in order to	provide	some compatibility with	regular	expressions in
       Python, .NET, and Oniguruma.

       Perl's  regular expressions are described in its	own documentation, and
       regular expressions in general are covered in a number of  books,  some
       of which	have copious examples. Jeffrey Friedl's	"Mastering Regular Ex-
       pressions", published by	O'Reilly, covers regular expressions in	 great
       detail.	This  description of PCRE's regular expressions	is intended as
       reference material.

       This document discusses the patterns that are supported	by  PCRE  when
       one    its    main   matching   functions,   pcre_exec()	  (8-bit)   or
       pcre[16|32]_exec() (16- or 32-bit), is used. PCRE also has  alternative
       matching	 functions,  pcre_dfa_exec()  and pcre[16|32_dfa_exec(), 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
       pcrematching page.

SPECIAL	START-OF-PATTERN ITEMS
       A number	of options that	can be passed to pcre_compile()	 can  also  be
       set by special items at the start of a pattern. These are not Perl-com-
       patible,	but are	provided to make these options accessible  to  pattern
       writers	who are	not able to change the program that processes the pat-
       tern. Any number	of these items may appear, but they must  all  be  to-
       gether  right  at the start of the pattern string, and the letters must
       be in upper case.

   UTF support
       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,	an 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 appropri-
       ate  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:

	 (*UTF8)
	 (*UTF16)
	 (*UTF32)
	 (*UTF)

       (*UTF)  is  a  generic  sequence	 that  can be used with	any of the li-
       braries.	 Starting a pattern with such a	sequence is equivalent to set-
       ting the	relevant option. How setting a UTF mode	affects	pattern	match-
       ing is mentioned	in several places below. There is also	a  summary  of
       features	in the pcreunicode page.

       Some applications that allow their users	to supply patterns may wish to
       restrict	 them  to  non-UTF  data  for	security   reasons.   If   the
       PCRE_NEVER_UTF  option  is set at compile time, (*UTF) etc. are not al-
       lowed, and their	appearance causes an error.

   Unicode property support
       Another special sequence	that may appear	at the start of	a  pattern  is
       (*UCP).	 This  has  the	same effect as setting the PCRE_UCP option: it
       causes sequences	such as	\d and \w to use Unicode properties to	deter-
       mine character types, instead of	recognizing only characters with codes
       less than 128 via a lookup table.

   Disabling auto-possessification
       If a pattern starts with	(*NO_AUTO_POSSESS), it has the same effect  as
       setting	the  PCRE_NO_AUTO_POSSESS  option  at compile time. This stops
       PCRE from making	quantifiers possessive when what follows cannot	 match
       the  repeated item. For example,	by default a+b is treated as a++b. For
       more details, see the pcreapi documentation.

   Disabling start-up optimizations
       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. This disables several  optimizations  for	quickly	 reaching  "no
       match" results. For more	details, see the pcreapi documentation.

   Newline conventions
       PCRE  supports five different conventions for indicating	line breaks in
       strings:	a single CR (carriage return) character, a  single  LF	(line-
       feed) character,	the two-character sequence CRLF, any of	the three pre-
       ceding, or any Unicode newline sequence.	The pcreapi page  has  further
       discussion  about newlines, and shows how to set	the newline convention
       in the options arguments	for the	compiling and matching functions.

       It is also possible to specify a	newline	convention by starting a  pat-
       tern string with	one of the following five sequences:

	 (*CR)	      carriage return
	 (*LF)	      linefeed
	 (*CRLF)      carriage return, followed	by linefeed
	 (*ANYCRLF)   any of the three above
	 (*ANY)	      all Unicode newline sequences

       These override the default and the options given	to the compiling func-
       tion. For example, on a Unix system where LF is the default newline se-
       quence, the pattern

	 (*CR)a.b

       changes the convention to CR. That pattern matches "a\nb" because LF is
       no longer a newline. If more than one of	these settings is present, the
       last one	is used.

       The  newline  convention	affects	where the circumflex and dollar	asser-
       tions are true. It also affects the interpretation of the dot metachar-
       acter when PCRE_DOTALL is not set, and the behaviour of \N. However, it
       does not	affect what the	\R escape sequence matches. By	default,  this
       is  any Unicode newline sequence, for Perl compatibility. However, this
       can be changed; see the description of \R in the	section	entitled "New-
       line  sequences"	 below.	 A change of \R	setting	can be combined	with a
       change of newline convention.

   Setting match and recursion limits
       The caller of pcre_exec() can set a limit on the	number	of  times  the
       internal	 match() function is called and	on the maximum depth of	recur-
       sive 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,	pcre_exec() gives an error return. The limits can also be  set
       by items	at the start of	the pattern of the form

	 (*LIMIT_MATCH=d)
	 (*LIMIT_RECURSION=d)

       where d is any number of	decimal	digits.	However, the value of the set-
       ting must be less than the value	set (or	defaulted) by  the  caller  of
       pcre_exec()  for	 it  to	 have  any effect. In other words, the pattern
       writer can lower	the limits set by the programmer, but not raise	 them.
       If  there  is  more  than one setting of	one of these limits, the lower
       value is	used.

EBCDIC CHARACTER CODES
       PCRE can	be compiled to run in an environment that uses EBCDIC  as  its
       character code rather than ASCII	or Unicode (typically a	mainframe sys-
       tem). In	the sections below, character code values are  ASCII  or  Uni-
       code; in	an EBCDIC environment these characters may have	different code
       values, and there are no	code points greater than 255.

CHARACTERS AND METACHARACTERS
       A regular expression is a pattern that is  matched  against  a  subject
       string  from  left  to right. Most characters stand for themselves in a
       pattern,	and match the corresponding characters in the  subject.	 As  a
       trivial example,	the pattern

	 The quick brown fox

       matches a portion of a subject string that is identical to itself. When
       caseless	matching is specified (the PCRE_CASELESS option), letters  are
       matched	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 always possible. For characters with higher val-
       ues, the	concept	of case	is 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	ensure	that  PCRE  is
       compiled	with Unicode property support as well as with UTF support.

       The  power of regular expressions comes from the	ability	to include al-
       ternatives and repetitions in the pattern. These	 are  encoded  in  the
       pattern by the use of metacharacters, which do not stand	for themselves
       but instead are interpreted in some special way.

       There are two different sets of metacharacters: those that  are	recog-
       nized  anywhere in the pattern except within square brackets, and those
       that are	recognized within square brackets.  Outside  square  brackets,
       the metacharacters are as follows:

	 \	general	escape character with several uses
	 ^	assert start of	string (or line, in multiline mode)
	 $	assert end of string (or line, in multiline mode)
	 .	match any character except newline (by default)
	 [	start character	class definition
	 |	start of alternative branch
	 (	start subpattern
	 )	end subpattern
	 ?	extends	the meaning of (
		also 0 or 1 quantifier
		also quantifier	minimizer
	 *	0 or more quantifier
	 +	1 or more quantifier
		also "possessive quantifier"
	 {	start min/max quantifier

       Part  of	 a  pattern  that is in	square brackets	is called a "character
       class". In a character class the	only metacharacters are:

	 \	general	escape character
	 ^	negate the class, but only if the first	character
	 -	indicates character range
	 [	POSIX character	class (only if followed	by POSIX
		  syntax)
	 ]	terminates the character class

       The following sections describe the use of each of the metacharacters.

BACKSLASH
       The backslash character has several uses. Firstly, if it	is followed by
       a character that	is not a number	or a letter, it	takes away any special
       meaning that character may have.	This use of  backslash	as  an	escape
       character applies both inside and outside character classes.

       For  example,  if  you want to match a *	character, you write \*	in the
       pattern.	 This escaping action applies whether  or  not	the  following
       character  would	 otherwise be interpreted as a metacharacter, so it is
       always safe to precede a	non-alphanumeric  with	backslash  to  specify
       that  it	stands for itself. In particular, if you want to match a back-
       slash, you write	\\.

       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 greater than 127)	are treated as literals.

       If a pattern is compiled	with  the  PCRE_EXTENDED  option,  most	 white
       space  in the pattern (other than in a character	class),	and characters
       between a # outside a character class and the next newline,  inclusive,
       are ignored. An escaping	backslash can be used to include a white space
       or # character as part of the pattern.

       If you want to remove the special meaning from a	 sequence  of  charac-
       ters,  you can do so by putting them between \Q and \E. This is differ-
       ent from	Perl in	that $ and @ are handled as literals  in  \Q...\E  se-
       quences in PCRE,	whereas	in Perl, $ and @ cause variable	interpolation.
       Note the	following examples:

	 Pattern	    PCRE matches   Perl	matches

	 \Qabc$xyz\E	    abc$xyz	   abc followed	by the
					     contents of $xyz
	 \Qabc\$xyz\E	    abc\$xyz	   abc\$xyz
	 \Qabc\E\$\Qxyz\E   abc$xyz	   abc$xyz

       The \Q...\E sequence is recognized both inside  and  outside  character
       classes.	  An  isolated \E that is not preceded by \Q is	ignored. If \Q
       is not followed by \E later in the pattern, the literal	interpretation
       continues  to  the  end	of  the	pattern	(that is, \E is	assumed	at the
       end). If	the isolated \Q	is inside a character class,  this  causes  an
       error, because the character class is not terminated.

   Non-printing	characters
       A second	use of backslash provides a way	of encoding non-printing char-
       acters in patterns in a visible manner. There is	no restriction on  the
       appearance  of non-printing characters, apart from the binary zero that
       terminates a pattern, but when a	pattern	 is  being  prepared  by  text
       editing,	 it  is	 often	easier	to use one of the following escape se-
       quences than the	binary character it represents.	 In an ASCII  or  Uni-
       code environment, these escapes are as follows:

	 \a	   alarm, that is, the BEL character (hex 07)
	 \cx	   "control-x",	where x	is any ASCII character
	 \e	   escape (hex 1B)
	 \f	   form	feed (hex 0C)
	 \n	   linefeed (hex 0A)
	 \r	   carriage return (hex	0D)
	 \t	   tab (hex 09)
	 \0dd	   character with octal	code 0dd
	 \ddd	   character with octal	code ddd, or back reference
	 \o{ddd..} character with octal	code ddd..
	 \xhh	   character with hex code hh
	 \x{hhh..} character with hex code hhh.. (non-JavaScript mode)
	 \uhhhh	   character with hex code hhhh	(JavaScript mode only)

       The  precise effect of \cx on ASCII characters is as follows: if	x is a
       lower case letter, it is	converted to upper case. Then  bit  6  of  the
       character (hex 40) is inverted. Thus \cA	to \cZ become hex 01 to	hex 1A
       (A is 41, Z is 5A), but \c{ becomes hex 3B ({ is	7B), and  \c;  becomes
       hex  7B (; is 3B). If the data item (byte or 16-bit value) following \c
       has a value greater than	127, a compile-time error occurs.  This	 locks
       out non-ASCII characters	in all modes.

       When PCRE is compiled in	EBCDIC mode, \a, \e, \f, \n, \r, and \t	gener-
       ate the appropriate EBCDIC code values. The \c escape is	 processed  as
       specified for Perl in the perlebcdic document. The only characters that
       are allowed after \c are	A-Z, a-z, or one of @, [, \, ],	^,  _,	or  ?.
       Any other character provokes a compile-time error. The sequence \c@ en-
       codes character code 0; after \c	the letters (in	 either	 case)	encode
       characters 1-26 (hex 01 to hex 1A); [, \, ], ^, and _ encode characters
       27-31 (hex 1B to	hex 1F), and \c? becomes either	255  (hex  FF)	or  95
       (hex 5F).

       Thus,  apart  from  \c?,	these escapes generate the same	character code
       values as they do in an ASCII environment, though the meanings  of  the
       values  mostly  differ. For example, \cG	always generates code value 7,
       which is	BEL in ASCII but DEL in	EBCDIC.

       The sequence \c?	generates DEL (127, hex	7F) in an  ASCII  environment,
       but  because  127  is  not a control character in EBCDIC, Perl makes it
       generate	the APC	character. Unfortunately, there	are  several  variants
       of  EBCDIC.  In	most  of them the APC character	has the	value 255 (hex
       FF), but	in the one Perl	calls POSIX-BC its value is 95	(hex  5F).  If
       certain	other characters have POSIX-BC values, PCRE makes \c? generate
       95; otherwise it	generates 255.

       After \0	up to two further octal	digits are read. If  there  are	 fewer
       than  two  digits,  just	 those that are	present	are used. Thus the se-
       quence \0\x\015 specifies two binary zeros followed by a	 CR  character
       (code value 13).	Make sure you supply two digits	after the initial zero
       if the pattern character	that follows is	itself an octal	digit.

       The escape \o must be followed by a sequence of octal digits,  enclosed
       in  braces.  An	error occurs if	this is	not the	case. This escape is a
       recent addition to Perl;	it provides way	of specifying  character  code
       points  as  octal  numbers  greater than	0777, and it also allows octal
       numbers and back	references to be unambiguously specified.

       For greater clarity and unambiguity, it is best to avoid	following \ by
       a digit greater than zero. Instead, use \o{} or \x{} to specify charac-
       ter numbers, and	\g{} to	specify	back references. The  following	 para-
       graphs describe the old,	ambiguous syntax.

       The handling of a backslash followed by a digit other than 0 is compli-
       cated, and Perl has changed in recent releases, causing	PCRE  also  to
       change. Outside a character class, PCRE reads the digit and any follow-
       ing digits as a decimal number. If the number is	less  than  8,	or  if
       there  have been	at least that many previous capturing left parentheses
       in the expression, the entire sequence is taken as a back reference.  A
       description  of how this	works is given later, following	the discussion
       of parenthesized	subpatterns.

       Inside a	character class, or if	the  decimal  number  following	 \  is
       greater than 7 and there	have not been that many	capturing subpatterns,
       PCRE handles \8 and \9 as the literal characters	"8" and	"9", and  oth-
       erwise re-reads up to three octal digits	following the backslash, using
       them to generate	a data character.  Any	subsequent  digits  stand  for
       themselves. For example:

	 \040	is another way of writing an ASCII space
	 \40	is the same, provided there are	fewer than 40
		   previous capturing subpatterns
	 \7	is always a back reference
	 \11	might be a back	reference, or another way of
		   writing a tab
	 \011	is always a tab
	 \0113	is a tab followed by the character "3"
	 \113	might be a back	reference, otherwise the
		   character with octal	code 113
	 \377	might be a back	reference, otherwise
		   the value 255 (decimal)
	 \81	is either a back reference, or the two
		   characters "8" and "1"

       Note  that octal	values of 100 or greater that are specified using this
       syntax must not be introduced by	a leading zero,	because	no  more  than
       three octal digits are ever read.

       By  default, after \x that is not followed by {,	from zero to two hexa-
       decimal digits are read (letters	can be in upper	or  lower  case).  Any
       number of hexadecimal digits may	appear between \x{ and }. If a charac-
       ter other than a	hexadecimal digit appears between \x{  and  },	or  if
       there is	no terminating }, an error occurs.

       If  the	PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \x
       is as just described only when it is followed by	two  hexadecimal  dig-
       its.   Otherwise,  it  matches  a  literal "x" character. In JavaScript
       mode, support for code points greater than 256 is provided by \u, which
       must  be	 followed  by  four hexadecimal	digits;	otherwise it matches a
       literal "u" character.

