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PCREPATTERN(3)                                                  PCREPATTERN(3)

       PCRE - Perl-compatible regular expressions


       The  syntax  and semantics of the regular expressions that are supported by PCRE are described in detail below.
       There is a quick-reference syntax 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 syntax) in order to provide some compatibility with regular  expressions  in  Python,  .NET,  and

       Perl's  regular expressions are described in its own documentation, and regular expressions in general are cov-
       ered in a number of books, some of which have copious examples. Jeffrey  Friedl's  "Mastering  Regular  Expres-
       sions",  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_com-
       pile() or pcre_compile2() with the PCRE_UTF8 option. There is also a special sequence that can be given at  the
       start of a pattern:


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

       Another special sequence that may appear at the start of a pattern or in combination with (*UTF8) is:


       This  has  the same effect as setting the PCRE_UCP option: it causes sequences such as \d and \w to use Unicode
       properties to determine character types, instead of recognizing only characters with codes less than 128 via  a
       lookup table.

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

       The  remainder  of this document discusses the patterns that are supported by PCRE when its main matching func-
       tion, 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.


       PCRE  supports  five different conventions for indicating line breaks in strings: a single CR (carriage return)
       character, a single LF (linefeed) character, the two-character sequence CRLF, any of the  three  preceding,  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 pattern string with one of the following five

         (*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


       changes the convention to CR. That pattern matches "a\nb" because LF is no longer a newline.  Note  that  these
       special  settings,  which are not Perl-compatible, are recognized only at the very start of a pattern, and that
       they must be in upper case. If more than one of them is present, the last one is used.

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


       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 values, the concept of case is supported if PCRE is compiled with Unicode property sup-
       port, 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 recognized anywhere in the pattern except within
       square brackets, and those that are recognized within square brackets. Outside square brackets, the metacharac-
       ters 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
         ]      terminates the character class

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


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

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

       In  UTF-8 mode, only ASCII numbers and letters have any special meaning after a backslash. All other characters
       (in particular, those whose codepoints are greater than 127) are treated as literals.

       If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the pattern (other than  in  a  character
       class) and characters between a # outside a character class and the next newline are ignored. An escaping back-
       slash 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 characters, you can do so by putting them  between
       \Q  and  \E.  This is different from Perl in that $ and @ are handled as literals in \Q...\E sequences in PCRE,
       whereas in Perl, $ and @ cause variable interpolation. Note the following examples:

         Pattern            PCRE matches   Perl matches

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

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

   Non-printing characters

       A  second  use of backslash provides a way of encoding non-printing characters in patterns in a visible manner.
       There is no restriction on the appearance of non-printing characters, apart from the binary  zero  that  termi-
       nates  a  pattern,  but  when a pattern is being prepared by text editing, it is often easier to use one of the
       following escape sequences than the binary character it represents:

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

       The precise effect of \cx is as follows: if x is a lower case letter, it is converted to upper case. Then bit 6
       of  the  character  (hex 40) is inverted.  Thus \cz becomes hex 1A (z is 7A), but \c{ becomes hex 3B ({ is 7B),
       while \c; becomes hex 7B (; is 3B). If the byte following \c has a value greater than 127, a compile-time error
       occurs.  This locks out non-ASCII characters in both byte mode and UTF-8 mode. (When PCRE is compiled in EBCDIC
       mode, all byte values are valid. A lower case letter is converted to upper case, and then  the  0xc0  bits  are

       By default, 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.

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

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

       After  \0  up  to  two  further  octal digits are read. If there are fewer than two digits, just those that are
       present are used. Thus the sequence \0\x\07 specifies two binary zeros followed by a BEL character (code  value
       7).  Make  sure you supply two digits after the initial zero if the pattern character that follows is itself an
       octal digit.

