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PCREPATTERN(3)							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
       Expressions", published by  O'Reilly,  covers  regular  expressions  in
       great  detail.  This  description  of  PCRE's  regular  expressions  is
       intended as reference material.

       The original operation of PCRE was on strings of	 one-byte  characters.
       However,	 there is now also support for UTF-8 character strings. To use
       this, PCRE must be built to include UTF-8 support, and  you  must  call
       pcre_compile()  or  pcre_compile2() with the PCRE_UTF8 option. There is
       also a special sequence that can be given at the start of a pattern:

	 (*UTF8)

       Starting a pattern with this sequence  is  equivalent  to  setting  the
       PCRE_UTF8  option.  This	 feature  is  not Perl-compatible. How setting
       UTF-8 mode affects pattern matching  is	mentioned  in  several	places
       below.  There  is  also	a summary of UTF-8 features in the pcreunicode
       page.

       Another special sequence that may appear at the start of a  pattern  or
       in combination with (*UTF8) 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  determine
       character types, instead of recognizing only characters with codes less
       than 128 via a lookup table.

       If a pattern starts with (*NO_START_OPT), it has	 the  same  effect  as
       setting the PCRE_NO_START_OPTIMIZE option either at compile or matching
       time. There are also some more of these special sequences that are con‐
       cerned with the handling of newlines; they are described below.

       The  remainder  of  this	 document discusses the patterns that are sup‐
       ported by PCRE when its main matching function, pcre_exec(),  is	 used.
       From   release	6.0,   PCRE   offers   a   second  matching  function,
       pcre_dfa_exec(), which matches using a different algorithm that is  not
       Perl-compatible. Some of the features discussed below are not available
       when pcre_dfa_exec() is used. The advantages and disadvantages  of  the
       alternative  function, and how it differs from the normal function, are
       discussed in the pcrematching page.

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  pcre_compile()  or
       pcre_compile2().	 For example, on a Unix system where LF is the default
       newline sequence, the pattern

	 (*CR)a.b

       changes the convention to CR. That pattern matches "a\nb" because LF is
       no  longer  a  newline. 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.

       The newline convention affects the interpretation of the dot  metachar‐
       acter  when  PCRE_DOTALL is not set, and also the behaviour of \N. How‐
       ever, 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	 "Newline sequences" below. A change of \R setting can be com‐
       bined with a change of newline convention.

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 UTF-8 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-8 support.

       The  power  of  regular	expressions  comes from the ability to include
       alternatives and repetitions in the pattern. These are encoded  in  the
       pattern by the use of 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 UTF-8 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,	whitespace  in
       the  pattern (other than in a character class) and characters between a
       # outside a character class and the next newline are ignored. An escap‐
       ing  backslash  can  be	used to include a whitespace 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
       sequences  in  PCRE, whereas in Perl, $ and @ cause variable interpola‐
       tion. 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
       sequences than the binary character it represents:

	 \a	   alarm, that is, the BEL character (hex 07)
	 \cx	   "control-x", where x is any ASCII character
	 \e	   escape (hex 1B)
	 \f	   formfeed (hex 0C)
	 \n	   linefeed (hex 0A)
	 \r	   carriage return (hex 0D)
	 \t	   tab (hex 09)
	 \ddd	   character with octal code ddd, or back reference
	 \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 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 \cz becomes hex 1A (z is 7A), but \c{ becomes hex 3B ({
       is  7B),	 while	\c; becomes hex 7B (; is 3B). If the byte following \c
       has a value greater than 127, a compile-time error occurs.  This	 locks
       out  non-ASCII  characters in both byte mode and UTF-8 mode. (When PCRE
       is compiled in EBCDIC mode, all byte values are	valid.	A  lower  case
       letter is converted to upper case, and then the 0xc0 bits are flipped.)