       Characters whose	value is less than 256 can be defined by either	of the
       two  syntaxes for \x (or	by \u in JavaScript mode). There is no differ-
       ence in the way they are	handled. For example, \xdc is exactly the same
       as \x{dc} (or \u00dc in JavaScript mode).

   Constraints on character values
       Characters  that	 are  specified	using octal or hexadecimal numbers are
       limited to certain values, as follows:

	 8-bit non-UTF mode    less than 0x100
	 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 0x100000000
	 32-bit	UTF-32 mode    less than 0x10ffff and a	valid codepoint

       Invalid Unicode codepoints are the range	 0xd800	 to  0xdfff  (the  so-
       called "surrogate" codepoints), and 0xffef.

   Escape sequences in character classes
       All the sequences that define a single character	value can be used both
       inside and outside character classes. In	addition, inside  a  character
       class, \b is interpreted	as the backspace character (hex	08).

       \N  is not allowed in a character class.	\B, \R,	and \X are not special
       inside a	character class. Like  other  unrecognized  escape  sequences,
       they  are  treated  as  the literal characters "B", "R",	and "X"	by de-
       fault, but cause	an error if the	PCRE_EXTRA option is  set.  Outside  a
       character class,	these sequences	have different meanings.

   Unsupported escape sequences
       In  Perl, the sequences \l, \L, \u, and \U are recognized by its	string
       handler and used	to modify the case of  following  characters.  By  de-
       fault,  PCRE  does  not support these escape sequences. However,	if the
       PCRE_JAVASCRIPT_COMPAT option is	set, \U	matches	a "U"  character,  and
       \u can be used to define	a character by code point, as described	in the
       previous	section.

   Absolute and	relative back references
       The sequence \g followed	by an unsigned or a negative  number,  option-
       ally  enclosed  in braces, is an	absolute or relative back reference. A
       named back reference can	be coded as \g{name}. Back references are dis-
       cussed later, following the discussion of parenthesized subpatterns.

   Absolute and	relative subroutine calls
       For  compatibility with Oniguruma, the non-Perl syntax \g followed by a
       name or a number	enclosed either	in angle brackets or single quotes, is
       an  alternative	syntax for referencing a subpattern as a "subroutine".
       Details are discussed later.   Note  that  \g{...}  (Perl  syntax)  and
       \g<...>	(Oniguruma  syntax)  are  not synonymous. The former is	a back
       reference; the latter is	a subroutine call.

   Generic character types
       Another use of backslash	is for specifying generic character types:

	 \d	any decimal digit
	 \D	any character that is not a decimal digit
	 \h	any horizontal white space character
	 \H	any character that is not a horizontal white space character
	 \s	any white space	character
	 \S	any character that is not a white space	character
	 \v	any vertical white space character
	 \V	any character that is not a vertical white space character
	 \w	any "word" character
	 \W	any "non-word" character

       There is	also the single	sequence \N, which matches a non-newline char-
       acter.	This  is the same as the "." metacharacter when	PCRE_DOTALL is
       not set.	Perl also uses \N to match characters by name; PCRE  does  not
       support this.

       Each  pair of lower and upper case escape sequences partitions the com-
       plete set of characters into two	disjoint  sets.	 Any  given  character
       matches	one, and only one, of each pair. The sequences can appear both
       inside and outside character classes. They each match one character  of
       the  appropriate	 type.	If the current matching	point is at the	end of
       the subject string, all of them fail, because there is no character  to
       match.

       For  compatibility with Perl, \s	did not	used to	match the VT character
       (code 11), which	made it	different from the the	POSIX  "space"	class.
       However,	 Perl  added VT	at release 5.18, and PCRE followed suit	at re-
       lease 8.34. The default \s characters are now HT	(9), LF	(10), VT (11),
       FF  (12),  CR (13), and space (32), which are defined as	white space in
       the "C" locale. This list may vary if locale-specific matching is  tak-
       ing  place. For example,	in some	locales	the "non-breaking space" char-
       acter (\xA0) is recognized as white space, and in others	the VT charac-
       ter is not.

       A  "word"  character is an underscore or	any character that is a	letter
       or digit.  By default, the definition of	letters	 and  digits  is  con-
       trolled	by PCRE's low-valued character tables, and may vary if locale-
       specific	matching is taking place (see "Locale support" in the  pcreapi
       page).  For  example,  in  a French locale such as "fr_FR" in Unix-like
       systems,	or "french" in Windows,	some character codes greater than  127
       are  used  for  accented	letters, and these are then matched by \w. The
       use of locales with Unicode is discouraged.

       By default, characters whose code points	are  greater  than  127	 never
       match \d, \s, or	\w, and	always match \D, \S, and \W, although this may
       vary for	characters in the range	128-255	when locale-specific  matching
       is  happening.	These  escape sequences	retain their original meanings
       from before Unicode support was available, mainly for  efficiency  rea-
       sons.  If  PCRE	is  compiled  with  Unicode  property support, and the
       PCRE_UCP	option is set, the behaviour is	changed	so that	Unicode	 prop-
       erties are used to determine character types, as	follows:

	 \d  any character that	matches	\p{Nd} (decimal	digit)
	 \s  any character that	matches	\p{Z} or \h or \v
	 \w  any character that	matches	\p{L} or \p{N},	plus underscore

       The  upper case escapes match the inverse sets of characters. Note that
       \d matches only decimal digits, whereas \w matches any  Unicode	digit,
       as  well	as any Unicode letter, and underscore. Note also that PCRE_UCP
       affects \b, and \B because they are defined in  terms  of  \w  and  \W.
       Matching	these sequences	is noticeably slower when PCRE_UCP is set.

       The  sequences  \h, \H, \v, and \V are features that were added to Perl
       at release 5.10.	In contrast to the other sequences, which  match  only
       ASCII  characters  by  default,	these always match certain high-valued
       code points, whether or not PCRE_UCP is set. The	horizontal space char-
       acters are:

	 U+0009	    Horizontal tab (HT)
	 U+0020	    Space
	 U+00A0	    Non-break space
	 U+1680	    Ogham space	mark
	 U+180E	    Mongolian vowel separator
	 U+2000	    En quad
	 U+2001	    Em quad
	 U+2002	    En space
	 U+2003	    Em space
	 U+2004	    Three-per-em space
	 U+2005	    Four-per-em	space
	 U+2006	    Six-per-em space
	 U+2007	    Figure space
	 U+2008	    Punctuation	space
	 U+2009	    Thin space
	 U+200A	    Hair space
	 U+202F	    Narrow no-break space
	 U+205F	    Medium mathematical	space
	 U+3000	    Ideographic	space

       The vertical space characters are:

	 U+000A	    Linefeed (LF)
	 U+000B	    Vertical tab (VT)
	 U+000C	    Form feed (FF)
	 U+000D	    Carriage return (CR)
	 U+0085	    Next line (NEL)
	 U+2028	    Line separator
	 U+2029	    Paragraph separator

       In 8-bit, non-UTF-8 mode, only the characters with codepoints less than
       256 are relevant.

   Newline sequences
       Outside a character class, by default, the escape sequence  \R  matches
       any  Unicode newline sequence. In 8-bit non-UTF-8 mode \R is equivalent
       to the following:

	 (?>\r\n|\n|\x0b|\f|\r|\x85)

       This is an example of an	"atomic	group",	details	of which are given be-
       low.   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, U+000B), FF (form feed, U+000C), CR (car-
       riage return, U+000D), or NEL (next line,  U+0085).  The	 two-character
       sequence	is treated as a	single unit that cannot	be split.

       In  other modes,	two additional characters whose	codepoints are greater
       than 255	are added: LS (line separator, U+2028) and PS (paragraph sepa-
       rator,  U+2029).	  Unicode character property support is	not needed for
       these characters	to be recognized.

       It is possible to restrict \R to	match only CR, LF, or CRLF (instead of
       the  complete  set  of  Unicode	line  endings)	by  setting the	option
       PCRE_BSR_ANYCRLF	either at compile time or when the pattern is matched.
       (BSR is an abbrevation for "backslash R".) This can be made the default
       when PCRE is built; if this is the case,	the other behaviour can	be re-
       quested	via the	PCRE_BSR_UNICODE option.  It is	also possible to spec-
       ify these settings by starting a	pattern	string with one	of the follow-
       ing sequences:

	 (*BSR_ANYCRLF)	  CR, LF, or CRLF only
	 (*BSR_UNICODE)	  any Unicode newline sequence

       These override the default and the options given	to the compiling func-
       tion, but they can themselves be	 overridden  by	 options  given	 to  a
       matching	 function.  Note  that	these  special settings, which are not
       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. They can be combined with	 a  change  of
       newline convention; for example,	a pattern can start with:

	 (*ANY)(*BSR_ANYCRLF)

       They  can also be combined with the (*UTF8), (*UTF16), (*UTF32),	(*UTF)
       or (*UCP) special sequences. Inside a character class, \R is treated as
       an  unrecognized	 escape	sequence, and so matches the letter "R"	by de-
       fault, but causes an error if PCRE_EXTRA	is set.

   Unicode character properties
       When PCRE is built with Unicode character property support, three addi-
       tional  escape sequences	that match characters with specific properties
       are available.  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.  The	extra escape sequences are:

	 \p{xx}	  a character with the xx property
	 \P{xx}	  a character without the xx property
	 \X	  a Unicode extended grapheme cluster

       The property names represented by xx above are limited to  the  Unicode
       script names, the general category properties, "Any", which matches any
       character (including newline), and some special	PCRE  properties  (de-
       scribed	in  the	next section).	Other Perl properties such as "InMusi-
       calSymbols" are not currently supported by PCRE.	Note that \P{Any} does
       not match any characters, so always causes a match failure.

       Sets of Unicode characters are defined as belonging to certain scripts.
       A character from	one of these sets can be matched using a script	 name.
       For example:

	 \p{Greek}
	 \P{Han}

       Those  that are not part	of an identified script	are lumped together as
       "Common". The current list of scripts is:

       Arabic, Armenian, Avestan, Balinese, Bamum, Bassa_Vah, Batak,  Bengali,
       Bopomofo,  Brahmi,  Braille, Buginese, Buhid, Canadian_Aboriginal, Car-
       ian, Caucasian_Albanian,	Chakma,	Cham, Cherokee,	Common,	Coptic,	Cunei-
       form, Cypriot, Cyrillic,	Deseret, Devanagari, Duployan, Egyptian_Hiero-
       glyphs,	Elbasan,  Ethiopic,  Georgian,	Glagolitic,  Gothic,  Grantha,
       Greek,  Gujarati, Gurmukhi, Han,	Hangul,	Hanunoo, Hebrew, Hiragana, Im-
       perial_Aramaic,	   Inherited,	  Inscriptional_Pahlavi,      Inscrip-
       tional_Parthian,	  Javanese,   Kaithi,	Kannada,  Katakana,  Kayah_Li,
       Kharoshthi, Khmer, Khojki, Khudawadi, Lao, Latin, Lepcha,  Limbu,  Lin-
       ear_A,  Linear_B,  Lisu,	 Lycian, Lydian, Mahajani, Malayalam, Mandaic,
       Manichaean, Meetei_Mayek, Mende_Kikakui,	Meroitic_Cursive, Meroitic_Hi-
       eroglyphs, Miao,	Modi, Mongolian, Mro, Myanmar, Nabataean, New_Tai_Lue,
       Nko,  Ogham,  Ol_Chiki,	Old_Italic,   Old_North_Arabian,   Old_Permic,
       Old_Persian,   Old_South_Arabian,   Old_Turkic,	 Oriya,	 Osmanya,  Pa-
       hawh_Hmong,    Palmyrene,    Pau_Cin_Hau,     Phags_Pa,	   Phoenician,
       Psalter_Pahlavi,	 Rejang,  Runic,  Samaritan, Saurashtra, Sharada, Sha-
       vian, Siddham, Sinhala, Sora_Sompeng, Sundanese,	Syloti_Nagri,  Syriac,
       Tagalog,	 Tagbanwa,  Tai_Le,  Tai_Tham, Tai_Viet, Takri,	Tamil, Telugu,
       Thaana, Thai, Tibetan, Tifinagh,	Tirhuta, Ugaritic,  Vai,  Warang_Citi,
       Yi.

       Each character has exactly one Unicode general category property, spec-
       ified by	a two-letter abbreviation. For compatibility with Perl,	 nega-
       tion  can  be  specified	 by including a	circumflex between the opening
       brace and the property name.  For  example,  \p{^Lu}  is	 the  same  as
       \P{Lu}.

       If only one letter is specified with \p or \P, it includes all the gen-
       eral category properties	that start with	that letter. In	this case,  in
       the  absence of negation, the curly brackets in the escape sequence are
       optional; these two examples have the same effect:

	 \p{L}
	 \pL

       The following general category property codes are supported:

	 C     Other
	 Cc    Control
	 Cf    Format
	 Cn    Unassigned
	 Co    Private use
	 Cs    Surrogate

	 L     Letter
	 Ll    Lower case letter
	 Lm    Modifier	letter
	 Lo    Other letter
	 Lt    Title case letter
	 Lu    Upper case letter

	 M     Mark
	 Mc    Spacing mark
	 Me    Enclosing mark
	 Mn    Non-spacing mark

	 N     Number
	 Nd    Decimal number
	 Nl    Letter number
	 No    Other number

	 P     Punctuation
	 Pc    Connector punctuation
	 Pd    Dash punctuation
	 Pe    Close punctuation
	 Pf    Final punctuation
	 Pi    Initial punctuation
	 Po    Other punctuation
	 Ps    Open punctuation

	 S     Symbol
	 Sc    Currency	symbol
	 Sk    Modifier	symbol
	 Sm    Mathematical symbol
	 So    Other symbol

	 Z     Separator
	 Zl    Line separator
	 Zp    Paragraph separator
	 Zs    Space separator

       The special property L& is also supported: it matches a character  that
       has  the	 Lu,  Ll, or Lt	property, in other words, a letter that	is not
       classified as a modifier	or "other".

       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 cannot be tested by PCRE, unless  UTF	 validity  checking  has  been
       turned	 off	(see	the    discussion    of	   PCRE_NO_UTF8_CHECK,
       PCRE_NO_UTF16_CHECK and PCRE_NO_UTF32_CHECK in the pcreapi page).  Perl
       does not	support	the Cs property.

       The  long  synonyms  for	 property  names  that	Perl supports (such as
       \p{Letter}) are not supported by	PCRE, nor is it	 permitted  to	prefix
       any of these properties with "Is".

       No character that is in the Unicode table has the Cn (unassigned) prop-
       erty.  Instead, this property is	assumed	for any	code point that	is not
       in the Unicode table.

       Specifying  caseless  matching  does not	affect these escape sequences.
       For example, \p{Lu} always matches only upper  case  letters.  This  is
       different from the behaviour of current versions	of Perl.

       Matching	 characters  by	Unicode	property is not	fast, because PCRE has
       to do a multistage table	lookup in order	to find	 a  character's	 prop-
       erty. That is why the traditional escape	sequences such as \d and \w 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).