       The handling of a backslash followed by a digit other than 0 is complicated.  Outside a character  class,  PCRE
       reads  it and any following digits 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 subpat-

       Inside a character class, or if the decimal number is greater than 9 and there have not been that many  captur-
       ing  subpatterns,  PCRE  re-reads up to three octal digits following the backslash, and uses them to generate a
       data character. Any subsequent digits stand for themselves. In non-UTF-8 mode, the value of a character  speci-
       fied in octal must be less than \400. In UTF-8 mode, values up to \777 are permitted. For example:

         \040   is another way of writing a space
         \40    is the same, provided there are fewer than 40
                   previous capturing subpatterns
         \7     is always a back reference
         \11    might be a back reference, or another way of
                   writing a tab
         \011   is always a tab
         \0113  is a tab followed by the character "3"
         \113   might be a back reference, otherwise the
                   character with octal code 113
         \377   might be a back reference, otherwise
                   the byte consisting entirely of 1 bits
         \81    is either a back reference, or a binary zero
                   followed by the two characters "8" and "1"

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

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

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

   Unsupported escape sequences

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

   Absolute and relative back references

       The  sequence \g followed by an unsigned or a negative number, optionally enclosed in braces, is an absolute or
       relative back reference. A named back reference can be coded as \g{name}. Back references are discussed  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 synony-
       mous. The former is a back reference; the latter is a subroutine call.

   Generic character types

       Another use of backslash is for specifying generic character types:

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

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

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

       For  compatibility  with  Perl, \s does not match the VT character (code 11).  This makes it different from the
       the POSIX "space" class. The \s characters are HT (9), LF (10), FF (12), CR  (13),  and  space  (32).  If  "use
       locale;" is included in a Perl script, \s may match the VT character. In PCRE, it never does.

       A  "word" character is an underscore or any character that is a letter or digit.  By default, the definition of
       letters and digits is controlled by PCRE's low-valued character tables, and may vary if locale-specific  match-
       ing 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  let-
       ters, and these are then matched by \w. The use of locales with Unicode is discouraged.

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

         \d  any character that \p{Nd} matches (decimal digit)
         \s  any character that \p{Z} matches, plus HT, LF, FF, CR
         \w  any character that \p{L} or \p{N} matches, plus underscore

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

       The sequences \h, \H, \v, and \V are features that were added to Perl at release 5.10. In contrast to the other
       sequences, which match only ASCII characters by default, these always match certain high-valued  codepoints  in
       UTF-8 mode, whether or not PCRE_UCP is set. The 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

   Newline sequences

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


       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 separator, U+2029).  Unicode character property support is not needed for these char-
       acters to be recognized.

       It  is  possible to restrict \R to match only CR, LF, or CRLF (instead of the complete set of Unicode line end-
       ings) 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 overrid-
       den 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:


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

   Unicode character properties

       When  PCRE is built with Unicode character property support, three additional escape sequences that match char-
       acters 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  prop-
       erties,  "Any", which matches any character (including newline), and some special PCRE properties (described in
       the next section).  Other Perl properties such as "InMusicalSymbols" 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:


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

       Arabic,  Armenian,  Avestan, Balinese, Bamum, Bengali, Bopomofo, Braille, Buginese, Buhid, Canadian_Aboriginal,
       Carian, Cham, Cherokee, Common, Coptic, Cuneiform,  Cypriot,  Cyrillic,  Deseret,  Devanagari,  Egyptian_Hiero-
       glyphs,  Ethiopic, Georgian, Glagolitic, Gothic, Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hira-
       gana, Imperial_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_Tur-
       kic,  Ol_Chiki,  Oriya,  Osmanya, Phags_Pa, Phoenician, Rejang, Runic, Samaritan, Saurashtra, Shavian, Sinhala,
       Sundanese, Syloti_Nagri, Syriac, Tagalog, Tagbanwa, Tai_Le, Tai_Tham, Tai_Viet, Tamil,  Telugu,  Thaana,  Thai,
       Tibetan, Tifinagh, Ugaritic, Vai, Yi.

       Each  character  has exactly one Unicode general category property, 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 general 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:


       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 checking has been
       turned off (see the discussion of PCRE_NO_UTF8_CHECK in the pcreapi page). Perl does not support the  Cs  prop-

       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) property.  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


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

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

       Matching  characters by Unicode property is not fast, because PCRE has to search a structure that contains data
       for over fifteen thousand characters. That is why the traditional escape sequences such as \d and \w do not use
       Unicode  properties  in  PCRE  by  default,  though  you can make them do so by setting the PCRE_UCP option for
       pcre_compile() or by starting the pattern with (*UCP).