       By  default,  after  \x,	 from  zero to two hexadecimal digits are read
       (letters can be in upper or lower case). Any number of hexadecimal dig‐
       its  may	 appear between \x{ and }, but the value of the character code
       must be less than 256 in non-UTF-8 mode, and less than 2**31  in	 UTF-8
       mode.  That is, the maximum value in hexadecimal is 7FFFFFFF. Note that
       this is bigger than the largest Unicode code point, which is 10FFFF.

       If characters other than hexadecimal digits appear between \x{  and  },
       or if there is no terminating }, this form of escape is not recognized.
       Instead, the initial \x will be	interpreted  as	 a  basic  hexadecimal
       escape,	with  no  following  digits, giving a character whose value is
       zero.

       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).

       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
       sequence \0\x\07 specifies two binary zeros followed by a BEL character
       (code  value 7). Make sure you supply two digits after the initial zero
       if the pattern character that follows is itself an octal digit.

       The handling of a backslash followed by a digit other than 0 is compli‐
       cated.  Outside a character class, PCRE reads it and any following dig‐
       its as a decimal number. If the number is less than  10,	 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 is  greater  than  9
       and  there have not been that many capturing subpatterns, PCRE re-reads
       up to three octal digits following the backslash, and uses them to gen‐
       erate  a data character. Any subsequent digits stand for themselves. In
       non-UTF-8 mode, the value of a character specified  in  octal  must  be
       less  than  \400.  In  UTF-8 mode, values up to \777 are permitted. For
       example:

	 \040	is another way of writing a 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 byte consisting entirely of 1 bits
	 \81	is either a back reference, or a binary zero
		   followed by the two characters "8" and "1"

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

       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
       default, 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
       default,	 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 whitespace character
	 \H	any character that is not a horizontal whitespace character
	 \s	any whitespace character
	 \S	any character that is not a whitespace character
	 \v	any vertical whitespace character
	 \V	any character that is not a vertical whitespace 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 does not match the VT character (code
       11).  This makes it different from the the POSIX "space" class. The  \s
       characters  are	HT  (9), LF (10), FF (12), CR (13), and space (32). If
       "use locale;" is included in a Perl script, \s may match the VT charac‐
       ter. In PCRE, it never does.

       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  128
       are  used  for  accented letters, and these are then matched by \w. The
       use of locales with Unicode is discouraged.

       By default, in UTF-8 mode, characters  with  values  greater  than  128
       never  match  \d,  \s,  or  \w,	and always match \D, \S, and \W. These
       sequences retain their original meanings from before UTF-8 support  was
       available,  mainly for efficiency reasons. However, if PCRE is compiled
       with Unicode property support, and the PCRE_UCP option is set, the  be‐
       haviour	is  changed  so	 that Unicode properties are used to determine
       character types, as follows:

	 \d  any character that \p{Nd} matches (decimal digit)
	 \s  any character that \p{Z} matches, plus HT, LF, FF, CR
	 \w  any character that \p{L} or \p{N} matches, 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
       codepoints  in UTF-8 mode, whether or not PCRE_UCP is set. The horizon‐
       tal space characters are:

	 U+0009	    Horizontal tab
	 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
	 U+000B	    Vertical tab
	 U+000C	    Formfeed
	 U+000D	    Carriage return
	 U+0085	    Next line
	 U+2028	    Line separator
	 U+2029	    Paragraph separator

   Newline sequences

       Outside a character class, by default, the escape sequence  \R  matches
       any Unicode newline sequence. In 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
       below.  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 (formfeed, U+000C), CR (carriage
       return, U+000D), or NEL (next line, U+0085). The two-character sequence
       is treated as a single unit that cannot be split.