   Extended grapheme clusters
       The \X escape matches any number	of Unicode  characters	that  form  an
       "extended grapheme cluster", and	treats the sequence as an atomic group
       (see below).  Up	to and including release 8.31, PCRE  matched  an  ear-
       lier, simpler definition	that was equivalent to

	 (?>\PM\pM*)

       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.

       This simple definition was extended in Unicode to include more  compli-
       cated  kinds of composite character by giving each character a grapheme
       breaking	property, and creating rules that use these properties to  de-
       fine  the boundaries of extended	grapheme clusters. In releases of PCRE
       later than 8.31,	\X matches one of these	clusters.

       \X always matches at least one character. Then it  decides  whether  to
       add additional characters according to the following rules for ending a
       cluster:

       1. End at the end of the	subject	string.

       2. Do not end between CR	and LF;	otherwise end after any	control	 char-
       acter.

       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.

       4.  Do not end before extending characters or spacing marks. Characters
       with the	"mark" property	always have  the  "extend"  grapheme  breaking
       property.

       5. Do not end after prepend characters.

       6. Otherwise, end the cluster.

   PCRE's additional properties
       As  well	 as the	standard Unicode properties described above, PCRE sup-
       ports four more that make it possible to	convert	traditional escape se-
       quences	such  as  \w and \s to use Unicode properties. PCRE uses these
       non-standard, non-Perl properties internally when PCRE_UCP is set. How-
       ever, they may also be used explicitly. These properties	are:

	 Xan   Any alphanumeric	character
	 Xps   Any POSIX space character
	 Xsp   Any Perl	space character
	 Xwd   Any Perl	"word" character

       Xan  matches  characters	that have either the L (letter)	or the N (num-
       ber) property. Xps matches the characters tab, linefeed,	vertical  tab,
       form  feed,  or carriage	return,	and any	other character	that has the Z
       (separator) property.  Xsp is the same as Xps; it used to exclude  ver-
       tical  tab,  for	Perl compatibility, but	Perl changed, and so PCRE fol-
       lowed at	release	8.34. Xwd matches the same characters as Xan, plus un-
       derscore.

       There  is another non-standard property,	Xuc, which matches any charac-
       ter 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 \uHHHH or \UHHHHHHHH where H is a hexadecimal	digit.
       Note that the Xuc property does not match these sequences but the char-
       acters that they	represent.)

   Resetting the match start
       The  escape sequence \K causes any previously matched characters	not to
       be included in the final	matched	sequence. For example, the pattern:

	 foo\Kbar

       matches "foobar", but reports that it has matched "bar".	 This  feature
       is  similar  to	a lookbehind assertion (described below).  However, in
       this case, the part of the subject before the real match	does not  have
       to  be of fixed length, as lookbehind assertions	do. The	use of \K does
       not interfere with the setting of captured  substrings.	 For  example,
       when the	pattern

	 (foo)\Kbar

       matches "foobar", the first substring is	still set to "foo".

       Perl  documents	that  the use of \K within assertions is "not well de-
       fined". In PCRE,	\K is acted upon when it occurs	inside positive	asser-
       tions,  but is ignored in negative assertions. Note that	when a pattern
       such as (?=ab\K)	matches, the  reported	start  of  the	match  can  be
       greater than the	end of the match.

   Simple assertions
       The  final use of backslash is for certain simple assertions. An	asser-
       tion specifies a	condition that has to be met at	a particular point  in
       a  match, without consuming any characters from the subject string. The
       use of subpatterns for more complicated assertions is described	below.
       The backslashed assertions are:

	 \b	matches	at a word boundary
	 \B	matches	when not at a word boundary
	 \A	matches	at the start of	the subject
	 \Z	matches	at the end of the subject
		 also matches before a newline at the end of the subject
	 \z	matches	only at	the end	of the subject
	 \G	matches	at the first matching position in the subject

       Inside  a  character  class, \b has a different meaning;	it matches the
       backspace character. If any other of  these  assertions	appears	 in  a
       character  class, by default it matches the corresponding literal char-
       acter (for example, \B matches the letter B). However, if the  PCRE_EX-
       TRA  option is set, an "invalid escape sequence"	error is generated in-
       stead.

       A word boundary is a position in	the subject string where  the  current
       character  and  the previous character do not both match	\w or \W (i.e.
       one matches \w and the other matches \W), or the	start or  end  of  the
       string  if  the	first or last character	matches	\w, respectively. In a
       UTF mode, the meanings of \w and	\W  can	 be  changed  by  setting  the
       PCRE_UCP	 option. When this is done, it also affects \b and \B. Neither
       PCRE nor	Perl has a separate "start of word" or "end of	word"  metase-
       quence.	However,  whatever follows \b normally determines which	it is.
       For example, the	fragment \ba matches "a" at the	start of a word.

       The \A, \Z, and \z assertions differ from  the  traditional  circumflex
       and dollar (described in	the next section) in that they only ever match
       at the very start and end of the	subject	string,	whatever  options  are
       set.  Thus,  they are independent of multiline mode. These three	asser-
       tions are not affected by the PCRE_NOTBOL or PCRE_NOTEOL	options, which
       affect  only the	behaviour of the circumflex and	dollar metacharacters.
       However,	if the startoffset argument of pcre_exec() is non-zero,	 indi-
       cating that matching is to start	at a point other than the beginning of
       the subject, \A can never match.	The difference between \Z  and	\z  is
       that \Z matches before a	newline	at the end of the string as well as at
       the very	end, whereas \z	matches	only at	the end.

       The \G assertion	is true	only when the current matching position	is  at
       the  start point	of the match, as specified by the startoffset argument
       of pcre_exec(). It differs from \A when the  value  of  startoffset  is
       non-zero.  By calling pcre_exec() multiple times	with appropriate argu-
       ments, you can mimic Perl's /g option, and it is	in this	kind of	imple-
       mentation where \G can be useful.

       Note,  however,	that  PCRE's interpretation of \G, as the start	of the
       current match, is subtly	different from Perl's, which defines it	as the
       end  of	the  previous  match. In Perl, these can be different when the
       previously matched string was empty. Because PCRE does just  one	 match
       at a time, it cannot reproduce this behaviour.

       If  all	the alternatives of a pattern begin with \G, the expression is
       anchored	to the starting	match position,	and the	"anchored" flag	is set
       in the compiled regular expression.

CIRCUMFLEX AND DOLLAR
       The  circumflex	and  dollar  metacharacters are	zero-width assertions.
       That is,	they test for a	particular condition being true	 without  con-
       suming any characters from the subject string.

       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  at the start of the subject string. If the startoffset argu-
       ment of pcre_exec() is non-zero,	circumflex  can	 never	match  if  the
       PCRE_MULTILINE  option  is  unset. Inside a character class, circumflex
       has an entirely different meaning (see below).

       Circumflex need not be the first	character of the pattern if  a	number
       of  alternatives	are involved, but it should be the first thing in each
       alternative in which it appears if the pattern is ever  to  match  that
       branch.	If all possible	alternatives start with	a circumflex, that is,
       if the pattern is constrained to	match only at the start	 of  the  sub-
       ject,  it  is  said  to be an "anchored"	pattern. (There	are also other
       constructs that can cause a pattern to be anchored.)

       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 be-
       fore a newline at the end of the	string (by  default).  Note,  however,
       that  it	 does  not  actually match the newline.	Dollar need not	be the
       last character of the pattern if	a number of alternatives are involved,
       but  it should be the last item in any branch in	which it appears. Dol-
       lar has no special meaning in a character class.

       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 does not affect the \Z assertion.

       The meanings of the circumflex and dollar characters are	changed	if the
       PCRE_MULTILINE  option  is  set.	 When  this  is	the case, a circumflex
       matches immediately after internal newlines as well as at the start  of
       the  subject  string.  It  does not match after a newline that ends the
       string. A dollar	matches	before any newlines in the string, as well  as
       at  the very end, when PCRE_MULTILINE is	set. When newline is specified
       as the two-character sequence CRLF, isolated CR and  LF	characters  do
       not indicate newlines.

       For  example, the pattern /^abc$/ matches the subject string "def\nabc"
       (where \n represents a newline) in multiline mode, but  not  otherwise.
       Consequently,  patterns	that  are anchored in single line mode because
       all branches start with ^ are not anchored in  multiline	 mode,	and  a
       match  for  circumflex  is  possible  when  the startoffset argument of
       pcre_exec() is non-zero.	The PCRE_DOLLAR_ENDONLY	option is  ignored  if
       PCRE_MULTILINE is set.

       Note  that  the sequences \A, \Z, and \z	can be used to match the start
       and end of the subject in both modes, and if all	branches of a  pattern
       start  with  \A it is always anchored, whether or not PCRE_MULTILINE is
       set.

FULL STOP (PERIOD, DOT)	AND \N
       Outside a character class, a dot	in the pattern matches any one charac-
       ter  in	the subject string except (by default) a character that	signi-
       fies the	end of a line.

       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 if it is immediately followed  by  LF,  but	 otherwise  it
       matches	all characters (including isolated CRs and LFs). When any Uni-
       code line endings are being recognized, dot does	not match CR or	LF  or
       any of the other	line ending characters.

       The  behaviour  of  dot	with regard to newlines	can be changed.	If the
       PCRE_DOTALL option is set, a dot	matches	any one	character, without ex-
       ception.	 If  the two-character sequence	CRLF is	present	in the subject
       string, it takes	two dots to match it.

       The handling of dot is entirely independent of the handling of  circum-
       flex  and  dollar,  the	only relationship being	that they both involve
       newlines. Dot has no special meaning in a character class.

       The escape sequence \N behaves like a dot, except that it  is  not  af-
       fected  by the PCRE_DOTALL option. In other words, it matches any char-
       acter except one	that signifies the end of a line. Perl also uses \N to
       match characters	by name; PCRE does not support this.

MATCHING A SINGLE DATA UNIT
       Outside	a character class, the escape sequence \C matches any one data
       unit, 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;	in the
       32-bit library it is a 32-bit unit. Unlike a  dot,  \C  always  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 use-
       fully  be  used.	 Because  \C breaks up characters into individual data
       units, matching one unit	with \C	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 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).

       PCRE  does  not	allow \C to appear in lookbehind assertions (described
       below) in a UTF mode, because this would	make it	impossible  to	calcu-
       late the	length of the lookbehind.

       In general, the \C escape sequence is best avoided. However, one	way of
       using it	that avoids the	problem	of malformed UTF characters is to  use
       a  lookahead to check the length	of the next character, as in this pat-
       tern, which could be used with a	UTF-8 string (ignore white  space  and
       line breaks):

	 (?| (?=[\x00-\x7f])(\C) |
	     (?=[\x80-\x{7ff}])(\C)(\C)	|
	     (?=[\x{800}-\x{ffff}])(\C)(\C)(\C)	|
	     (?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C))

       A  group	 that starts with (?| resets the capturing parentheses numbers
       in each alternative (see	"Duplicate Subpattern Numbers" below). The as-
       sertions	at the start of	each branch check the next UTF-8 character for
       values whose encoding uses 1, 2,	3, or 4	bytes, respectively. The char-
       acter's individual bytes	are then captured by the appropriate number of
       groups.

SQUARE BRACKETS	AND CHARACTER CLASSES
       An opening square bracket introduces a character	class, terminated by a
       closing square bracket. A closing square	bracket	on its own is not spe-
       cial by default.	 However, if the PCRE_JAVASCRIPT_COMPAT	option is set,
       a lone closing square bracket causes a compile-time error. If a closing
       square bracket is required as 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.

       A character class matches a single character in the subject. In	a  UTF
       mode,  the  character  may  be  more than one data unit long. A matched
       character must be in the	set of characters defined by the class,	unless
       the  first  character in	the class definition is	a circumflex, in which
       case the	subject	character must not be in the set defined by the	class.
       If  a  circumflex is actually required as a member of the class,	ensure
       it is not the first character, or escape	it with	a backslash.

       For example, the	character class	[aeiou]	matches	any lower case	vowel,
       while  [^aeiou]	matches	 any character that is not a lower case	vowel.
       Note that a circumflex is just a	convenient notation for	specifying the
       characters  that	 are in	the class by enumerating those that are	not. A
       class that starts with a	circumflex is not an assertion;	it still  con-
       sumes  a	 character  from the subject string, and therefore it fails if
       the current pointer is at the end of the	string.

       In UTF-8	(UTF-16, UTF-32) mode, characters with values greater than 255
       (0xffff)	 can be	included in a class as a literal string	of data	units,
       or by using the \x{ escaping mechanism.

       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 "A" as well as "a", and a caseless  [^aeiou]  does  not
       match  "A", whereas a 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 always possible. For characters with
       higher values, the concept of case is supported	if  PCRE  is  compiled
       with  Unicode  property support,	but not	otherwise.  If you want	to use
       caseless	matching in a UTF mode for characters 128 and above, you  must
       ensure  that  PCRE is compiled with Unicode property support as well as
       with UTF	support.

       Characters that might indicate line breaks are  never  treated  in  any
       special	way  when matching character classes, whatever line-ending se-
       quence is in use, and whatever setting of the PCRE_DOTALL and PCRE_MUL-
       TILINE  options	is  used.  A  class such as [^a] always	matches	one of
       these characters.

       The minus (hyphen) character can	be used	to specify a range of  charac-
       ters  in	 a  character class. For example, [d-m]	matches	any letter be-
       tween d and m, inclusive. If a minus character is required in a	class,
       it  must	 be  escaped with a backslash or appear	in a position where it
       cannot be interpreted as	indicating a range, typically as the first  or
       last character in the class, or immediately after a range. For example,
       [b-d-z] matches letters in the range b to d, a hyphen character,	or z.

       It is not possible to have the literal character	"]" as the end charac-
       ter  of a range.	A pattern such as [W-]46] is interpreted as a class of
       two characters ("W" and "-") followed by	a literal string "46]",	so  it
       would  match  "W46]"  or	 "-46]". However, if the "]" is	escaped	with a
       backslash it is interpreted as the end of range,	so [W-\]46] is	inter-
       preted  as a class containing a range followed by two other characters.
       The octal or hexadecimal	representation of "]" can also be used to  end
       a range.

       An  error is generated if a POSIX character class (see below) or	an es-
       cape sequence other than	one that defines a single character appears at
       a  point	 where	a  range  ending  character  is	expected. For example,
       [z-\xff]	is valid, but [A-\d] and [A-[:digit:]] are not.

       Ranges operate in the collating sequence	of character values. They  can
       also   be  used	for  characters	 specified  numerically,  for  example
       [\000-\037]. Ranges can include any characters that are valid  for  the
       current mode.

       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  [][\\^_`wxyzabc],  matched  caselessly,  and	 in a non-UTF mode, if
       character tables	for a French locale are	in  use,  [\xc8-\xcb]  matches
       accented	 E  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 property support.

       The  character escape sequences \d, \D, \h, \H, \p, \P, \s, \S, \v, \V,
       \w, and \W may appear in	a character class, and add the characters that
       they  match to the class. For example, [\dABCDEF] matches any hexadeci-
       mal digit. In UTF modes,	the PCRE_UCP option affects  the  meanings  of
       \d, \s, \w and their upper case partners, just as it does when they ap-
       pear outside a character	class, as described in	the  section  entitled
       "Generic	character types" above.	The escape sequence \b has a different
       meaning inside a	character class; it matches the	 backspace  character.
       The  sequences  \B,  \N,	 \R, and \X are	not special inside a character
       class. Like any other unrecognized escape sequences, they  are  treated
       as  the literal characters "B", "N", "R", and "X" by default, but cause
       an error	if the PCRE_EXTRA option is set.