   PCRE's additional properties

       As well as the standard Unicode properties described in the previous section, PCRE supports four more that make
       it  possible  to convert traditional escape sequences such as \w and \s and POSIX character classes to use Uni-
       code properties. PCRE uses these non-standard, non-Perl properties internally when PCRE_UCP is set. They are:

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

       Xan matches characters that have either the L (letter) or the N (number) property. Xps matches  the  characters
       tab,  linefeed,  vertical tab, formfeed, or carriage return, and any other character that has the Z (separator)
       property.  Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches  the  same  characters  as
       Xan, plus underscore.

   Resetting the match start

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


       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  sub-
       strings.  For example, when the pattern


       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 assertion 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 sub-
       patterns for more complicated assertions is described below.  The backslashed assertions are:

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

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

       A  word  boundary is a position in the subject string where the current character and the previous character do
       not both match \w or \W (i.e. one matches \w and the other matches \W), or the start or end of  the  string  if
       the  first  or last character matches \w, respectively. In UTF-8 mode, the meanings of \w and \W can be changed
       by setting the PCRE_UCP option. When this is done, it also affects \b and \B. Neither PCRE nor Perl has a sepa-
       rate  "start  of word" or "end of word" metasequence. 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  assertions  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, indicating 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  speci-
       fied  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 arguments, you can mimic Perl's /g option, and it is  in
       this kind of implementation 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.


       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 argument 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 subject, 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 mul-
       tiline 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

       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.


       Outside a character class, a dot in the pattern matches any one character in  the  subject  string  except  (by
       default)  a  character  that signifies the end of a 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 Unicode 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 circumflex and dollar, the only relationship
       being that they both involve newlines. Dot has no special meaning in a character class.

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


       Outside a character class, the escape sequence \C matches any one byte, both in and out of UTF-8 mode. Unlike a
       dot,  it  always  matches  line-ending characters. The feature is provided in Perl in order to match individual
       bytes in UTF-8 mode, but it is unclear how it can usefully be used. Because \C breaks up characters into  indi-
       vidual  bytes,  matching one byte with \C in UTF-8 mode means that the rest of the string may start with a mal-
       formed UTF-8 character. This has undefined results, because PCRE assumes that it is dealing  with  valid  UTF-8
       strings  (and  by  default  it  checks  this at the start of processing unless the PCRE_NO_UTF8_CHECK option is

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

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

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

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


       An opening square bracket introduces a character class, terminated by  a  closing  square  bracket.  A  closing
       square  bracket  on its own is not special 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 charac-
       ter 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 charac-
       ter, 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 charac-
       ters that are in the class by enumerating those that are not. A class that starts with a circumflex is  not  an
       assertion; it still consumes 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 characters 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, typi-
       cally as the first or last character in the class.

       It is not possible to have the literal character "]" as the end character 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 interpreted as a class containing a range followed by two other characters. The octal or hexadeci-
       mal representation of "]" can also be used to end a range.

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

       If a range that includes letters is used when caseless matching is set, it matches the letters in either  case.
       For  example,  [W-c] is equivalent to [][\\^_'wxyzabc], matched caselessly, and in non-UTF-8 mode, if character
       tables for a French locale are in use, [\xc8-\xcb] matches accented E characters in both cases. In UTF-8  mode,
       PCRE  supports  the  concept  of case for characters with values greater than 128 only when it is compiled with
       Unicode property support.

       The character escape sequences \d, \D, \h, \H, \p, \P, \s, \S, \v, \V, \w, and \W may  appear  in  a  character
       class,  and  add  the  characters that they match to the class. For example, [\dABCDEF] matches any hexadecimal
       digit. In UTF-8 mode, the PCRE_UCP option affects the meanings of \d, \s, \w and  their  upper  case  partners,
       just as it does when they appear outside a character class, as described in the section entitled "Generic char-
       acter types" above. The escape sequence \b has a different meaning inside a character  class;  it  matches  the
       backspace  character. The sequences \B, \N, \R, and \X are not special inside a character class. Like any other
       unrecognized escape sequences, they are treated as the literal characters "B", "N", "R", and  "X"  by  default,
       but cause an error if the PCRE_EXTRA option is set.