       In  UTF-8  mode, 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
       requested  via  the  PCRE_BSR_UNICODE  option.	It is also possible to
       specify these settings by starting a pattern string  with  one  of  the
       following sequences:

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

       These  override	the default and the options given to pcre_compile() or
       pcre_compile2(), but  they  can	be  overridden	by  options  given  to
       pcre_exec() or pcre_dfa_exec(). 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) or (*UCP) special sequences.
       Inside a character class, \R  is	 treated  as  an  unrecognized	escape
       sequence, and so matches the letter "R" by default, 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 not in 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	  an extended Unicode sequence

       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
       (described  in the next section).  Other Perl properties such as "InMu‐
       sicalSymbols" 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, Bengali, Bopomofo, Braille,
       Buginese,  Buhid,  Canadian_Aboriginal, Carian, Cham, Cherokee, Common,
       Coptic,	Cuneiform,  Cypriot,  Cyrillic,	 Deseret,  Devanagari,	 Egyp‐
       tian_Hieroglyphs,   Ethiopic,   Georgian,  Glagolitic,  Gothic,	Greek,
       Gujarati, Gurmukhi,  Han,  Hangul,  Hanunoo,  Hebrew,  Hiragana,	 Impe‐
       rial_Aramaic, Inherited, Inscriptional_Pahlavi, Inscriptional_Parthian,
       Javanese, Kaithi, Kannada, Katakana, Kayah_Li, Kharoshthi, Khmer,  Lao,
       Latin,  Lepcha,	Limbu,	Linear_B,  Lisu,  Lycian,  Lydian,  Malayalam,
       Meetei_Mayek, Mongolian, Myanmar, New_Tai_Lue, Nko, Ogham,  Old_Italic,
       Old_Persian,  Old_South_Arabian,	 Old_Turkic, Ol_Chiki, Oriya, Osmanya,
       Phags_Pa, Phoenician, Rejang, Runic,  Samaritan,	 Saurashtra,  Shavian,
       Sinhala,	 Sundanese,  Syloti_Nagri,  Syriac, Tagalog, Tagbanwa, Tai_Le,
       Tai_Tham, Tai_Viet, Tamil, Telugu,  Thaana,  Thai,  Tibetan,  Tifinagh,
       Ugaritic, Vai, 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 UTF-8 strings (see
       RFC 3629) and so cannot be tested by PCRE, unless UTF-8 validity check‐
       ing  has	 been  turned off (see the discussion of PCRE_NO_UTF8_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.

       The  \X	escape	matches	 any number of Unicode characters that form an
       extended Unicode sequence. \X is equivalent to

	 (?>\PM\pM*)

       That is, it matches a character without the "mark"  property,  followed
       by  zero	 or  more  characters with the "mark" property, and treats the
       sequence as an atomic group (see below).	 Characters  with  the	"mark"
       property	 are  typically	 accents  that affect the preceding character.
       None of them have codepoints less than 256, so  in  non-UTF-8  mode  \X
       matches any one character.

       Note that recent versions of Perl have changed \X to match what Unicode
       calls an "extended grapheme cluster", which has a more complicated def‐
       inition.

       Matching	 characters  by Unicode property is not fast, because PCRE has
       to search a structure that contains  data  for  over  fifteen  thousand
       characters. 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 for pcre_compile() or by
       starting the pattern with (*UCP).

   PCRE's additional properties

       As well as the standard Unicode properties described  in	 the  previous
       section,	 PCRE supports four more that make it possible to convert tra‐
       ditional escape sequences such as \w and \s and POSIX character classes
       to use Unicode properties. PCRE uses these non-standard, non-Perl prop‐
       erties internally when PCRE_UCP is set. They 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,
       formfeed, or carriage return, and any other character that  has	the  Z
       (separator) property.  Xsp is the same as Xps, except that vertical tab
       is excluded. Xwd matches the same characters as Xan, plus underscore.

   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
       defined".  In  PCRE,  \K	 is  acted upon when it occurs inside positive
       assertions, but is ignored in negative assertions.

   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_EXTRA option is set, an "invalid escape sequence" error is	gener‐
       ated instead.

       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
       UTF-8  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

       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.)

       A  dollar  character  is	 an assertion that is true only if the current
       matching point is at the end of	the  subject  string,  or  immediately
       before a newline at the end of the string (by default). 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. Dollar 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. In UTF-8 mode, the  matched  character  may  be
       more than one byte long.