       A circumflex can	conveniently be	used with  the	upper  case  character
       types  to specify a more	restricted set of characters than the matching
       lower case type.	 For example, the class	[^\W_] matches any  letter  or
       digit, but not underscore, whereas [\w] includes	underscore. A positive
       character class should be read as "something OR something OR ..." and a
       negative	class as "NOT something	AND NOT	something AND NOT ...".

       The  only  metacharacters  that are recognized in character classes are
       backslash, hyphen (only where it	can be	interpreted  as	 specifying  a
       range),	circumflex  (only  at the start), opening square bracket (only
       when it can be interpreted as introducing a POSIX class name, or	for  a
       special	compatibility  feature	-  see the next	two sections), and the
       terminating closing square bracket.  However,  escaping	other  non-al-
       phanumeric characters does no harm.

POSIX CHARACTER	CLASSES
       Perl supports the POSIX notation	for character classes. This uses names
       enclosed	by [: and :] within the	enclosing square brackets.  PCRE  also
       supports	this notation. For example,

	 [01[:alpha:]%]

       matches "0", "1", any alphabetic	character, or "%". The supported class
       names are:

	 alnum	  letters and digits
	 alpha	  letters
	 ascii	  character codes 0 - 127
	 blank	  space	or tab only
	 cntrl	  control characters
	 digit	  decimal digits (same as \d)
	 graph	  printing characters, excluding space
	 lower	  lower	case letters
	 print	  printing characters, including space
	 punct	  printing characters, excluding letters and digits and	space
	 space	  white	space (the same	as \s from PCRE	8.34)
	 upper	  upper	case letters
	 word	  "word" characters (same as \w)
	 xdigit	  hexadecimal digits

       The default "space" characters are HT (9), LF (10), VT (11),  FF	 (12),
       CR  (13),  and space (32). If locale-specific matching is taking	place,
       the list	of space characters may	be different; there may	 be  fewer  or
       more of them. "Space" used to be	different to \s, which did not include
       VT, for Perl compatibility.  However, Perl changed at release 5.18, and
       PCRE  followed  at release 8.34.	 "Space" and \s	now match the same set
       of characters.

       The name	"word" is a Perl extension, and	"blank"	 is  a	GNU  extension
       from  Perl  5.8.	Another	Perl extension is negation, which is indicated
       by a ^ character	after the colon. For example,

	 [12[:^digit:]]

       matches "1", "2", or any	non-digit. PCRE	(and Perl) also	recognize  the
       POSIX syntax [.ch.] and [=ch=] where "ch" is a "collating element", but
       these are not supported,	and an error is	given if they are encountered.

       By default, characters with values greater than 128 do not match	any of
       the  POSIX character classes. However, if the PCRE_UCP option is	passed
       to pcre_compile(), some of the classes  are  changed  so	 that  Unicode
       character  properties  are  used. This is achieved by replacing certain
       POSIX classes by	other sequences, as follows:

	 [:alnum:]  becomes  \p{Xan}
	 [:alpha:]  becomes  \p{L}
	 [:blank:]  becomes  \h
	 [:digit:]  becomes  \p{Nd}
	 [:lower:]  becomes  \p{Ll}
	 [:space:]  becomes  \p{Xps}
	 [:upper:]  becomes  \p{Lu}
	 [:word:]   becomes  \p{Xwd}

       Negated versions, such as [:^alpha:] use	\P instead of \p. Three	 other
       POSIX classes are handled specially in UCP mode:

       [:graph:] This  matches	characters that	have glyphs that mark the page
		 when printed. In Unicode property terms, it matches all char-
		 acters	with the L, M, N, P, S,	or Cf properties, except for:

		   U+061C	    Arabic Letter Mark
		   U+180E	    Mongolian Vowel Separator
		   U+2066 - U+2069  Various "isolate"s

       [:print:] This  matches	the  same  characters  as [:graph:] plus space
		 characters that are not controls, that	 is,  characters  with
		 the Zs	property.

       [:punct:] This matches all characters that have the Unicode P (punctua-
		 tion) property, plus those characters whose code  points  are
		 less than 128 that have the S (Symbol)	property.

       The  other  POSIX classes are unchanged,	and match only characters with
       code points less	than 128.

COMPATIBILITY FEATURE FOR WORD BOUNDARIES
       In the POSIX.2 compliant	library	that was included in 4.4BSD Unix,  the
       ugly  syntax  [[:<:]]  and [[:>:]] is used for matching "start of word"
       and "end	of word". PCRE treats these items as follows:

	 [[:<:]]  is converted to  \b(?=\w)
	 [[:>:]]  is converted to  \b(?<=\w)

       Only these exact	character sequences are	recognized. A sequence such as
       [a[:<:]b]  provokes  error  for	an unrecognized	POSIX class name. This
       support is not compatible with Perl. It is provided to help  migrations
       from other environments,	and is best not	used in	any new	patterns. Note
       that \b matches at the start and	the end	of a word (see "Simple	asser-
       tions"  above),	and in a Perl-style pattern the	preceding or following
       character normally shows	which is wanted, without the need for the  as-
       sertions	 that are used above in	order to give exactly the POSIX	behav-
       iour.

VERTICAL BAR
       Vertical	bar characters are used	to separate alternative	patterns.  For
       example,	the pattern

	 gilbert|sullivan

       matches	either "gilbert" or "sullivan".	Any number of alternatives may
       appear, and an empty  alternative  is  permitted	 (matching  the	 empty
       string).	The matching process tries each	alternative in turn, from left
       to right, and the first one that	succeeds is used. If the  alternatives
       are  within a subpattern	(defined below), "succeeds" means matching the
       rest of the main	pattern	as well	as the alternative in the subpattern.

INTERNAL OPTION	SETTING
       The settings of the  PCRE_CASELESS,  PCRE_MULTILINE,  PCRE_DOTALL,  and
       PCRE_EXTENDED  options  (which are Perl-compatible) can be changed from
       within the pattern by a sequence	of Perl	option	letters	 enclosed  be-
       tween "(?" and ")".  The	option letters are

	 i  for	PCRE_CASELESS
	 m  for	PCRE_MULTILINE
	 s  for	PCRE_DOTALL
	 x  for	PCRE_EXTENDED

       For example, (?im) sets caseless, multiline matching. It	is also	possi-
       ble to unset these options by preceding the letter with a hyphen, and a
       combined	 setting and unsetting such as (?im-sx), which sets PCRE_CASE-
       LESS and	PCRE_MULTILINE while unsetting PCRE_DOTALL and	PCRE_EXTENDED,
       is  also	 permitted.  If	a letter appears both before and after the hy-
       phen, the option	is unset.

       The PCRE-specific options PCRE_DUPNAMES,	PCRE_UNGREEDY, and  PCRE_EXTRA
       can  be changed in the same way as the Perl-compatible options by using
       the characters J, U and X respectively.

       When one	of these option	changes	occurs at top level (that is, not  in-
       side  subpattern	 parentheses),	the change applies to the remainder of
       the pattern that	follows. An option change within a subpattern (see be-
       low  for	 a  description	 of subpatterns) affects only that part	of the
       subpattern that follows it, so

	 (a(?i)b)c

       matches abc and aBc and no other	strings	(assuming PCRE_CASELESS	is not
       used).	By  this means,	options	can be made to have different settings
       in different parts of the pattern. Any changes made in one  alternative
       do  carry  on  into subsequent branches within the same subpattern. For
       example,

	 (a(?i)b|c)

       matches "ab", "aB", "c",	and "C", even though  when  matching  "C"  the
       first  branch  is  abandoned before the option setting. This is because
       the effects of option settings happen at	compile	time. There  would  be
       some very weird behaviour otherwise.

       Note:  There are	other PCRE-specific options that can be	set by the ap-
       plication when the compiling or matching	functions are called. In  some
       cases the pattern can contain special leading sequences such as (*CRLF)
       to override what	the application	has set	or what	 has  been  defaulted.
       Details	are  given  in the section entitled "Newline sequences"	above.
       There are also the (*UTF8), (*UTF16),(*UTF32), and (*UCP)  leading  se-
       quences	that  can  be used to set UTF and Unicode property modes; they
       are equivalent to setting the PCRE_UTF8,	PCRE_UTF16, PCRE_UTF32 and the
       PCRE_UCP	 options,  respectively. The (*UTF) sequence is	a generic ver-
       sion that can be	used with any of the libraries.	However, the  applica-
       tion  can set the PCRE_NEVER_UTF	option,	which locks out	the use	of the
       (*UTF) sequences.

SUBPATTERNS
       Subpatterns are delimited by parentheses	(round brackets), which	can be
       nested.	Turning	part of	a pattern into a subpattern does two things:

       1. It localizes a set of	alternatives. For example, the pattern

	 cat(aract|erpillar|)

       matches	"cataract",  "caterpillar", or "cat". Without the parentheses,
       it would	match "cataract", "erpillar" or	an empty string.

       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 subpattern is passed back to the	caller via the
       ovector	argument  of  the matching function. (This applies only	to the
       traditional matching functions; the DFA matching	functions do not  sup-
       port capturing.)

       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

	 the ((red|white) (king|queen))

       the captured substrings are "red	king", "red", and "king", and are num-
       bered 1,	2, and 3, respectively.

       The fact	that plain parentheses fulfil  two  functions  is  not	always
       helpful.	  There	are often times	when a grouping	subpattern is required
       without a capturing requirement.	If an opening parenthesis is  followed
       by  a question mark and a colon,	the subpattern does not	do any captur-
       ing, and	is not counted when computing the  number  of  any  subsequent
       capturing  subpatterns. For example, if the string "the white queen" is
       matched against the pattern

	 the ((?:red|white) (king|queen))

       the captured substrings are "white queen" and "queen", and are numbered
       1 and 2.	The maximum number of capturing	subpatterns is 65535.

       As  a  convenient shorthand, if any option settings are required	at the
       start of	a non-capturing	subpattern, the	option letters may appear  be-
       tween the "?" and the ":". Thus the two patterns

	 (?i:saturday|sunday)
	 (?:(?i)saturday|sunday)

       match exactly the same set of strings. Because alternative branches are
       tried from left to right, and options are not reset until  the  end  of
       the  subpattern is reached, an option setting in	one branch does	affect
       subsequent branches, so the above patterns match	"SUNDAY"  as  well  as
       "Saturday".

DUPLICATE SUBPATTERN NUMBERS
       Perl 5.10 introduced a feature whereby each alternative in a subpattern
       uses the	same numbers for its capturing parentheses. Such a  subpattern
       starts  with (?|	and is itself a	non-capturing subpattern. For example,
       consider	this pattern:

	 (?|(Sat)ur|(Sun))day

       Because the two alternatives are	inside a (?| group, both sets of  cap-
       turing  parentheses  are	 numbered one. Thus, when the pattern matches,
       you can look at captured	substring number  one,	whichever  alternative
       matched.	 This  construct  is useful when you want to capture part, but
       not all,	of one of a number of alternatives. Inside a (?| group,	paren-
       theses  are  numbered as	usual, but the number is reset at the start of
       each branch. The	numbers	of any capturing parentheses that  follow  the
       subpattern  start after the highest number used in any branch. The fol-
       lowing example is taken from the	Perl documentation. The	numbers	under-
       neath show in which buffer the captured content will be stored.

	 # before  ---------------branch-reset----------- after
	 / ( a )  (?| x	( y ) z	| (p (q) r) | (t) u (v)	) ( z )	/x
	 # 1		2	  2  3	      2	    3	  4

       A  back	reference  to a	numbered subpattern uses the most recent value
       that is set for that number by any subpattern.  The  following  pattern
       matches "abcabc"	or "defdef":

	 /(?|(abc)|(def))\1/

       In  contrast,  a	subroutine call	to a numbered subpattern always	refers
       to the first one	in the pattern with the	given  number.	The  following
       pattern matches "abcabc"	or "defabc":

	 /(?|(abc)|(def))(?1)/

       If  a condition test for	a subpattern's having matched refers to	a non-
       unique number, the test is true if any of the subpatterns of that  num-
       ber have	matched.

       An  alternative approach	to using this "branch reset" feature is	to use
       duplicate named subpatterns, as described in the	next section.

NAMED SUBPATTERNS
       Identifying capturing parentheses by number is simple, but  it  can  be
       very  hard  to keep track of the	numbers	in complicated regular expres-
       sions. Furthermore, if an  expression  is  modified,  the  numbers  may
       change.	To help	with this difficulty, PCRE supports the	naming of sub-
       patterns. This feature was not added to Perl until release 5.10.	Python
       had  the	 feature earlier, and PCRE introduced it at release 4.0, using
       the Python syntax. PCRE now supports both the Perl and the Python  syn-
       tax.  Perl  allows  identically	numbered subpatterns to	have different
       names, but PCRE does not.

       In PCRE,	a subpattern can be named in one of three  ways:  (?<name>...)
       or  (?'name'...)	 as in Perl, or	(?P<name>...) as in Python. References
       to capturing parentheses	from other parts of the	pattern, such as  back
       references,  recursion,	and conditions,	can be made by name as well as
       by number.

       Names consist of	up to 32 alphanumeric characters and underscores,  but
       must  start with	a non-digit. Named capturing parentheses are still al-
       located numbers as well as names, exactly as  if	 the  names  were  not
       present.	 The PCRE API provides function	calls for extracting the name-
       to-number translation table from	a compiled pattern. There  is  also  a
       convenience function for	extracting a captured substring	by name.

       By  default, a name must	be unique within a pattern, but	it is possible
       to relax	this constraint	by setting the PCRE_DUPNAMES option at compile
       time.  (Duplicate  names	are also always	permitted for subpatterns with
       the same	number,	set up as described in the previous  section.)	Dupli-
       cate  names  can	 be useful for patterns	where only one instance	of the
       named parentheses can match. Suppose you	want to	match the  name	 of  a
       weekday,	 either	as a 3-letter abbreviation or as the full name,	and in
       both cases you want to extract the abbreviation.	This pattern (ignoring
       the line	breaks)	does the job:

	 (?<DN>Mon|Fri|Sun)(?:day)?|
	 (?<DN>Tue)(?:sday)?|
	 (?<DN>Wed)(?:nesday)?|
	 (?<DN>Thu)(?:rsday)?|
	 (?<DN>Sat)(?:urday)?

       There  are  five	capturing substrings, but only one is ever set after a
       match.  (An alternative way of solving this problem is to use a "branch
       reset" subpattern, as described in the previous section.)

       The  convenience	 function  for extracting the data by name returns the
       substring for the first (and in this example, the only)	subpattern  of
       that  name  that	 matched.  This	saves searching	to find	which numbered
       subpattern it was.