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

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


       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,


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

         alnum    letters and digits
         alpha    letters
         ascii    character codes 0 - 127
         blank    space or tab only
         cntrl    control characters
         digit    decimal digits (same as \d)
         graph    printing characters, excluding space
         lower    lower case letters
         print    printing characters, including space
         punct    printing characters, excluding letters and digits and space
         space    white space (not quite the same as \s)
         upper    upper case letters
         word     "word" characters (same as \w)
         xdigit   hexadecimal digits

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

       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,


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

       By default, in UTF-8 mode, characters with values greater than 128 do not match  any  of  the  POSIX  character
       classes.  However,  if the PCRE_UCP option is passed to pcre_compile(), some of the classes are changed so that
       Unicode character properties are used. This is achieved by replacing the POSIX classes by other  sequences,  as

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

       Negated  versions,  such  as  [:^alpha:] use \P instead of \p. The other POSIX classes are unchanged, and match
       only characters with code points less than 128.


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


       matches either "gilbert" or "sullivan". Any number of alternatives may appear, and an empty alternative is per-
       mitted  (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),  "suc-
       ceeds" means matching the rest of the main pattern as well as the alternative in the subpattern.


       The  settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and PCRE_EXTENDED options (which are Perl-com-
       patible) 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 possible to unset these options by preceding
       the letter with a hyphen, and a combined setting and unsetting such as (?im-sx), which sets  PCRE_CASELESS  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  therefore  show  up  in  data  extracted by the
       pcre_fullinfo() function).

       An option change within a subpattern (see below for a description of subpatterns) affects only that part of the
       subpattern that follows it, so


       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,


       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 func-
       tions are called. In some cases the pattern can contain special leading sequences such as (*CRLF)  to  override
       what  the  application  has  set or what has been defaulted. Details are given in the section entitled "Newline
       sequences" above. There are also the (*UTF8) and (*UCP) leading sequences that can be used  to  set  UTF-8  and
       Unicode property modes; they are equivalent to setting the PCRE_UTF8 and the PCRE_UCP options, respectively.


       Subpatterns 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


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

       2.  It  sets up the subpattern as a capturing subpattern. This means that, when the whole pattern matches, that
       portion of the subject string that matched the subpattern is passed back to the caller via the ovector argument
       of  pcre_exec(). Opening parentheses are counted from left to right (starting from 1) to obtain numbers for the
       capturing subpatterns. For example, if the string "the red king" is matched against the pattern

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

       the captured substrings are "red king", "red", and "king", and are numbered 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 capturing, and is not counted when computing the num-
       ber  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  captur-
       ing 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


       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 subse-
       quent branches, so the above patterns match "SUNDAY" as well as "Saturday".


       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:


       Because the two alternatives are inside a (?| group, both sets of capturing 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, parentheses are numbered as usual, but the number is reset at the start of each branch. The numbers
       of any capturing parentheses that follow the subpattern start after the highest number used in any branch.  The
       following  example  is  taken from the Perl documentation. The numbers underneath show in which buffer the cap-
       tured 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 subpat-
       tern. The following pattern matches "abcabc" or "defdef":


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


       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 number have matched.

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


       Identifying capturing parentheses by number is simple, but it can be very hard to keep track of the numbers  in
       complicated  regular  expressions.  Furthermore,  if an expression is modified, the numbers may change. To help
       with this difficulty, PCRE supports the naming of subpatterns. 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 syntax. 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 allo-
       cated 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 func-
       tion 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.) Duplicate 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:


       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 exam-
       ple,  the only) subpattern of that name that matched. This saves searching to find which numbered subpattern it

       If you make a back reference to a non-unique named subpattern from elsewhere in the pattern, the one that  cor-
       responds  to  the  first  occurrence of the name is used. In the absence of duplicate numbers (see the previous
       section) this is the one with the lowest number. If you use a named reference in a condition test (see the sec-
       tion  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 documentation.

