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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 section on
       UTF-8 support in the main pcre page.

       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	 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  set‐
       ting can be combined 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 non-alphanumeric character, 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 \\.

       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.

   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 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..

       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, but \c{ becomes hex 3B, while \c;
       becomes hex 7B.

       After \x, from zero to two hexadecimal digits are read (letters can  be
       in  upper  or  lower case). Any number of hexadecimal digits 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.

       Characters whose value is less than 256 can be defined by either of the
       two syntaxes for \x. There is no difference in the way  they  are  han‐
       dled. For example, \xdc is exactly the same as \x{dc}.

       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, the sequence \b is interpreted as the backspace  character  (hex
       08),  and the sequences \R and \X are interpreted as the characters "R"
       and "X", respectively. Outside a character class, these sequences  have
       different meanings (see below).

   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. The
       following are always recognized:

	 \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

       Each pair of escape sequences partitions the complete set of characters
       into  two disjoint sets. Any given character matches one, and only one,
       of each pair.

       These character type sequences can appear both inside and outside char‐
       acter  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, since 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.

       In  UTF-8 mode, characters with values greater than 128 never match \d,
       \s, or \w, and always match \D, \S, and \W. This is true even when Uni‐
       code  character	property  support is available. These sequences retain
       their original meanings from before UTF-8 support was available, mainly
       for  efficiency	reasons. Note that this also affects \b, because it is
       defined in terms of \w and \W.

       The sequences \h, \H, \v, and \V are Perl 5.10 features. In contrast to
       the  other  sequences, these do match certain high-valued codepoints in
       UTF-8 mode.  The horizontal 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

       A "word" character is an underscore or any character less than 256 that
       is  a  letter  or  digit.  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 matched by \w. The use  of
       locales with Unicode is discouraged.

   Newline sequences

       Outside	a  character class, by default, the escape sequence \R matches
       any Unicode newline sequence. This is a Perl 5.10 feature. 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)

       Inside a character class, \R matches the letter "R".

   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, and "Any", which matches
       any character (including newline). Other properties such as "InMusical‐
       Symbols"	 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 general category property, specified  by
       a two-letter abbreviation. For compatibility with Perl, negation 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.

       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.

   Resetting the match start

       The escape sequence \K, which is a Perl 5.10 feature, causes any previ‐
       ously  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

       These  assertions may not appear in character classes (but note that \b
       has a different meaning, namely the backspace character, inside a char‐
       acter class).

       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. Neither
       PCRE  nor  Perl	has a separte "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)

       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.

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  any
       line-ending  characters.	 The  feature  is provided in Perl in order to
       match individual bytes in UTF-8 mode. Because it breaks up UTF-8	 char‐
       acters  into individual bytes, what remains in the string may be a mal‐
       formed UTF-8 string. For this reason, the \C escape  sequence  is  best
       avoided.

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

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 types \d, \D, \p, \P, \s, \S, \w, and \W may also	appear
       in  a  character	 class,	 and add the characters that they match to the
       class. For example, [\dABCDEF] matches any hexadecimal digit. A circum‐
       flex  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.

       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
	 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.

       In UTF-8 mode, characters with values greater than 128 do not match any
       of the POSIX character classes.

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 current pattern 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 is also the (*UTF8) leading sequence that	can  be	 used  to  set
       UTF-8 mode; this is equivalent to setting the PCRE_UTF8 option.

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 one of the words "cat", "cataract", or  "caterpillar".  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 pat‐
       tern

	 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 buffers that follow the sub‐
       pattern start after the highest number used in any branch. The  follow‐
       ing  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 recursive or "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 that matches a single character
	 a character class
	 a back reference (see next section)
	 a parenthesized subpattern (unless it is an assertion)
	 a recursive or "subroutine" call to a subpattern

       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. 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, which is a fea‐
       ture introduced in Perl 5.10.  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.  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, and may not be
       repeated, because it makes no sense to assert the  same	thing  several
       times.  If  any kind of assertion contains capturing subpatterns within
       it, these are counted for the purposes of numbering the capturing  sub‐
       patterns in the whole pattern.  However, substring capturing is carried
       out only for positive assertions, because it does not  make  sense  for
       negative assertions.

   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  Perl  5.10  backtracking	 control  verb
       (*FAIL) or (*F) is essentially 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 (5.8 and 5.10), 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 Perl 5.10 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.