       If you make a back reference to	a  non-unique  named  subpattern  from
       elsewhere  in the pattern, the subpatterns to which the name refers are
       checked in the order in which they appear in the	overall	 pattern.  The
       first one that is set is	used for the reference.	For example, this pat-
       tern matches both "foofoo" and "barbar" but not "foobar"	or "barfoo":

	 (?:(?<n>foo)|(?<n>bar))\k<n>

       If you make a subroutine	call to	a non-unique named subpattern, the one
       that  corresponds  to  the first	occurrence of the name is used.	In the
       absence of duplicate numbers (see the previous section) this is the one
       with the	lowest number.

       If you use a named reference in a condition test	(see the section about
       conditions below), either to check whether a subpattern has matched, or
       to  check for recursion,	all subpatterns	with the same name are tested.
       If the condition	is true	for any	one of them, the overall condition  is
       true.  This is the same behaviour as testing by number. For further de-
       tails of	the interfaces for handling named subpatterns, see the pcreapi
       documentation.

       Warning:	You cannot use different names to distinguish between two sub-
       patterns	with the same number because PCRE uses only the	 numbers  when
       matching. For this reason, an error is given at compile time if differ-
       ent names are given to subpatterns with the same	number.	 However,  you
       can always give the same	name to	subpatterns with the same number, even
       when PCRE_DUPNAMES is not set.

REPETITION
       Repetition is specified by quantifiers, which can  follow  any  of  the
       following items:

	 a literal data	character
	 the dot metacharacter
	 the \C	escape sequence
	 the \X	escape sequence
	 the \R	escape sequence
	 an escape such	as \d or \pL that matches a single character
	 a character class
	 a back	reference (see next section)
	 a parenthesized subpattern (including assertions)
	 a subroutine call to a	subpattern (recursive or otherwise)

       The  general repetition quantifier specifies a minimum and maximum num-
       ber of permitted	matches, by giving the two numbers in  curly  brackets
       (braces),  separated  by	 a comma. The numbers must be less than	65536,
       and the first must be less than or equal	to the second. For example:

	 z{2,4}

       matches "zz", "zzz", or "zzzz". A closing brace on its  own  is	not  a
       special	character.  If	the second number is omitted, but the comma is
       present,	there is no upper limit; if the	second number  and  the	 comma
       are  both omitted, the quantifier specifies an exact number of required
       matches.	Thus

	 [aeiou]{3,}

       matches at least	3 successive vowels, but may match many	more, while

	 \d{8}

       matches exactly 8 digits. An opening curly bracket that	appears	 in  a
       position	 where a quantifier is not allowed, or one that	does not match
       the syntax of a quantifier, is taken as a literal character. For	 exam-
       ple, {,6} is not	a quantifier, but a literal string of four characters.

       In UTF modes, quantifiers apply to characters rather than to individual
       data units. Thus, for example, \x{100}{2} matches two characters,  each
       of which	is represented by a two-byte sequence in a UTF-8 string. Simi-
       larly, \X{3} matches three Unicode extended grapheme clusters, each  of
       which  may  be  several	data  units long (and they may be of different
       lengths).

       The quantifier {0} is permitted,	causing	the expression to behave as if
       the previous item and the quantifier were not present. This may be use-
       ful for subpatterns that	are referenced as subroutines  from  elsewhere
       in the pattern (but see also the	section	entitled "Defining subpatterns
       for use by reference only" below). Items	other  than  subpatterns  that
       have a {0} quantifier are omitted from the compiled pattern.

       For  convenience, the three most	common quantifiers have	single-charac-
       ter abbreviations:

	 *    is equivalent to {0,}
	 +    is equivalent to {1,}
	 ?    is equivalent to {0,1}

       It is possible to construct infinite loops by  following	 a  subpattern
       that can	match no characters with a quantifier that has no upper	limit,
       for example:

	 (a?)*

       Earlier versions	of Perl	and PCRE used to give an error at compile time
       for  such  patterns. However, because there are cases where this	can be
       useful, such patterns are now accepted, but if any  repetition  of  the
       subpattern  does	in fact	match no characters, the loop is forcibly bro-
       ken.

       By default, the quantifiers are "greedy", that is, they match  as  much
       as  possible  (up  to  the  maximum number of permitted times), without
       causing the rest	of the pattern to fail.	The classic example  of	 where
       this gives problems is in trying	to match comments in C programs. These
       appear between /* and */	and within the comment,	 individual  *	and  /
       characters  may	appear.	An attempt to match C comments by applying the
       pattern

	 /\*.*\*/

       to the string

	 /* first comment */  not comment  /* second comment */

       fails, because it matches the entire string owing to the	greediness  of
       the .*  item.

       However,	 if  a quantifier is followed by a question mark, it ceases to
       be greedy, and instead matches the minimum number of times possible, so
       the pattern

	 /\*.*?\*/

       does  the  right	 thing with the	C comments. The	meaning	of the various
       quantifiers is not otherwise changed,  just  the	 preferred  number  of
       matches.	  Do  not  confuse this	use of question	mark with its use as a
       quantifier in its own right. Because it has two uses, it	can  sometimes
       appear doubled, as in

	 \d??\d

       which matches one digit by preference, but can match two	if that	is the
       only way	the rest of the	pattern	matches.

       If the PCRE_UNGREEDY option is set (an option that is not available  in
       Perl),  the  quantifiers	are not	greedy by default, but individual ones
       can be made greedy by following them with a  question  mark.  In	 other
       words, it inverts the default behaviour.

       When  a	parenthesized  subpattern  is quantified with a	minimum	repeat
       count that is greater than 1 or with a limited maximum, more memory  is
       required	 for  the  compiled  pattern, in proportion to the size	of the
       minimum or maximum.

       If a pattern starts with	.* or .{0,} and	the PCRE_DOTALL	option (equiv-
       alent  to  Perl's  /s) is set, thus allowing the	dot to match newlines,
       the pattern is implicitly anchored, because whatever  follows  will  be
       tried  against every character position in the subject string, so there
       is no point in retrying the overall match at  any  position  after  the
       first.  PCRE  normally treats such a pattern as though it were preceded
       by \A.

       In cases	where it is known that the subject  string  contains  no  new-
       lines,  it  is  worth setting PCRE_DOTALL in order to obtain this opti-
       mization, or alternatively using	^ to indicate anchoring	explicitly.

       However,	there are some cases where the optimization  cannot  be	 used.
       When .*	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 succeeds. Consider, for example:

	 (.*)abc\1

       If  the subject is "xyz123abc123" the match point is the	fourth charac-
       ter. For	this reason, such a pattern is not implicitly anchored.

       Another case where implicit anchoring is	not applied is when the	 lead-
       ing  .* is inside an atomic group. Once again, a	match at the start may
       fail where a later one succeeds.	Consider this pattern:

	 (?>.*?a)b

       It matches "ab" in the subject "aab". The use of	the backtracking  con-
       trol verbs (*PRUNE) and (*SKIP) also disable this optimization.

       When a capturing	subpattern is repeated,	the value captured is the sub-
       string that matched the final iteration.	For example, after

	 (tweedle[dume]{3}\s*)+

       has matched "tweedledum tweedledee" the value of	the captured substring
       is  "tweedledee".  However,  if there are nested	capturing subpatterns,
       the corresponding captured values may have been set in previous	itera-
       tions. For example, after

	 /(a|(b))+/

       matches "aba" the value of the second captured substring	is "b".

ATOMIC GROUPING	AND POSSESSIVE QUANTIFIERS
       With  both  maximizing ("greedy") and minimizing	("ungreedy" or "lazy")
       repetition, failure of what follows normally causes the	repeated  item
       to  be  re-evaluated to see if a	different number of repeats allows the
       rest of the pattern to match. Sometimes it is useful to	prevent	 this,
       either  to  change the nature of	the match, or to cause it fail earlier
       than it otherwise might,	when the author	of the pattern knows there  is
       no point	in carrying on.

       Consider,  for  example,	the pattern \d+foo when	applied	to the subject
       line

	 123456bar

       After matching all 6 digits and then failing to match "foo", the	normal
       action  of  the matcher is to try again with only 5 digits matching the
       \d+ item, and then with	4,  and	 so  on,  before  ultimately  failing.
       "Atomic	grouping"  (a  term taken from Jeffrey Friedl's	book) provides
       the means for specifying	that once a subpattern has matched, it is  not
       to be re-evaluated in this way.

       If  we  use atomic grouping for the previous example, the matcher gives
       up immediately on failing to match "foo"	the first time.	 The  notation
       is a kind of special parenthesis, starting with (?> as in this example:

	 (?>\d+)foo

       This  kind  of  parenthesis "locks up" the  part	of the pattern it con-
       tains once it has matched, and a	failure	further	into  the  pattern  is
       prevented  from	backtracking into it. Backtracking past	it to previous
       items, however, works as	normal.

       An alternative description is that a subpattern of  this	 type  matches
       the  string  of	characters  that an identical standalone pattern would
       match, if anchored at the current point in the subject string.

       Atomic grouping subpatterns are not capturing subpatterns. Simple cases
       such as the above example can be	thought	of as a	maximizing repeat that
       must swallow everything it can. So, while both \d+ and  \d+?  are  pre-
       pared  to  adjust  the number of	digits they match in order to make the
       rest of the pattern match, (?>\d+) can only match an entire sequence of
       digits.

       Atomic  groups in general can of	course contain arbitrarily complicated
       subpatterns, and	can be nested. However,	when  the  subpattern  for  an
       atomic group is just a single repeated item, as in the example above, a
       simpler notation, called	a "possessive quantifier" can  be  used.  This
       consists	 of  an	 additional  + character following a quantifier. Using
       this notation, the previous example can be rewritten as

	 \d++foo

       Note that a possessive quantifier can be	used with an entire group, for
       example:

	 (abc|xyz){2,3}+

       Possessive  quantifiers	are always greedy; the setting of the PCRE_UN-
       GREEDY option is	ignored. They are a convenient notation	for  the  sim-
       pler  forms  of	atomic	group.	However, there is no difference	in the
       meaning of a possessive quantifier and  the  equivalent	atomic	group,
       though  there  may  be a	performance difference;	possessive quantifiers
       should be slightly faster.

       The possessive quantifier syntax	is an extension	to the Perl  5.8  syn-
       tax.   Jeffrey  Friedl  originated the idea (and	the name) in the first
       edition of his book. Mike McCloskey liked it, so	implemented it when he
       built  Sun's Java package, and PCRE copied it from there. It ultimately
       found its way into Perl at release 5.10.

       PCRE has	an optimization	that automatically "possessifies" certain sim-
       ple  pattern  constructs.  For  example,	the sequence A+B is treated as
       A++B because there is no	point in backtracking into a sequence  of  A's
       when B must follow.

       When  a	pattern	 contains an unlimited repeat inside a subpattern that
       can itself be repeated an unlimited number of  times,  the  use	of  an
       atomic  group  is  the  only way	to avoid some failing matches taking a
       very long time indeed. The pattern

	 (\D+|<\d+>)*[!?]

       matches an unlimited number of substrings that either consist  of  non-
       digits,	or  digits  enclosed in	<>, followed by	either ! or ?. When it
       matches,	it runs	quickly. However, if it	is applied to

	 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

       it takes	a long time before reporting  failure.	This  is  because  the
       string  can be divided between the internal \D+ repeat and the external
       * repeat	in a large number of ways, and all have	to be tried. (The  ex-
       ample uses [!?] rather than a single character at the end, because both
       PCRE and	Perl have an optimization that allows for fast failure when  a
       single  character is used. They remember	the last single	character that
       is required for a match,	and fail early if it is	 not  present  in  the
       string.)	 If  the  pattern  is changed so that it uses an atomic	group,
       like this:

	 ((?>\D+)|<\d+>)*[!?]

       sequences of non-digits cannot be broken, and failure happens quickly.

BACK REFERENCES
       Outside a character class, a backslash followed by a digit greater than
       0 (and possibly further digits) is a back reference to a	capturing sub-
       pattern earlier (that is, to its	left) in the pattern,  provided	 there
       have been that many previous capturing left parentheses.

       However,	if the decimal number following	the backslash is less than 10,
       it is always taken as a back reference, and causes  an  error  only  if
       there  are  not that many capturing left	parentheses in the entire pat-
       tern. In	other words, the parentheses that are referenced need  not  be
       to  the left of the reference for numbers less than 10. A "forward back
       reference" of this type can make	sense when a  repetition  is  involved
       and  the	 subpattern to the right has participated in an	earlier	itera-
       tion.

       It is not possible to have a numerical "forward back  reference"	 to  a
       subpattern  whose  number is 10 or more using this syntax because a se-
       quence such as \50 is interpreted as a character	defined	in octal.  See
       the subsection entitled "Non-printing characters" above for further de-
       tails of	the handling of	digits following a backslash. There is no such
       problem	when  named parentheses	are used. A back reference to any sub-
       pattern is possible using named parentheses (see	below).

       Another way of avoiding the ambiguity inherent in  the  use  of	digits
       following  a  backslash	is  to use the \g escape sequence. This	escape
       must be followed	by an unsigned number or a negative number, optionally
       enclosed	in braces. These examples are all identical:

	 (ring), \1
	 (ring), \g1
	 (ring), \g{1}

       An  unsigned number specifies an	absolute reference without the ambigu-
       ity that	is present in the older	syntax.	It is also useful when literal
       digits follow the reference. A negative number is a relative reference.
       Consider	this example:

	 (abc(def)ghi)\g{-1}

       The sequence \g{-1} is a	reference to the most recently started captur-
       ing subpattern before \g, that is, is it	equivalent to \2 in this exam-
       ple.  Similarly,	\g{-2} would be	equivalent to \1. The use of  relative
       references  can	be helpful in long patterns, and also in patterns that
       are created by  joining	together  fragments  that  contain  references
       within themselves.

       A  back	reference matches whatever actually matched the	capturing sub-
       pattern in the current subject string, rather  than  anything  matching
       the subpattern itself (see "Subpatterns as subroutines" below for a way
       of doing	that). So the pattern

	 (sens|respons)e and \1ibility

       matches "sense and sensibility" and "response and responsibility",  but
       not  "sense and responsibility".	If caseful matching is in force	at the
       time of the back	reference, the case of letters is relevant. For	 exam-
       ple,

	 ((?i)rah)\s+\1

       matches	"rah  rah"  and	 "RAH RAH", but	not "RAH rah", even though the
       original	capturing subpattern is	matched	caselessly.

       There are several different ways	of writing back	 references  to	 named
       subpatterns.  The  .NET syntax \k{name} and the Perl syntax \k<name> or
       \k'name'	are supported, as is the Python	syntax (?P=name). Perl	5.10's
       unified back reference syntax, in which \g can be used for both numeric
       and named references, is	also supported.	We could rewrite the above ex-
       ample in	any of the following ways:

	 (?<p1>(?i)rah)\s+\k<p1>
	 (?'p1'(?i)rah)\s+\k{p1}
	 (?P<p1>(?i)rah)\s+(?P=p1)
	 (?<p1>(?i)rah)\s+\g{p1}

       A  subpattern  that is referenced by name may appear in the pattern be-
       fore or after the reference.

       There may be more than one back reference to the	same subpattern. If  a
       subpattern  has	not actually been used in a particular match, any back
       references to it	always fail by default.	For example, the pattern

	 (a|(bc))\2

       always fails if it starts to match "a" rather than  "bc".  However,  if
       the PCRE_JAVASCRIPT_COMPAT option is set	at compile time, a back	refer-
       ence to an unset	value matches an empty string.