       Warning: You cannot use different names to distinguish between two subpatterns with  the  same  number  because
       PCRE uses only the numbers when matching. For this reason, an error is given at compile time if different 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 is specified by quantifiers, which can follow any of the following items:

         a literal data character
         the dot metacharacter
         the \C escape sequence
         the \X escape sequence (in UTF-8 mode with Unicode properties)
         the \R escape sequence
         an escape such as \d or \pL that matches a single character
         a character class
         a back reference (see next section)
         a parenthesized subpattern (including assertions)
         a subroutine call to a subpattern (recursive or otherwise)

       The  general  repetition  quantifier specifies a minimum and maximum number 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:


       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  omit-
       ted, the quantifier specifies an exact number of required matches. Thus


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


       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  example,
       {,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 characters, 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 useful for subpatterns that are referenced as subroutines from elsewhere in the
       pattern (but see also the section entitled "Defining subpatterns for use by reference only" below). Items other
       than subpatterns that have a {0} quantifier are omitted from the compiled pattern.

       For convenience, the three most common quantifiers have single-character 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 quan-
       tifier that has no upper limit, for example:


       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 subpat-
       tern does in fact match no characters, the loop is forcibly broken.

       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 mini-
       mum 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


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

       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 lim-
       ited maximum, more memory is required for the compiled pattern, in proportion to the size  of  the  minimum  or

       If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent 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 posi-
       tion 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 newlines, it is  worth  setting  PCRE_DOTALL  in
       order to obtain this optimization, 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:


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

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


       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  iterations.  For
       example, after


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


       With  both maximizing ("greedy") and minimizing ("ungreedy" or "lazy") repetition, failure of what follows nor-
       mally 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 carry-
       ing on.

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


       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 group-
       ing"  (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:


       This  kind  of  parenthesis  "locks up" the  part of the pattern it contains once it has matched, and a failure
       further into the pattern is prevented from backtracking into it. Backtracking past it to previous  items,  how-
       ever, 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 prepared
       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


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


       Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY option is ignored. They are a conve-
       nient  notation for the simpler forms of atomic group. However, there is no difference in the meaning of a pos-
       sessive 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 syntax.  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 simple 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

       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


       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


       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 character that is required for a
       match, and fail early if it is not present in the string.) If the pattern is changed so that it uses an  atomic
       group, like this:


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


       Outside  a  character  class, a backslash followed by a digit greater than 0 (and possibly further digits) is a
       back reference to a capturing subpattern 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 pattern. 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 iteration.

       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  sub-
       section  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 pos-
       sible using named parentheses (see below).

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

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

       An  unsigned  number specifies an absolute reference without the ambiguity 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:


       The  sequence \g{-1} is a reference to the most recently started capturing subpattern before \g, that is, is it
       equivalent to \2 in this example.  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 con-
       tain references within themselves.

       A back reference matches whatever actually matched the capturing subpattern  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 case-
       ful matching is in force at the time of the back reference, the case of letters is relevant. For example,


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

       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:


       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


       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 reference to an unset value matches an empty string.

       Because  there  may  be  many capturing parentheses in a pattern, all digits following a backslash are taken as
       part of a potential back reference number.  If the pattern continues with a  digit  character,  some  delimiter
       must  be used to terminate the back reference. If the PCRE_EXTENDED option is set, this can be whitespace. Oth-
       erwise, 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 subpatterns. For
       example, the pattern


       matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of the subpattern, the back  refer-
       ence  matches the character string corresponding to the previous iteration. In order for this to work, the pat-
       tern 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


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

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

       Assertion subpatterns are not capturing subpatterns. If such an assertion contains capturing subpatterns within
       it, these are counted for the purposes of numbering the capturing subpatterns in the  whole  pattern.  However,
       substring  capturing  is  carried out only for positive assertions, because it does not make sense for negative

       For compatibility with Perl, assertion subpatterns may be repeated; though it makes no sense to assert the same
       thing  several  times,  the side effect of capturing parentheses may occasionally be useful. In practice, there
       only three cases:

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

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

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

   Lookahead assertions

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


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


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


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

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

   Lookbehind assertions

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


       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 several top-
       level alternatives, they do not all have to have the same fixed length. Thus


       is permitted, but


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


       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:


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

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

       In UTF-8 mode, PCRE does not allow the \C escape (which matches a single byte, even in UTF-8 mode) to appear in
       lookbehind  assertions, because it makes it impossible 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


       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


       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


       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,


       matches  "foo" preceded by three digits that are not "999". Notice that each of the assertions is applied inde-
       pendently 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" preceded by six characters, the first of which are digits and the  last  three  of  which  are  not
       "999". For example, it doesn't match "123abcfoo". A pattern to do that is