       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
       exception. 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
       affected by the PCRE_DOTALL option. In  other  words,  it  matches  any
       character  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 BYTE

       Outside a character class, the escape sequence \C matches any one byte,
       both  in	 and  out of UTF-8 mode. Unlike a dot, it 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 usefully
       be used. Because \C breaks up characters into individual bytes,	match‐
       ing  one	 byte  with \C in UTF-8 mode means that the rest of the string
       may start with a malformed UTF-8 character. This has undefined results,
       because	PCRE  assumes that it is dealing with valid UTF-8 strings (and
       by default it checks  this  at  the  start  of  processing  unless  the
       PCRE_NO_UTF8_CHECK option is used).

       PCRE  does  not	allow \C to appear in lookbehind assertions (described
       below) in UTF-8 mode, because this would make it impossible  to	calcu‐
       late the length of the lookbehind.

       In  general, the \C escape sequence is best avoided in UTF-8 mode. How‐
       ever, one way of using it that avoids the problem  of  malformed	 UTF-8
       characters  is to use a lookahead to check the length of the next char‐
       acter, as in this pattern (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
       assertions 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
       character's individual bytes are then captured by the appropriate  num‐
       ber 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 UTF-8
       mode, the character may be more than one byte 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 mode, characters with values greater than 255 can be included
       in a class as a literal string of bytes, 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 UTF-8 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 UTF8-mode for characters 128 and above, you must
       ensure that PCRE is compiled with Unicode property support as  well  as
       with UTF-8 support.

       Characters  that	 might	indicate  line breaks are never treated in any
       special way  when  matching  character  classes,	 whatever  line-ending
       sequence	 is  in	 use,  and  whatever  setting  of  the PCRE_DOTALL and
       PCRE_MULTILINE 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
       between	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.

       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.

       Ranges  operate in the collating sequence of character values. They can
       also  be	 used  for  characters	specified  numerically,	 for   example
       [\000-\037].  In UTF-8 mode, ranges can include characters whose values
       are greater than 255, for example [\x{100}-\x{2ff}].

       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  non-UTF-8  mode,  if
       character  tables  for  a French locale are in use, [\xc8-\xcb] matches
       accented E characters in both cases. In UTF-8 mode, 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-8 mode, the PCRE_UCP option affects the meanings of
       \d, \s, \w and their upper case partners, just as  it  does  when  they
       appear  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 - see the
       next section), and the terminating  closing  square  bracket.  However,
       escaping other non-alphanumeric 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 (not quite the same as \s)
	 upper	  upper case letters
	 word	  "word" characters (same as \w)
	 xdigit	  hexadecimal digits

       The "space" characters are HT (9), LF (10), VT (11), FF (12), CR	 (13),
       and  space  (32). Notice that this list includes the VT character (code
       11). This makes "space" different to \s, which does not include VT (for
       Perl compatibility).

       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,  in UTF-8 mode, 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 replac‐
       ing the 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. The other
       POSIX classes are unchanged, and match only characters with code points
       less than 128.

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
       between "(?" 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
       hyphen, 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
       inside subpattern parentheses), the change applies to the remainder  of
       the pattern that follows. If the change is placed right at the start of
       a pattern, PCRE extracts it into the global options (and it will there‐
       fore show up in data extracted by the pcre_fullinfo() function).

       An  option  change  within a subpattern (see below 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
       application  when  the  compile	or match 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) and (*UCP) leading sequences that can be
       used to set UTF-8 and Unicode property modes; they  are	equivalent  to
       setting the PCRE_UTF8 and the PCRE_UCP options, respectively.

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 pcre_exec(). 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
       between 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.
       Named  capturing	 parentheses  are  still  allocated 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 one that corresponds to the first occur‐
       rence 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 details of
       the interfaces for handling named subpatterns, see the pcreapi documen‐
       tation.

       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 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 (in UTF-8 mode with Unicode properties)
	 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-8 mode, quantifiers apply to UTF-8  characters  rather  than  to
       individual bytes. Thus, for example, \x{100}{2} matches two UTF-8 char‐
       acters, each of which is represented by a two-byte sequence. Similarly,
       when Unicode property support is available, \X{3} matches three Unicode
       extended sequences, each of which may be several bytes 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 is one situation 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.