       Because there may be many capturing parentheses in a pattern, all  dig-
       its  following a	backslash are taken as part of a potential back	refer-
       ence number.  If	the pattern continues with a digit character, some de-
       limiter	must  be used to terminate the back reference. If the PCRE_EX-
       TENDED option is	set, this can be white space. Otherwise, the \g{  syn-
       tax or an empty comment (see "Comments" below) can be used.

   Recursive back references
       A  back reference that occurs inside the	parentheses to which it	refers
       fails when the subpattern is first used,	so, for	example,  (a\1)	 never
       matches.	  However,  such references can	be useful inside repeated sub-
       patterns. For example, the pattern

	 (a|b\1)+

       matches any number of "a"s and also "aba", "ababbaa" etc. At each iter-
       ation  of  the  subpattern,  the	 back  reference matches the character
       string corresponding to the previous iteration. In order	 for  this  to
       work,  the  pattern must	be such	that the first iteration does not need
       to match	the back reference. This can be	done using alternation,	as  in
       the example above, or by	a quantifier with a minimum of zero.

       Back  references	of this	type cause the group that they reference to be
       treated as an atomic group.  Once the whole group has been  matched,  a
       subsequent  matching  failure cannot cause backtracking into the	middle
       of the group.

ASSERTIONS
       An assertion is a test on the characters	 following  or	preceding  the
       current	matching  point	that does not actually consume any characters.
       The simple assertions coded as \b, \B, \A, \G, \Z, \z, ^	and $ are  de-
       scribed above.

       More  complicated  assertions  are  coded as subpatterns. There are two
       kinds: those that look ahead of the current  position  in  the  subject
       string,	and  those  that  look	behind	it. An assertion subpattern is
       matched in the normal way, except that it does not  cause  the  current
       matching	position to be changed.

       Assertion  subpatterns are not capturing	subpatterns. If	such an	asser-
       tion contains capturing subpatterns within it, these  are  counted  for
       the  purposes  of numbering the capturing subpatterns in	the whole pat-
       tern. However, substring	capturing is carried out only for positive as-
       sertions.  (Perl	 sometimes, but	not always, does do capturing in nega-
       tive assertions.)

       WARNING:	If a positive assertion	containing one or more capturing  sub-
       patterns	 succeeds,  but	 failure  to match later in the	pattern	causes
       backtracking over this assertion, the captures within the assertion are
       reset only if no	higher numbered	captures are already set. This is, un-
       fortunately, a fundamental limitation of	 the  current  implementation,
       and  as PCRE1 is	now in maintenance-only	status,	it is unlikely ever to
       change.

       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 capturing	parentheses may	 occasionally  be  useful.  In
       practice, there only three cases:

       (1)  If	the  quantifier	 is  {0}, the assertion	is never obeyed	during
       matching.  However, it may  contain  internal  capturing	 parenthesized
       groups that are called from elsewhere via the subroutine	mechanism.

       (2)  If quantifier is {0,n} where n is greater than zero, it is treated
       as if it	were {0,1}. At run time, the rest  of  the  pattern  match  is
       tried with and without the assertion, the order depending on the	greed-
       iness of	the quantifier.

       (3) If the minimum repetition is	greater	than zero, the	quantifier  is
       ignored.	  The  assertion  is  obeyed just once when encountered	during
       matching.

   Lookahead assertions
       Lookahead assertions start with (?= for positive	assertions and (?! for
       negative	assertions. For	example,

	 \w+(?=;)

       matches	a word followed	by a semicolon,	but does not include the semi-
       colon in	the match, and

	 foo(?!bar)

       matches any occurrence of "foo" that is not  followed  by  "bar".  Note
       that the	apparently similar pattern

	 (?!foo)bar

       does  not  find	an  occurrence	of "bar" that is preceded by something
       other than "foo"; it finds any occurrence of "bar" whatsoever,  because
       the assertion (?!foo) is	always true when the next three	characters are
       "bar". A	lookbehind assertion is	needed to achieve the other effect.

       If you want to force a matching failure at some point in	a pattern, the
       most  convenient	 way to	do it is with (?!) because an empty string al-
       ways matches, so	an assertion that requires there not to	 be  an	 empty
       string must always fail.	 The backtracking control verb (*FAIL) or (*F)
       is a synonym for	(?!).

   Lookbehind assertions
       Lookbehind assertions start with	(?<= for positive assertions and  (?<!
       for negative assertions.	For example,

	 (?<!foo)bar

       does  find  an  occurrence  of "bar" that is not	preceded by "foo". The
       contents	of a lookbehind	assertion are restricted  such	that  all  the
       strings it matches must have a fixed length. However, if	there are sev-
       eral top-level alternatives, they do not	all  have  to  have  the  same
       fixed length. Thus

	 (?<=bullock|donkey)

       is permitted, but

	 (?<!dogs?|cats?)

       causes  an  error at compile time. Branches that	match different	length
       strings are permitted only at the top level of a	lookbehind  assertion.
       This is an extension compared with Perl,	which requires all branches to
       match the same length of	string.	An assertion such as

	 (?<=ab(c|de))

       is not permitted, because its single top-level  branch  can  match  two
       different lengths, but it is acceptable to PCRE if rewritten to use two
       top-level branches:

	 (?<=abc|abde)

       In some cases, the escape sequence \K (see above) can be	 used  instead
       of a lookbehind assertion to get	round the fixed-length restriction.

       The  implementation  of lookbehind assertions is, for each alternative,
       to temporarily move the current position	back by	the fixed  length  and
       then try	to match. If there are insufficient characters before the cur-
       rent position, the assertion fails.

       In a UTF	mode, PCRE does	not allow the \C escape	(which matches a  sin-
       gle  data  unit even in a UTF mode) to appear in	lookbehind assertions,
       because it makes	it impossible to calculate the length of  the  lookbe-
       hind.  The \X and \R escapes, which can match different numbers of data
       units, are also not permitted.

       "Subroutine" calls (see below) such as (?2) or (?&X) are	 permitted  in
       lookbehinds,  as	 long as the subpattern	matches	a fixed-length string.
       Recursion, however, is not supported.

       Possessive quantifiers can be used in conjunction with  lookbehind  as-
       sertions	 to  specify efficient matching	of fixed-length	strings	at the
       end of subject strings. Consider	a simple pattern such as

	 abcd$

       when applied to a long string that does	not  match.  Because  matching
       proceeds	from left to right, PCRE will look for each "a"	in the subject
       and then	see if what follows matches the	rest of	the  pattern.  If  the
       pattern is specified as

	 ^.*abcd$

       the  initial .* matches the entire string at first, but when this fails
       (because	there is no following "a"), it backtracks to match all but the
       last  character,	 then all but the last two characters, and so on. Once
       again the search	for "a"	covers the entire string, from right to	 left,
       so we are no better off.	However, if the	pattern	is written as

	 ^.*+(?<=abcd)

       there  can  be  no backtracking for the .*+ item; it can	match only the
       entire string. The subsequent lookbehind	assertion does a  single  test
       on  the last four characters. If	it fails, the match fails immediately.
       For long	strings, this approach makes a significant difference  to  the
       processing time.

   Using multiple assertions
       Several assertions (of any sort)	may occur in succession. For example,

	 (?<=\d{3})(?<!999)foo

       matches	"foo" preceded by three	digits that are	not "999". Notice that
       each of the assertions is applied independently at the  same  point  in
       the  subject  string.  First  there  is a check that the	previous three
       characters are all digits, and then there is  a	check  that  the  same
       three characters	are not	"999".	This pattern does not match "foo" pre-
       ceded by	six characters,	the first of which are	digits	and  the  last
       three  of  which	 are not "999".	For example, it	doesn't	match "123abc-
       foo". A pattern to do that is

	 (?<=\d{3}...)(?<!999)foo

       This time the first assertion looks at the  preceding  six  characters,
       checking	that the first three are digits, and then the second assertion
       checks that the preceding three characters are not "999".

       Assertions can be nested	in any combination. For	example,

	 (?<=(?<!foo)bar)baz

       matches an occurrence of	"baz" that is preceded by "bar"	which in  turn
       is not preceded by "foo", while

	 (?<=\d{3}(?!999)...)foo

       is  another pattern that	matches	"foo" preceded by three	digits and any
       three characters	that are not "999".

CONDITIONAL SUBPATTERNS
       It is possible to cause the matching process to obey a subpattern  con-
       ditionally  or to choose	between	two alternative	subpatterns, depending
       on the result of	an assertion, or whether a specific capturing  subpat-
       tern  has  already  been	matched. The two possible forms	of conditional
       subpattern are:

	 (?(condition)yes-pattern)
	 (?(condition)yes-pattern|no-pattern)

       If the condition	is satisfied, the yes-pattern is used;	otherwise  the
       no-pattern  (if	present)  is used. If there are	more than two alterna-
       tives in	the subpattern,	a compile-time error occurs. Each of  the  two
       alternatives may	itself contain nested subpatterns of any form, includ-
       ing conditional subpatterns; the	restriction to	two  alternatives  ap-
       plies  only  at the level of the	condition. This	pattern	fragment is an
       example where the alternatives are complex:

	 (?(1) (A|B|C) | (D | (?(2)E|F)	| E) )

       There are four kinds of condition: references  to  subpatterns,	refer-
       ences to	recursion, a pseudo-condition called DEFINE, and assertions.

   Checking for	a used subpattern by number
       If  the	text between the parentheses consists of a sequence of digits,
       the condition is	true if	a capturing subpattern of that number has pre-
       viously	matched.  If  there is more than one capturing subpattern with
       the same	number (see the	earlier	 section  about	 duplicate  subpattern
       numbers),  the condition	is true	if any of them have matched. An	alter-
       native notation is to precede the digits	with a plus or minus sign.  In
       this  case, the subpattern number is relative rather than absolute. The
       most recently opened parentheses	can be referenced by (?(-1), the  next
       most  recent  by	(?(-2),	and so on. Inside loops	it can also make sense
       to refer	to subsequent groups. The next parentheses to be opened	can be
       referenced  as (?(+1), and so on. (The value zero in any	of these forms
       is not used; it provokes	a compile-time error.)

       Consider	the following pattern, which  contains	non-significant	 white
       space to	make it	more readable (assume the PCRE_EXTENDED	option)	and to
       divide it into three parts for ease of discussion:

	 ( \( )?    [^()]+    (?(1) \) )

       The first part matches an optional opening  parenthesis,	 and  if  that
       character is present, sets it as	the first captured substring. The sec-
       ond part	matches	one or more characters that are	not  parentheses.  The
       third  part  is	a conditional subpattern that tests whether or not the
       first set of parentheses	matched. If they  did,	that  is,  if  subject
       started	with an	opening	parenthesis, the condition is true, and	so the
       yes-pattern is executed and a closing parenthesis is  required.	Other-
       wise,  since no-pattern is not present, the subpattern matches nothing.
       In other	words, this pattern matches a sequence of non-parentheses, op-
       tionally	enclosed in parentheses.

       If  you	were  embedding	 this pattern in a larger one, you could use a
       relative	reference:

	 ...other stuff... ( \(	)?    [^()]+	(?(-1) \) ) ...

       This makes the fragment independent of the parentheses  in  the	larger
       pattern.

   Checking for	a used subpattern by name
       Perl  uses  the	syntax	(?(<name>)...) or (?('name')...) to test for a
       used subpattern by name.	For compatibility  with	 earlier  versions  of
       PCRE,  which  had this facility before Perl, the	syntax (?(name)...) is
       also recognized.

       Rewriting the above example to use a named subpattern gives this:

	 (?<OPEN> \( )?	   [^()]+    (?(<OPEN>)	\) )

       If the name used	in a condition of this kind is a duplicate,  the  test
       is  applied to all subpatterns of the same name,	and is true if any one
       of them has matched.

   Checking for	pattern	recursion
       If the condition	is the string (R), and there is	no subpattern with the
       name  R,	the condition is true if a recursive call to the whole pattern
       or any subpattern has been made.	If digits or a name preceded by	amper-
       sand follow the letter R, for example:

	 (?(R3)...) or (?(R&name)...)

       the condition is	true if	the most recent	recursion is into a subpattern
       whose number or name is given. This condition does not check the	entire
       recursion  stack. If the	name used in a condition of this kind is a du-
       plicate,	the test is applied to all subpatterns of the same  name,  and
       is true if any one of them is the most recent recursion.

       At  "top	 level",  all  these recursion test conditions are false.  The
       syntax for recursive patterns is	described below.

   Defining subpatterns	for use	by reference only
       If the condition	is the string (DEFINE),	and  there  is	no  subpattern
       with  the  name	DEFINE,	 the  condition	is always false. In this case,
       there may be only one alternative  in  the  subpattern.	It  is	always
       skipped	if  control reaches this point in the pattern; the idea	of DE-
       FINE is that it can be used to define subroutines that  can  be	refer-
       enced  from elsewhere. (The use of subroutines is described below.) For
       example,	a pattern to match an IPv4 address  such  as  "192.168.23.245"
       could be	written	like this (ignore white	space and line breaks):

	 (?(DEFINE) (?<byte> 2[0-4]\d |	25[0-5]	| 1\d\d	| [1-9]?\d) )
	 \b (?&byte) (\.(?&byte)){3} \b

       The  first part of the pattern is a DEFINE group	inside which a another
       group named "byte" is defined. This matches an individual component  of
       an  IPv4	 address  (a number less than 256). When matching takes	place,
       this part of the	pattern	is skipped because DEFINE acts	like  a	 false
       condition.  The	rest of	the pattern uses references to the named group
       to match	the four dot-separated components of an	IPv4 address,  insist-
       ing on a	word boundary at each end.

   Assertion conditions
       If  the condition is not	in any of the above formats, it	must be	an as-
       sertion.	 This may be a positive	or negative  lookahead	or  lookbehind
       assertion.  Consider  this  pattern,  again  containing non-significant
       white space, and	with the two alternatives on the second	line:

	 (?(?=[^a-z]*[a-z])
	 \d{2}-[a-z]{3}-\d{2}  |  \d{2}-\d{2}-\d{2} )

       The condition is	a positive lookahead assertion	that  matches  an  op-
       tional sequence of non-letters followed by a letter. In other words, it
       tests for the presence of at least one letter in	the subject. If	a let-
       ter  is	found,	the  subject is	matched	against	the first alternative;
       otherwise it is	matched	 against  the  second.	This  pattern  matches
       strings	in  one	 of the	two forms dd-aaa-dd or dd-dd-dd, where aaa are
       letters and dd are digits.

COMMENTS
       There are two ways of including comments	in patterns that are processed
       by PCRE.	In both	cases, the start of the	comment	must not be in a char-
       acter class, nor	in the middle of any other sequence of related charac-
       ters  such  as  (?: or a	subpattern name	or number. The characters that
       make up a comment play no part in the pattern matching.