       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,


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


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


       It is possible to cause the matching process to obey a subpattern conditionally or to choose between two alter-
       native  subpatterns,  depending  on  the result of an assertion, or whether a specific capturing subpattern has
       already been matched. The two possible forms of conditional subpattern are:


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

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

       There are four kinds of condition: references to  subpatterns,  references  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 sub-
       pattern of that number has previously matched. If there is more than one capturing  subpattern  with  the  same
       number  (see the earlier section about duplicate subpattern numbers), the condition is true if any of them have
       matched. An alternative notation is to precede the digits with a plus or minus sign. In this case, the  subpat-
       tern number is relative rather than absolute. The most recently opened parentheses can be referenced by (?(-1),
       the next most recent by (?(-2), and so on. Inside loops it can also make sense to refer to  subsequent  groups.
       The next parentheses to be opened can be referenced as (?(+1), and so on. (The value zero in any of these forms
       is not used; it provokes a compile-time error.)

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

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

       The  first part matches an optional opening parenthesis, and if that character is present, sets it as the first
       captured substring. The second part matches one or more characters that are not parentheses. The third part  is
       a  conditional subpattern that tests whether or not the first set of parentheses matched. If they did, that is,
       if subject started with an opening parenthesis, the condition is true, and so the yes-pattern is executed and a
       closing parenthesis is required. 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 syntax, because subpattern names may consist entirely of  dig-
       its.  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 number, 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 ampersand
       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 dupli-
       cate, 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

   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 con-
       trol reaches this point in the pattern; the idea of DEFINE is that it can be used to  define  subroutines  that
       can  be referenced from elsewhere. (The use of subroutines is described below.) For example, a pattern to match
       an IPv4 address such as "" 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  refer-
       ences  to  the  named  group to match the four dot-separated components of an IPv4 address, insisting 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  nega-
       tive  lookahead  or  lookbehind assertion. Consider this pattern, again containing non-significant white space,
       and with the two alternatives on the second line:

         \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.


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

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

         abc #comment \n still comment

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


       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 expressions 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  recur-
       sion  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 recur-
       sive subroutine call of the subpattern of the given number, provided that it occurs inside that subpattern. (If
       not, it is a non-recursive subroutine call, which is described in the next section.) The special item  (?R)  or
       (?0) is a recursive call of the entire regular expression.

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

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

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

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

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

       An alternative approach is to use named parentheses instead. The Perl syntax for this is (?&name); PCRE's  ear-
       lier 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  pattern  to
       strings that do not match. For example, when this pattern is applied to


       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  documentation).  If
       the pattern above is matched against


       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 subpattern is not matched at the top level, its final captured value is unset, even if it
       was (temporarily) set at a deeper level during the matching process.

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

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

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

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

   Differences in recursion processing between PCRE and Perl

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


       The idea is that it either matches a single character, or two identical  characters  surrounding  a  sub-palin-
       drome.  In  Perl,  this pattern works; in PCRE it does not if the pattern is longer than three characters. Con-
       sider 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 alterna-
       tive fails; the second alternative is taken and the recursion kicks in. The recursive call to subpattern 1 suc-
       cessfully matches the next character ("b"). (Note that the beginning and end of line tests are not part of  the

       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:


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

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


       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 alternatives at the higher level:


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


       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  backtracking
       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 subject string does not start  with  a  palin-
       drome  that is shorter than the entire string.  For example, although "abcba" is correctly matched, if the sub-
       ject 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 alternatives, so the
       entire match fails.

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


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


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


       An earlier example pointed out that the pattern

         (sens|respons)e and \1ibility

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

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

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

       All subroutine calls, whether recursive or not, are always treated as atomic groups. That is, once a subroutine
       has  matched  some  of the 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.

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


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


       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,  possi-
       bly recursively. Here are two of the examples used above, rewritten 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:


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


       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  substrings
       that match the same pair of parentheses when there is a repetition.

       PCRE provides a similar feature, but of course it cannot obey arbitrary Perl code. The feature is called "call-
       out". 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:


       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  back-
       track,  or  to fail altogether. A complete description of the interface to the callout function is given in the
       pcrecallout documentation.