       PCRE does not allow the \C escape (which matches a single byte in UTF-8
       mode)  to appear in lookbehind assertions, because it makes it impossi‐
       ble to calculate the length of the lookbehind. 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.

       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 been set. 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. In looping  constructs	 it  can  also
       make  sense  to	refer  to  subsequent  groups  with constructs such as
       (?(+2).

       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 the first set
       of parentheses matched or not. If they did, that is, if subject started
       with an opening parenthesis, the condition is true, and so the yes-pat‐
       tern is executed and a  closing	parenthesis  is	 required.  Otherwise,
       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 ref‐
       erenced from elsewhere. (The use of "subroutines" is described  below.)
       For  example,  a pattern to match an IPv4 address 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

       The  sequence (?# marks the start of a comment that continues up to the
       next closing parenthesis. Nested parentheses  are  not  permitted.  The
       characters  that make up a comment play no part in the pattern matching
       at all.

       If the PCRE_EXTENDED option is set, an unescaped # character outside  a
       character  class	 introduces  a	comment	 that continues to immediately
       after the next newline in the pattern.

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 call of the subpattern of
       the  given  number, provided that it occurs inside that subpattern. (If
       not, it is a "subroutine" call, which is described  in  the  next  sec‐
       tion.)  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 (a Perl
       5.10 feature).  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 "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 value is unset, even
       if it is (temporarily) set at a deeper level.

       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.

   Recursion difference from Perl

       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 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.

SUBPATTERNS AS SUBROUTINES

       If the syntax for a recursive subpattern reference (either by number or
       by  name)  is used outside the parentheses to which it refers, it oper‐
       ates like a subroutine in a programming language. The "called"  subpat‐
       tern 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.

       Like  recursive	subpatterns, a subroutine 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. Any capturing parentheses  that
       are  set	 during	 the  subroutine  call revert to their previous values
       afterwards.

       When a subpattern is used as a subroutine, processing options  such  as
       case-independence are fixed when the subpattern is defined. They 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 subroutine subpattern
       (including recursive subpatterns), their effect	is  confined  to  that
       subpattern;  it	does  not extend to the surrounding pattern. Note that
       such subpatterns are processed as anchored at the point where they  are
       tested.

       The  new verbs make use of what was previously invalid syntax: an open‐
       ing parenthesis followed by an asterisk. In Perl, they are generally of
       the form (*VERB:ARG) but PCRE does not support the use of arguments, so
       its general form is just (*VERB). Any number of these verbs  may	 occur
       in a pattern. There are two kinds:

   Verbs that act immediately

       The following verbs act as soon as they are encountered:

	  (*ACCEPT)

       This  verb causes the match to end successfully, skipping the remainder
       of the pattern. When inside a recursion, only the innermost pattern  is
       ended  immediately.  If	(*ACCEPT) is inside capturing parentheses, the
       data so far is captured. (This feature was added	 to  PCRE  at  release
       8.00.) 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 the match to fail, 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).

   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, a fail‐
       ure is forced.  The verbs  differ  in  exactly  what  kind  of  failure
       occurs.

	 (*COMMIT)

       This  verb  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."

	 (*PRUNE)

       This verb causes the match to fail at the current position 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.	 Back‐
       tracking	 can  occur as usual to the left of (*PRUNE), or when matching
       to the right of (*PRUNE), but if there is no match to the right,	 back‐
       tracking	 cannot	 cross (*PRUNE).  In simple cases, the use of (*PRUNE)
       is just an alternative to an atomic group or possessive quantifier, but
       there  are  some uses of (*PRUNE) that cannot be expressed in any other
       way.

	 (*SKIP)

       This verb is like (*PRUNE), except that if the pattern  is  unanchored,
       the  "bumpalong" advance is not to the next character, but to the posi‐
       tion in the subject where (*SKIP) was  encountered.  (*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".

	 (*THEN)

       This verb causes a skip to the next alternation if the rest of the pat‐
       tern does not match. That is, it cancels pending backtracking, but only
       within the current alternation. Its name	 comes	from  the  observation
       that it can be used for a pattern-based if-then-else block:

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

       If  the COND1 pattern matches, FOO is tried (and possibly further items
       after the end of the group if FOO succeeds);  on	 failure  the  matcher
       skips  to  the second alternative and tries COND2, without backtracking
       into COND1. If (*THEN) is used outside  of  any	alternation,  it  acts
       exactly like (*PRUNE).

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: 06 March 2010
       Copyright (c) 1997-2010 University of Cambridge.

								PCREPATTERN(3)
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