       The sequence (?#	marks the start	of a comment that continues up to  the
       next  closing parenthesis. Nested parentheses are not permitted.	If the
       PCRE_EXTENDED option is set, an unescaped # character also introduces a
       comment,	 which	in  this  case continues to immediately	after the next
       newline character or character sequence in the pattern.	Which  charac-
       ters are	interpreted as newlines	is controlled by the options passed to
       a compiling function or by a special sequence at	the start of the  pat-
       tern, as	described in the section entitled "Newline conventions"	above.
       Note that the end of this type of comment is a literal newline sequence
       in  the pattern;	escape sequences that happen to	represent a newline do
       not count. For example, consider	this  pattern  when  PCRE_EXTENDED  is
       set, and	the default newline convention is in force:

	 abc #comment \n still comment

       On  encountering	 the  #	character, pcre_compile() skips	along, looking
       for a newline in	the pattern. The sequence \n is	still literal at  this
       stage,  so  it does not terminate the comment. Only an actual character
       with the	code value 0x0a	(the default newline) does so.

RECURSIVE PATTERNS
       Consider	the problem of matching	a string in parentheses, allowing  for
       unlimited  nested  parentheses.	Without	the use	of recursion, the best
       that can	be done	is to use a pattern that  matches  up  to  some	 fixed
       depth  of  nesting.  It	is not possible	to handle an arbitrary nesting
       depth.

       For some	time, Perl has provided	a facility that	allows regular expres-
       sions  to recurse (amongst other	things). It does this by interpolating
       Perl code in the	expression at run time,	and the	code can refer to  the
       expression itself. A Perl pattern using code interpolation to solve the
       parentheses problem can be created like this:

	 $re = qr{\( (?: (?>[^()]+) | (?p{$re})	)* \)}x;

       The (?p{...}) item interpolates Perl code at run	time, and in this case
       refers recursively to the pattern in which it appears.

       Obviously, PCRE cannot support the interpolation	of Perl	code. Instead,
       it supports special syntax for recursion	of  the	 entire	 pattern,  and
       also  for  individual  subpattern  recursion. After its introduction in
       PCRE and	Python,	this kind of  recursion	 was  subsequently  introduced
       into Perl at release 5.10.

       A  special  item	 that consists of (? followed by a number greater than
       zero and	a closing parenthesis is a recursive subroutine	 call  of  the
       subpattern  of  the  given  number, provided that it occurs inside that
       subpattern. (If not, it is a non-recursive subroutine  call,  which  is
       described  in the next section.)	The special item (?R) or (?0) is a re-
       cursive call of the entire regular expression.

       This PCRE pattern solves	the nested  parentheses	 problem  (assume  the
       PCRE_EXTENDED option is set so that white space is ignored):

	 \( ( [^()]++ |	(?R) )*	\)

       First  it matches an opening parenthesis. Then it matches any number of
       substrings which	can either be a	sequence of non-parentheses, or	a  re-
       cursive match of	the pattern itself (that is, a correctly parenthesized
       substring).  Finally there is a closing parenthesis. Note the use of  a
       possessive  quantifier  to  avoid  backtracking	into sequences of non-
       parentheses.

       If this were part of a larger pattern, you would	not  want  to  recurse
       the entire pattern, so instead you could	use this:

	 ( \( (	[^()]++	| (?1) )* \) )

       We  have	 put the pattern into parentheses, and caused the recursion to
       refer to	them instead of	the whole pattern.

       In a larger pattern,  keeping  track  of	 parenthesis  numbers  can  be
       tricky.	This is	made easier by the use of relative references. Instead
       of (?1) in the pattern above you	can write (?-2)	to refer to the	second
       most  recently  opened  parentheses  preceding  the recursion. In other
       words, a	negative number	counts capturing  parentheses  leftwards  from
       the point at which it is	encountered.

       It  is  also  possible  to refer	to subsequently	opened parentheses, by
       writing references such as (?+2). However, these	 cannot	 be  recursive
       because	the  reference	is  not	inside the parentheses that are	refer-
       enced. They are always non-recursive subroutine calls, as described  in
       the next	section.

       An  alternative	approach is to use named parentheses instead. The Perl
       syntax for this is (?&name); PCRE's earlier syntax  (?P>name)  is  also
       supported. We could rewrite the above example as	follows:

	 (?<pn>	\( ( [^()]++ | (?&pn) )* \) )

       If  there  is more than one subpattern with the same name, the earliest
       one is used.

       This particular example pattern that we have been looking  at  contains
       nested unlimited	repeats, and so	the use	of a possessive	quantifier for
       matching	strings	of non-parentheses is important	when applying the pat-
       tern  to	 strings  that do not match. For example, when this pattern is
       applied to

	 (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()

       it yields "no match" quickly. However, if a  possessive	quantifier  is
       not  used, the match runs for a very long time indeed because there are
       so many different ways the + and	* repeats can carve  up	 the  subject,
       and all have to be tested before	failure	can be reported.

       At  the	end  of	a match, the values of capturing parentheses are those
       from the	outermost level. If you	want to	obtain intermediate values,  a
       callout	function can be	used (see below	and the	pcrecallout documenta-
       tion). If the pattern above is matched against

	 (ab(cd)ef)

       the value for the inner capturing parentheses  (numbered	 2)  is	 "ef",
       which  is the last value	taken on at the	top level. If a	capturing sub-
       pattern is not matched at the top level,	its final  captured  value  is
       unset,  even  if	 it was	(temporarily) set at a deeper level during the
       matching	process.

       If there	are more than 15 capturing parentheses in a pattern, PCRE  has
       to  obtain extra	memory to store	data during a recursion, which it does
       by using	pcre_malloc, freeing it	via pcre_free afterwards. If no	memory
       can be obtained,	the match fails	with the PCRE_ERROR_NOMEMORY error.

       Do  not	confuse	 the (?R) item with the	condition (R), which tests for
       recursion.  Consider this pattern, which	matches	text in	 angle	brack-
       ets,  allowing for arbitrary nesting. Only digits are allowed in	nested
       brackets	(that is, when recursing), whereas any characters are  permit-
       ted at the outer	level.

	 < (?: (?(R) \d++  | [^<>]*+) |	(?R)) *	>

       In  this	 pattern, (?(R)	is the start of	a conditional subpattern, with
       two different alternatives for the recursive and	 non-recursive	cases.
       The (?R)	item is	the actual recursive call.

   Differences in recursion processing between PCRE and	Perl
       Recursion  processing  in PCRE differs from Perl	in two important ways.
       In PCRE (like Python, but unlike	Perl), a recursive subpattern call  is
       always treated as an atomic group. That is, once	it has matched some of
       the subject string, it is never re-entered, even	if it contains untried
       alternatives  and  there	 is a subsequent matching failure. This	can be
       illustrated by the following pattern, which purports to match a	palin-
       dromic  string  that contains an	odd number of characters (for example,
       "a", "aba", "abcba", "abcdcba"):

	 ^(.|(.)(?1)\2)$

       The idea	is that	it either matches a single character, or two identical
       characters  surrounding	a sub-palindrome. In Perl, this	pattern	works;
       in PCRE it does not if the pattern is  longer  than  three  characters.
       Consider	the subject string "abcba":

       At  the	top level, the first character is matched, but as it is	not at
       the end of the string, the first	alternative fails; the second alterna-
       tive is taken and the recursion kicks in. The recursive call to subpat-
       tern 1 successfully matches the next character ("b").  (Note  that  the
       beginning and end of line tests are not part of the recursion).

       Back  at	 the top level,	the next character ("c") is compared with what
       subpattern 2 matched, which was "a". This fails.	Because	the  recursion
       is  treated  as	an atomic group, there are now no backtracking points,
       and so the entire match fails. (Perl is able, at	this point, to	re-en-
       ter the recursion and try the second alternative.) However, if the pat-
       tern is written with the	alternatives in	the other  order,  things  are
       different:

	 ^((.)(?1)\2|.)$

       This  time,  the	recursing alternative is tried first, and continues to
       recurse until it	runs out of characters,	at which point	the  recursion
       fails.  But  this  time	we  do	have another alternative to try	at the
       higher level. That is the big difference: in the	previous case the  re-
       maining	alternative  is	at a deeper recursion level, which PCRE	cannot
       use.

       To change the pattern so	that it	matches	all palindromic	 strings,  not
       just  those  with an odd	number of characters, it is tempting to	change
       the pattern to this:

	 ^((.)(?1)\2|.?)$

       Again, this works in Perl, but not in PCRE, and for  the	 same  reason.
       When  a	deeper	recursion has matched a	single character, it cannot be
       entered again in	order to match an empty	string.	 The  solution	is  to
       separate	 the two cases,	and write out the odd and even cases as	alter-
       natives at the higher level:

	 ^(?:((.)(?1)\2|)|((.)(?3)\4|.))

       If you want to match typical palindromic	phrases, the  pattern  has  to
       ignore all non-word characters, which can be done like this:

	 ^\W*+(?:((.)\W*+(?1)\W*+\2|)|((.)\W*+(?3)\W*+\4|\W*+.\W*+))\W*+$

       If run with the PCRE_CASELESS option, this pattern matches phrases such
       as "A man, a plan, a canal: Panama!" and	it works well in both PCRE and
       Perl.  Note the use of the possessive quantifier	*+ to avoid backtrack-
       ing into	sequences of non-word characters. Without this,	PCRE  takes  a
       great  deal  longer  (ten  times	or more) to match typical phrases, and
       Perl takes so long that you think it has	gone into a loop.

       WARNING:	The palindrome-matching	patterns above work only if  the  sub-
       ject  string  does not start with a palindrome that is shorter than the
       entire string.  For example, although "abcba" is	correctly matched,  if
       the  subject  is	"ababa", PCRE finds the	palindrome "aba" at the	start,
       then fails at top level because the end of the string does not  follow.
       Once  again, it cannot jump back	into the recursion to try other	alter-
       natives,	so the entire match fails.

       The second way in which PCRE and	Perl differ in	their  recursion  pro-
       cessing	is in the handling of captured values. In Perl,	when a subpat-
       tern is called recursively or as	a subpattern (see the  next  section),
       it  has	no  access to any values that were captured outside the	recur-
       sion, whereas in	PCRE these values can  be  referenced.	Consider  this
       pattern:

	 ^(.)(\1|a(?2))

       In  PCRE,  this	pattern	matches	"bab". The first capturing parentheses
       match "b", then in the second group, when the back reference  \1	 fails
       to  match "b", the second alternative matches "a" and then recurses. In
       the recursion, \1 does now match	"b" and	so the whole  match  succeeds.
       In  Perl,  the pattern fails to match because inside the	recursive call
       \1 cannot access	the externally set value.

SUBPATTERNS AS SUBROUTINES
       If the syntax for a recursive subpattern	call (either by	number	or  by
       name)  is  used outside the parentheses to which	it refers, it operates
       like a subroutine in a programming language. The	called subpattern  may
       be  defined  before or after the	reference. A numbered reference	can be
       absolute	or relative, as	in these examples:

	 (...(absolute)...)...(?2)...
	 (...(relative)...)...(?-1)...
	 (...(?+1)...(relative)...

       An earlier example pointed out that the pattern

	 (sens|respons)e and \1ibility

       matches "sense and sensibility" and "response and responsibility",  but
       not "sense and responsibility". If instead the pattern

	 (sens|respons)e and (?1)ibility

       is  used, it does match "sense and responsibility" as well as the other
       two strings. Another example is	given  in  the	discussion  of	DEFINE
       above.

       All  subroutine	calls, whether recursive or not, are always treated as
       atomic groups. That is, once a subroutine has matched some of the  sub-
       ject string, it is never	re-entered, even if it contains	untried	alter-
       natives and there is  a	subsequent  matching  failure.	Any  capturing
       parentheses  that  are  set  during the subroutine call revert to their
       previous	values afterwards.

       Processing options such as case-independence are	fixed when  a  subpat-
       tern  is	defined, so if it is used as a subroutine, such	options	cannot
       be changed for different	calls. For example, consider this pattern:

	 (abc)(?i:(?-1))

       It matches "abcabc". It does not	match "abcABC" because the  change  of
       processing option does not affect the called subpattern.

ONIGURUMA SUBROUTINE SYNTAX
       For  compatibility with Oniguruma, the non-Perl syntax \g followed by a
       name or a number	enclosed either	in angle brackets or single quotes, is
       an  alternative	syntax	for  referencing a subpattern as a subroutine,
       possibly	recursively. Here are two of the examples used above,  rewrit-
       ten using this syntax:

	 (?<pn>	\( ( (?>[^()]+)	| \g<pn> )* \) )
	 (sens|respons)e and \g'1'ibility

       PCRE  supports  an extension to Oniguruma: if a number is preceded by a
       plus or a minus sign it is taken	as a relative reference. For example:

	 (abc)(?i:\g<-1>)

       Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are  not
       synonymous.  The	former is a back reference; the	latter is a subroutine
       call.

CALLOUTS
       Perl has	a feature whereby using	the sequence (?{...}) causes arbitrary
       Perl  code to be	obeyed in the middle of	matching a regular expression.
       This makes it possible, amongst other things, to	extract	different sub-
       strings that match the same pair	of parentheses when there is a repeti-
       tion.

       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 function by putting its entry point in the global  variable
       pcre_callout  (8-bit  library) or pcre[16|32]_callout (16-bit or	32-bit
       library).  By default, this variable contains NULL, which disables  all
       calling out.

       Within a	regular	expression, (?C) indicates the points at which the ex-
       ternal function is to be	called.	If  you	 want  to  identify  different
       callout	points,	you can	put a number less than 256 after the letter C.
       The default value is zero.  For example,	this pattern has  two  callout
       points:

	 (?C1)abc(?C2)def

       If  the PCRE_AUTO_CALLOUT flag is passed	to a compiling function, call-
       outs are	automatically installed	before each item in the	pattern.  They
       are  all	 numbered  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 posi-
       tion, as	in this	example:

	 (?(?C9)(?=a)abc|def)

       Note that this applies only to assertion	conditions, not	to other types
       of condition.

       During  matching, when PCRE reaches a callout point, the	external func-
       tion is called. It is provided with the number of the callout, the  po-
       sition  in  the	pattern,  and, optionally, one item of data originally
       supplied	by the caller of the matching function.	The  callout  function
       may cause matching to proceed, to backtrack, or to fail altogether.

       By  default,  PCRE implements a number of optimizations at compile time
       and matching time, and one side-effect is that sometimes	 callouts  are
       skipped.	 If  you need all possible callouts to happen, you need	to set
       options that disable the	relevant optimizations.	More  details,	and  a
       complete	 description  of  the  interface  to the callout function, are
       given in	the pcrecallout	documentation.

BACKTRACKING CONTROL
       Perl 5.10 introduced a number of	"Special Backtracking Control  Verbs",
       which  are  still  described in the Perl	documentation as "experimental
       and subject to change or	removal	in a future version of Perl". It  goes
       on  to  say:  "Their  usage in production code should be	noted to avoid
       problems	during upgrades." The same remarks apply to the	PCRE  features
       described in this section.

       The  new	verbs make use of what was previously invalid syntax: an open-
       ing 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.

       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.

       The  behaviour  of  these  verbs	in repeated groups, assertions,	and in
       subpatterns called as subroutines (whether or not recursively) is docu-
       mented below.

   Optimizations that affect backtracking verbs
       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 minimum length of	matching subject, or that a particular
       character must be 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 the start-of-match optimizations
       by  setting  the	 PCRE_NO_START_OPTIMIZE	 option	when calling pcre_com-
       pile() or pcre_exec(), or by starting the pattern with (*NO_START_OPT).
       There is	more discussion	of this	option in the section entitled "Option
       bits for	pcre_exec()" in	the pcreapi documentation.