       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_UNGREEDY option is ignored. They are a convenient notation for the
       simpler	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
       example	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 charac‐
       ter 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
       sequence such as \50 is interpreted as a character  defined  in	octal.
       See the subsection entitled "Non-printing characters" above for further
       details of the handling of digits following a backslash.	 There	is  no
       such  problem  when named parentheses are used. A back reference to any
       subpattern 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
       example 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
       before 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
       delimiter  must	be  used  to  terminate	 the  back  reference.	If the
       PCRE_EXTENDED option is set, this can be whitespace. Otherwise, the \g{
       syntax 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
       described 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
       assertions, because it does not make sense for negative assertions.

       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
       always  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  UTF-8 mode, PCRE does not allow the \C escape (which matches a sin‐
       gle byte, even in UTF-8	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
       bytes, 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
       assertions 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
       applies 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,
       optionally 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. However, there is a possible ambiguity with this syn‐
       tax, because subpattern names may  consist  entirely  of	 digits.  PCRE
       looks  first for a named subpattern; if it cannot find one and the name
       consists entirely of digits, PCRE looks for a subpattern of  that  num‐
       ber,  which must be greater than zero. Using subpattern names that con‐
       sist entirely of digits is not recommended.

       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
       duplicate, 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
       DEFINE 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 whitespace 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
       assertion.  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
       optional	 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
       letter  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
       pcre_compile() or by a special sequence at the start of the pattern, 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
       recursive 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
       recursive  match	 of the pattern itself (that is, a correctly parenthe‐
       sized 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-
       enter  the  recursion  and try the second alternative.) However, if the
       pattern 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
       remaining 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.   By default, this variable contains NULL, which disables
       all calling out.

       Within a regular expression, (?C) indicates the	points	at  which  the
       external	 function  is  to be called. If you want to identify different
       callout points, you 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 pcre_compile(), callouts are
       automatically  installed	 before each item in the pattern. They are all
       numbered 255.

       During matching, when PCRE reaches a callout point (and pcre_callout is
       set),  the  external function is called. It is provided with the number
       of the callout, the position in the pattern, and, optionally, one  item
       of  data	 originally supplied by the caller of pcre_exec(). The callout
       function may cause matching to proceed, to backtrack, or to fail	 alto‐
       gether. A complete description of the interface to the callout function
       is given in the pcrecallout documentation.

BACKTRACKING CONTROL

       Perl 5.10 introduced a number of "Special Backtracking Control  Verbs",
       which are described in the Perl documentation as "experimental and sub‐
       ject 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.

       Since  these  verbs  are	 specifically related to backtracking, most of
       them can be  used  only	when  the  pattern  is	to  be	matched	 using
       pcre_exec(), which uses a backtracking algorithm. With the exception of
       (*FAIL), which behaves like a failing negative assertion, they cause an
       error if encountered by pcre_dfa_exec().

       If  any of these verbs are used in an assertion or in a subpattern that
       is called as a subroutine (whether or not recursively), their effect is
       confined to that subpattern; it does not extend to the surrounding pat‐
       tern, with one exception: the name from a *(MARK), (*PRUNE), or (*THEN)
       that  is	 encountered in a successful positive assertion is passed back
       when a match succeeds (compare capturing	 parentheses  in  assertions).
       Note that such subpatterns are processed as anchored at the point where
       they are tested. Note also that Perl's treatment of subroutines is dif‐
       ferent in some cases.

       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, with differing be‐
       haviour, depending on whether or not an argument is present. A name  is
       any sequence of characters that does not include a closing parenthesis.
       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.

       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 suppresses
       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).

       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)	 is  inside  capturing
       parentheses, the data so far is captured. 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
       arrived	at,  though  it	 also  has a secondary use in conjunction with
       advancing the match starting point (see (*SKIP) below).