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

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

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

       The new verbs make use of what was previously invalid syntax: an opening parenthesis followed by  an  asterisk.
       They  are  generally  of the form (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,
       depending on whether or not an argument is present. A name is any sequence of characters that does not  include
       a closing parenthesis. If the name is empty, that is, if the closing parenthesis immediately follows the colon,
       the effect is as if the colon were not there. Any number of these verbs may occur in a pattern.

       PCRE contains some optimizations that are used to speed up matching by running some checks at the start of each
       match  attempt. For example, it may know the minimum length of matching subject, or that a particular character
       must be present. When one of these optimizations suppresses the running of a match, any  included  backtracking
       verbs  will  not,  of  course,  be  processed. You can suppress the start-of-match optimizations by setting the
       PCRE_NO_START_OPTIMIZE option when calling pcre_compile() or pcre_exec(),  or  by  starting  the  pattern  with

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

   Verbs that act immediately

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


       This  verb  causes  the  match  to end successfully, skipping the remainder of the pattern. However, when it is
       inside a subpattern that is called as a subroutine, only that subpattern is ended successfully.  Matching  then
       continues  at  the  outer level. If (*ACCEPT) is inside capturing parentheses, the data so far is captured. For


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

         (*FAIL) or (*F)

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


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

   Recording which path was taken

       There is one verb whose main purpose is to track how a match was arrived at, though it also has a secondary use
       in conjunction with advancing the match starting point (see (*SKIP) below).

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

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

       When a match succeeds, the name of the last-encountered (*MARK) on the matching path  is  passed  back  to  the
       caller  via  the pcre_extra data structure, as described in the section on pcre_extra in the pcreapi documenta-
       tion. Here is an example of pcretest output, where the /K modifier requests the  retrieval  and  outputting  of
       (*MARK) data:

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

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

       If  (*MARK)  is  encountered  in  a positive assertion, its name is recorded and passed back if it is the last-
       encountered. This does not happen for negative assertions.

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

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

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

   Verbs that act after backtracking

       The following verbs do nothing when they are encountered. Matching continues with what follows, but if there is
       no subsequent match, causing a backtrack to the verb, a failure is forced. That is, backtracking cannot pass to
       the  left  of the verb. However, when one of these verbs appears inside an atomic group, its effect is confined
       to that group, because once the group has been matched, there is never any backtracking into it. In this situa-
       tion,  backtracking  can  "jump  back" to the left of the entire atomic group. (Remember also, as stated above,
       that this localization also applies in subroutine calls and assertions.)

       These verbs differ in exactly what kind of failure occurs when backtracking reaches them.


       This verb, which may not be followed by a name, causes the whole match to fail outright if the rest of the pat-
       tern  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:


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

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

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

       PCRE knows that any match must start with "a", so the optimization skips along the subject to "a"  before  run-
       ning  the first match attempt, which succeeds. When the optimization is disabled by the \Y escape in the second
       subject, the match starts at "x" and so the (*COMMIT) causes it to  fail  without  trying  any  other  starting

         (*PRUNE) or (*PRUNE:NAME)

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


       This  verb,  when given without a name, is like (*PRUNE), except that if the pattern is unanchored, the "bumpa-
       long" advance is not to the next character, but to the position 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. Con-


       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 quantifer 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".


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

         (*THEN) or (*THEN:NAME)

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

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

       If the COND1 pattern matches, FOO is tried (and possibly further items after the end of the group if  FOO  suc-
       ceeds);  on  failure,  the  matcher  skips to the second alternative and tries COND2, without backtracking into
       COND1. The behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN).  If (*THEN) is not  inside  an
       alternation, it acts like (*PRUNE).

       Note  that  a subpattern that does not contain a | character is just a part of the enclosing alternative; it is
       not a nested alternation with only one alternative. The effect of (*THEN) extends beyond such a  subpattern  to
       the  enclosing  alternative.  Consider this pattern, where A, B, etc. are complex pattern fragments that do not
       contain any | characters at this level:

         A (B(*THEN)C) | D

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

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

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

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

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

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

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

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


       Once  A  has  matched,  PCRE  is  committed  to this match, at the current starting position. If subsequently B
       matches, but C does not, the normal (*THEN) action of trying the next alternative (that is, D) does not  happen
       because (*COMMIT) overrides.


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


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


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