       Experiments with	Perl suggest that it too  has  similar	optimizations,
       sometimes leading to anomalous results.

   Verbs that act immediately
       The  following  verbs act as soon as they are encountered. They may not
       be followed by a	name.

	  (*ACCEPT)

       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 subroutine,	only that subpattern is	ended  successfully.  Matching
       then continues at the outer level. If (*ACCEPT) in triggered in a posi-
       tive assertion, the assertion succeeds; in a  negative  assertion,  the
       assertion fails.

       If  (*ACCEPT)  is inside	capturing parentheses, the data	so far is cap-
       tured. For example:

	 A((?:A|B(*ACCEPT)|C)D)

       This matches "AB", "AAD", or "ACD"; when	it matches "AB", "B"  is  cap-
       tured by	the outer parentheses.

	 (*FAIL) or (*F)

       This  verb causes a matching failure, forcing backtracking to occur. It
       is equivalent to	(?!) but easier	to read. The Perl documentation	 notes
       that  it	 is  probably  useful only when	combined with (?{}) or (??{}).
       Those are, of course, Perl features that	are not	present	in  PCRE.  The
       nearest	equivalent is the callout feature, as for example in this pat-
       tern:

	 a+(?C)(*FAIL)

       A match with the	string "aaaa" always fails, but	the callout  is	 taken
       before each backtrack happens (in this example, 10 times).

   Recording which path	was taken
       There  is  one  verb whose main purpose is to track how a match was ar-
       rived at, though	it also	has a secondary	use in	conjunction  with  ad-
       vancing the match starting point	(see (*SKIP) below).

	 (*MARK:NAME) or (*:NAME)

       A  name	is  always  required  with this	verb. There may	be as many in-
       stances of (*MARK) as you like in a pattern, and	 their	names  do  not
       have to be unique.

       When  a	match succeeds,	the name of the	last-encountered (*MARK:NAME),
       (*PRUNE:NAME), or (*THEN:NAME) on the matching path is passed  back  to
       the  caller  as	described  in  the  section  entitled  "Extra data for
       pcre_exec()" in the  pcreapi  documentation.  Here  is  an  example  of
       pcretest	 output, where the /K modifier requests	the retrieval and out-
       putting of (*MARK) data:

	   re> /X(*MARK:A)Y|X(*MARK:B)Z/K
	 data> XY
	  0: XY
	 MK: A
	 XZ
	  0: XZ
	 MK: B

       The (*MARK) name	is tagged with "MK:" in	this output, and in this exam-
       ple  it indicates which of the two alternatives matched.	This is	a more
       efficient way of	obtaining this information than	putting	each  alterna-
       tive in its own capturing parentheses.

       If  a  verb  with a name	is encountered in a positive assertion that is
       true, the name is recorded and passed back if it	 is  the  last-encoun-
       tered. This does	not happen for negative	assertions or failing positive
       assertions.

       After a partial match or	a failed match,	the last encountered  name  in
       the entire match	process	is returned. For example:

	   re> /X(*MARK:A)Y|X(*MARK:B)Z/K
	 data> XP
	 No match, mark	= B

       Note  that  in  this  unanchored	 example the mark is retained from the
       match attempt that started at the letter	"X" in the subject. Subsequent
       match attempts starting at "P" and then with an empty string do not get
       as far as the (*MARK) item, but nevertheless do not reset it.

       If you are interested in	 (*MARK)  values  after	 failed	 matches,  you
       should  probably	 set  the PCRE_NO_START_OPTIMIZE option	(see above) to
       ensure that the match is	always attempted.

   Verbs that act after	backtracking
       The following verbs do nothing when they	are encountered. Matching con-
       tinues  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. However, when one	of these verbs
       appears inside an atomic	group or an assertion that is true, its	effect
       is  confined  to	 that  group, because once the group has been matched,
       there is	never any backtracking into it.	In this	situation,  backtrack-
       ing  can	 "jump	back" to the left of the entire	atomic group or	asser-
       tion. (Remember also, as	stated above, that this	localization also  ap-
       plies in	subroutine calls.)

       These  verbs  differ  in	exactly	what kind of failure occurs when back-
       tracking	reaches	them. The behaviour described below  is	 what  happens
       when  the  verb is not in a subroutine or an assertion. Subsequent sec-
       tions cover these special cases.

	 (*COMMIT)

       This verb, which	may not	be followed by a name, causes the whole	 match
       to fail outright	if there is a later matching failure that causes back-
       tracking	to reach it. Even if the pattern is unanchored,	no further at-
       tempts  to  find	a match	by advancing the starting point	take place. If
       (*COMMIT) is the	only backtracking verb that is	encountered,  once  it
       has been	passed pcre_exec() is committed	to finding a match at the cur-
       rent starting point, or not at all. For example:

	 a+(*COMMIT)b

       This matches "xxaab" but	not "aacaab". It can be	thought	of as  a  kind
       of dynamic anchor, or "I've started, so I must finish." The name	of the
       most recently passed (*MARK) in the path	is passed back when  (*COMMIT)
       forces a	match failure.

       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.

       Note that (*COMMIT) at the start	of a pattern is	not the	same as	an an-
       chor,  unless  PCRE's  start-of-match  optimizations are	turned off, as
       shown in	this output from pcretest:

	   re> /(*COMMIT)abc/
	 data> xyzabc
	  0: abc
	 data> xyzabc\Y
	 No match

       For this	pattern, PCRE knows that any match must	start with "a",	so the
       optimization skips along	the subject to "a" before applying the pattern
       to the first set	of data. The match attempt then	succeeds. In the  sec-
       ond  set	of data, the escape sequence \Y	is interpreted by the pcretest
       program.	It causes the PCRE_NO_START_OPTIMIZE option  to	 be  set  when
       pcre_exec() is called.  This disables the optimization that skips along
       to the first character. The pattern is now applied starting at "x", and
       so  the	(*COMMIT)  causes  the	match to fail without trying any other
       starting	points.

	 (*PRUNE) or (*PRUNE:NAME)

       This verb causes	the match to fail at the current starting position  in
       the subject if there is a later matching	failure	that causes backtrack-
       ing to reach it.	If the pattern is unanchored, the  normal  "bumpalong"
       advance	to  the	next starting character	then happens. Backtracking can
       occur as	usual to the left of (*PRUNE), before it is reached,  or  when
       matching	 to  the  right	 of  (*PRUNE), but if there is no match	to the
       right, backtracking cannot cross	(*PRUNE). In simple cases, the use  of
       (*PRUNE)	 is just an alternative	to an atomic group or possessive quan-
       tifier, but there are some uses of (*PRUNE) that	cannot be expressed in
       any  other  way.	In an anchored pattern (*PRUNE)	has the	same effect as
       (*COMMIT).

       The   behaviour	 of   (*PRUNE:NAME)   is   the	 not   the   same   as
       (*MARK:NAME)(*PRUNE).   It is like (*MARK:NAME) in that the name	is re-
       membered	for passing back to the	caller.	However, (*SKIP:NAME) searches
       only for	names set with (*MARK).

	 (*SKIP)

       This  verb, when	given without a	name, is like (*PRUNE),	except that if
       the pattern is unanchored, the "bumpalong" advance is not to  the  next
       character, but to the position in the subject where (*SKIP) was encoun-
       tered. (*SKIP) signifies	that whatever text was matched leading	up  to
       it cannot be part of a successful match.	Consider:

	 a+(*SKIP)b

       If  the	subject	 is  "aaaac...",  after	 the first match attempt fails
       (starting at the	first character	in the	string),  the  starting	 point
       skips on	to start the next attempt at "c". Note that a possessive quan-
       tifer does not have the same effect as this example; although it	 would
       suppress	 backtracking  during  the first match attempt,	the second at-
       tempt would start at the	second character instead  of  skipping	on  to
       "c".

	 (*SKIP:NAME)

       When (*SKIP) has	an associated name, its	behaviour is modified. When it
       is triggered, the previous path through the pattern is searched for the
       most  recent  (*MARK)  that  has	 the  same  name. If one is found, the
       "bumpalong" advance is to the subject position that corresponds to that
       (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with
       a matching name is found, the (*SKIP) is	ignored.

       Note that (*SKIP:NAME) searches only for	names set by (*MARK:NAME).  It
       ignores names that are set by (*PRUNE:NAME) or (*THEN:NAME).

	 (*THEN) or (*THEN:NAME)

       This  verb  causes  a skip to the next innermost	alternative when back-
       tracking	reaches	it. That  is,  it  cancels  any	 further  backtracking
       within  the  current  alternative.  Its name comes from the observation
       that it can be used for a pattern-based if-then-else block:

	 ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...

       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 second alternative and tries COND2,	 without  backtracking
       into  COND1.  If	that succeeds and BAR fails, COND3 is tried. If	subse-
       quently BAZ fails, there	are no more alternatives, so there is a	 back-
       track  to  whatever came	before the entire group. If (*THEN) is not in-
       side an alternation, it acts like (*PRUNE).

       The   behaviour	 of   (*THEN:NAME)   is	  the	not   the   same    as
       (*MARK:NAME)(*THEN).   It  is like (*MARK:NAME) in that the name	is re-
       membered	for passing back to the	caller.	However, (*SKIP:NAME) searches
       only for	names set with (*MARK).

       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 al-
       ternative.  The	effect	of (*THEN) extends beyond such a subpattern to
       the enclosing alternative. Consider this	pattern, where A, B, etc.  are
       complex	pattern	fragments that do not contain any | characters at this
       level:

	 A (B(*THEN)C) | D

       If A and	B are matched, but there is a failure in C, matching does  not
       backtrack into A; instead it moves to the next alternative, that	is, D.
       However,	if the subpattern containing (*THEN) is	given an  alternative,
       it behaves differently:

	 A (B(*THEN)C |	(*FAIL)) | D

       The  effect of (*THEN) is now confined to the inner subpattern. After a
       failure in C, matching moves to (*FAIL),	which causes the whole subpat-
       tern  to	 fail  because	there are no more alternatives to try. In this
       case, matching does now backtrack into A.

       Note that a conditional subpattern is not considered as having two  al-
       ternatives,  because only one is	ever used. In other words, the | char-
       acter in	a conditional subpattern has  a	 different  meaning.  Ignoring
       white space, consider:

	 ^.*? (?(?=a) a	| b(*THEN)c )

       If the subject is "ba", this pattern does not match. Because .*?	is un-
       greedy, it initially matches zero characters. The condition (?=a)  then
       fails,  the  character  "b"  is matched,	but "c"	is not.	At this	point,
       matching	does not backtrack to .*? as might perhaps  be	expected  from
       the  presence of	the | character. The conditional subpattern is part of
       the single alternative that comprises the whole	pattern,  and  so  the
       match  fails.  (If there	was a backtrack	into .*?, allowing it to match
       "b", the	match would succeed.)

       The verbs just described	provide	four different "strengths" of  control
       when subsequent matching	fails. (*THEN) is the weakest, carrying	on the
       match at	the next alternative. (*PRUNE) comes next, failing  the	 match
       at  the	current	starting position, but allowing	an advance to the next
       character (for an unanchored pattern). (*SKIP) is similar, except  that
       the advance may be more than one	character. (*COMMIT) is	the strongest,
       causing the entire match	to fail.

   More	than one backtracking verb
       If more than one	backtracking verb is present in	 a  pattern,  the  one
       that  is	 backtracked  onto first acts. For example, consider this pat-
       tern, where A, B, etc. are complex pattern fragments:

	 (A(*COMMIT)B(*THEN)C|ABD)

       If A matches but	B fails, the backtrack to (*COMMIT) causes the	entire
       match to	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:

	 ...(*COMMIT)(*PRUNE)...

       If there	is a matching failure to the right, backtracking onto (*PRUNE)
       causes it to be triggered, and its action is taken. There can never  be
       a backtrack onto	(*COMMIT).

   Backtracking	verbs in repeated groups
       PCRE  differs  from  Perl  in its handling of backtracking verbs	in re-
       peated groups. For example, consider:

	 /(a(*COMMIT)b)+ac/

       If the subject is "abac", Perl matches,	but  PCRE  fails  because  the
       (*COMMIT) in the	second repeat of the group acts.

   Backtracking	verbs in assertions
       (*FAIL)	in  an assertion has its normal	effect:	it forces an immediate
       backtrack.

       (*ACCEPT) in a positive assertion causes	the assertion to succeed with-
       out  any	 further processing. In	a negative assertion, (*ACCEPT)	causes
       the assertion to	fail without any further processing.

       The other backtracking verbs are	not treated specially if  they	appear
       in  a  positive assertion. In particular, (*THEN) skips to the next al-
       ternative in the	 innermost  enclosing  group  that  has	 alternations,
       whether or not this is within the assertion.

       Negative	 assertions  are,  however, different, in order	to ensure that
       changing	a positive assertion into a negative assertion changes its re-
       sult.  Backtracking into	(*COMMIT), (*SKIP), or (*PRUNE)	causes a nega-
       tive 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  be-
       haviour),  but  if  the	assertion  does	 not have such an alternative,
       (*THEN) behaves like (*PRUNE).

   Backtracking	verbs in subroutines
       These behaviours	occur whether or not the subpattern is	called	recur-
       sively.	Perl's treatment of subroutines	is different in	some cases.

       (*FAIL)	in  a subpattern called	as a subroutine	has its	normal effect:
       it forces an immediate backtrack.

       (*ACCEPT) in a subpattern called	as a subroutine	causes the  subroutine
       match  to succeed without any further processing. Matching then contin-
       ues after the subroutine	call.

       (*COMMIT), (*SKIP), and (*PRUNE)	in a subpattern	called as a subroutine
       cause the subroutine match to fail.

       (*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.

SEE ALSO
       pcreapi(3),  pcrecallout(3),  pcrematching(3),  pcresyntax(3), pcre(3),
       pcre16(3), pcre32(3).

AUTHOR
       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.

REVISION
       Last updated: 23	October	2016
       Copyright (c) 1997-2016 University of Cambridge.

PCRE 8.40			23 October 2016			PCREPATTERN(3)

NAME | PCRE REGULAR EXPRESSION DETAILS | SPECIAL START-OF-PATTERN ITEMS | EBCDIC CHARACTER CODES | CHARACTERS AND METACHARACTERS | BACKSLASH | CIRCUMFLEX AND DOLLAR | FULL STOP (PERIOD, DOT) AND \N | MATCHING A SINGLE DATA UNIT | SQUARE BRACKETS AND CHARACTER CLASSES | POSIX CHARACTER CLASSES | COMPATIBILITY FEATURE FOR WORD BOUNDARIES | VERTICAL BAR | INTERNAL OPTION SETTING | SUBPATTERNS | DUPLICATE SUBPATTERN NUMBERS | NAMED SUBPATTERNS | REPETITION | ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS | BACK REFERENCES | ASSERTIONS | CONDITIONAL SUBPATTERNS | COMMENTS | RECURSIVE PATTERNS | SUBPATTERNS AS SUBROUTINES | ONIGURUMA SUBROUTINE SYNTAX | CALLOUTS | BACKTRACKING CONTROL | SEE ALSO | AUTHOR | REVISION

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