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

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

       When a match succeeds, the name of the last-encountered (*MARK) on  the
       matching	 path  is  passed  back	 to the caller via the pcre_extra data
       structure, as described in the section on  pcre_extra  in  the  pcreapi
       documentation. Here is an example of pcretest output, where the /K mod‐
       ifier requests the retrieval and outputting 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 (*MARK) is encountered in a positive assertion, its name is recorded
       and passed back if it is the last-encountered. This does not happen for
       negative assertions.

       After  a	 partial match or a failed match, the name of the last encoun‐
       tered (*MARK) 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". 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.

   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, its effect is confined to that group,
       because once the group has been matched, there is never any  backtrack‐
       ing  into  it.  In  this situation, backtracking can "jump back" to the
       left of the entire atomic group. (Remember also, as stated above,  that
       this localization also applies in subroutine calls and assertions.)

       These  verbs  differ  in exactly what kind of failure occurs when back‐
       tracking reaches them.

	 (*COMMIT)

       This verb, which may not be followed by a name, causes the whole	 match
       to fail outright if the rest of the pattern does not match. Even if the
       pattern is unanchored, no further attempts to find a match by advancing
       the  starting  point  take  place.  Once	 (*COMMIT)  has	 been  passed,
       pcre_exec() is committed to finding a match  at	the  current  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.

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

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

       PCRE knows that any match must start  with  "a",	 so  the  optimization
       skips  along the subject to "a" before running the first match attempt,
       which succeeds. When the optimization is disabled by the \Y  escape  in
       the second subject, the match starts at "x" and so the (*COMMIT) causes
       it 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 the rest of the pattern does not match. 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	alter‐
       native  to an atomic group or possessive quantifier, but there are some
       uses of (*PRUNE) that cannot be expressed in any other way.  The behav‐
       iour  of	 (*PRUNE:NAME)	is  the	 same  as  (*MARK:NAME)(*PRUNE). In an
       anchored pattern (*PRUNE) has the same effect as (*COMMIT).

	 (*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
       attempt	would  start at the second character instead of skipping on to
       "c".

	 (*SKIP:NAME)

       When (*SKIP) has an associated name, its behaviour is modified. If  the
       following pattern fails to match, 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 cor‐
       responds to that (*MARK) instead of to where (*SKIP)  was  encountered.
       If no (*MARK) with a matching name is found, the (*SKIP) is ignored.

	 (*THEN) or (*THEN:NAME)

       This  verb  causes a skip to the next innermost alternative if the rest
       of the pattern does not match. That is, it cancels  pending  backtrack‐
       ing,  but  only 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.  The  behaviour  of	 (*THEN:NAME)  is  exactly the same as
       (*MARK:NAME)(*THEN).  If (*THEN) is not inside an alternation, it  acts
       like (*PRUNE).

       Note  that  a  subpattern that does not contain a | character is just a
       part of the enclosing alternative; it is not a nested alternation  with
       only  one alternative. The effect of (*THEN) extends beyond such a sub‐
       pattern to the enclosing alternative. Consider this pattern,  where  A,
       B, etc. are complex pattern fragments that do not contain any | charac‐
       ters 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 also that a conditional subpattern is not considered as having two
       alternatives,  because  only  one  is  ever used. In other words, the |
       character 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
       ungreedy, 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.

       If more than one such verb is present in a pattern, the "strongest" one
       wins.  For example, consider this pattern, where A, B, etc. are complex
       pattern fragments:

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

       Once A has matched, PCRE is committed to this  match,  at  the  current
       starting	 position. If subsequently B matches, but C does not, the nor‐
       mal (*THEN) action of trying the next alternative (that is, D) does not
       happen because (*COMMIT) overrides.

SEE ALSO

       pcreapi(3), pcrecallout(3), pcrematching(3), pcresyntax(3), pcre(3).

AUTHOR

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

REVISION

       Last updated: 29 November 2011
       Copyright (c) 1997-2011 University of Cambridge.

								PCREPATTERN(3)
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