pcre2pattern


       The  syntax and semantics of the regular expressions that are supported
       by PCRE2 are described in detail below. There is a quick-reference syn-
       tax  summary  in the pcre2syntax page. PCRE2 tries to match Perl syntax
       and semantics as closely as it can.  PCRE2 also supports some  alterna-
       tive  regular  expression syntax (which does not conflict with the Perl
       syntax) in order to provide some compatibility with regular expressions
       in Python, .NET, and Oniguruma.

       Perl's  regular expressions are described in its own documentation, and
       regular expressions in general are covered in a number of  books,  some
       of  which  have  copious  examples. Jeffrey Friedl's "Mastering Regular
       Expressions", published by  O'Reilly,  covers  regular  expressions  in
       great  detail.  This  description  of  PCRE2's  regular  expressions is
       intended as reference material.

       This document discusses the regular expression patterns that  are  sup-
       ported  by  PCRE2  when  its  main matching function, pcre2_match(), is
       used.   PCRE2   also   has   an    alternative    matching    function,
       pcre2_dfa_match(),  which  matches  using a different algorithm that is
       not Perl-compatible. Some of  the  features  discussed  below  are  not
       available  when  DFA matching is used. The advantages and disadvantages
       of the alternative function, and how it differs from the  normal  func-
       tion, are discussed in the pcre2matching page.

SPECIAL START-OF-PATTERN ITEMS

       A  number  of options that can be passed to pcre2_compile() can also be
       set by special items at the start of a pattern. These are not Perl-com-
       patible,  but  are provided to make these options accessible to pattern
       writers who are not able to change the program that processes the  pat-
       tern.  Any  number  of  these  items  may  appear, but they must all be
       together right at the start of the pattern string, and the letters must
       be in upper case.

   UTF support

       In the 8-bit and 16-bit PCRE2 libraries, characters may be coded either
       as single code units, or as multiple UTF-8 or UTF-16 code units. UTF-32
       can  be  specified  for the 32-bit library, in which case it constrains
       the character values to valid  Unicode  code  points.  To  process  UTF
       strings,  PCRE2  must be built to include Unicode support (which is the
       default). When using UTF strings you must  either  call  the  compiling
       function  with the PCRE2_UTF option, or the pattern must start with the
       special sequence (*UTF), which is equivalent to  setting  the  relevant
       option. How setting a UTF mode affects pattern matching is mentioned in
       several places below. There is  also  a  summary  of  features  in  the
       pcre2unicode page.

       Some applications that allow their users to supply patterns may wish to
       restrict  them  to  non-UTF  data  for   security   reasons.   If   the
       PCRE2_NEVER_UTF  option  is  passed  to  pcre2_compile(), (*UTF) is not
       allowed, and its appearance in a pattern causes an error.


   Locking out empty string matching

       Starting a pattern with (*NOTEMPTY) or (*NOTEMPTY_ATSTART) has the same
       effect as passing the PCRE2_NOTEMPTY or  PCRE2_NOTEMPTY_ATSTART  option
       to whichever matching function is subsequently called to match the pat-
       tern. These options lock out the  matching  of  empty  strings,  either
       entirely, or only at the start of the subject.

   Disabling auto-possessification

       If  a pattern starts with (*NO_AUTO_POSSESS), it has the same effect as
       setting the PCRE2_NO_AUTO_POSSESS option. This stops PCRE2 from  making
       quantifiers  possessive  when  what  follows  cannot match the repeated
       item. For example, by default a+b is treated as a++b. For more details,
       see the pcre2api documentation.

   Disabling start-up optimizations

       If  a  pattern  starts  with (*NO_START_OPT), it has the same effect as
       setting the PCRE2_NO_START_OPTIMIZE option. This disables several opti-
       mizations  for  quickly  reaching "no match" results. For more details,
       see the pcre2api documentation.

   Disabling automatic anchoring

       If a pattern starts with (*NO_DOTSTAR_ANCHOR), it has the  same  effect
       as  setting the PCRE2_NO_DOTSTAR_ANCHOR option. This disables optimiza-
       tions that apply to patterns whose top-level branches all start with .*
       (match  any  number of arbitrary characters). For more details, see the
       pcre2api documentation.

   Disabling JIT compilation

       If a pattern that starts with (*NO_JIT) is  successfully  compiled,  an
       attempt  by  the  application  to apply the JIT optimization by calling
       pcre2_jit_compile() is ignored.

   Setting match resource limits

       The pcre2_match() function contains a counter that is incremented every
       time it goes round its main loop. The caller of pcre2_match() can set a
       limit on this counter, which therefore limits the amount  of  computing
       resource used for a match. The maximum depth of nested backtracking can
       also be limited; this indirectly restricts the amount  of  heap  memory
       that  is  used,  but there is also an explicit memory limit that can be
       set.

       These facilities are provided to catch runaway matches  that  are  pro-
       voked  by patterns with huge matching trees. A common example is a pat-
       tern with nested unlimited repeats applied to a long string  that  does
       not  match. When one of these limits is reached, pcre2_match() gives an
       error return. The limits can also be set by items at the start  of  the
       pattern of the form

       Prior  to  release  10.30, LIMIT_DEPTH was called LIMIT_RECURSION. This
       name is still recognized for backwards compatibility.

       The heap limit applies only when the pcre2_match() or pcre2_dfa_match()
       interpreters are used for matching. It does not apply to JIT. The match
       limit is used (but in a different way) when JIT is being used, or  when
       pcre2_dfa_match() is called, to limit computing resource usage by those
       matching functions. The depth limit is ignored by JIT but  is  relevant
       for  DFA  matching, which uses function recursion for recursions within
       the pattern and for lookaround assertions and atomic  groups.  In  this
       case, the depth limit controls the depth of such recursion.

   Newline conventions

       PCRE2  supports six different conventions for indicating line breaks in
       strings: a single CR (carriage return) character, a  single  LF  (line-
       feed) character, the two-character sequence CRLF, any of the three pre-
       ceding, any Unicode newline sequence,  or  the  NUL  character  (binary
       zero).  The  pcre2api  page  has further discussion about newlines, and
       shows how to set the newline convention when calling pcre2_compile().

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

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

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

         (*CR)a.b

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

       The  newline  convention affects where the circumflex and dollar asser-
       tions are true. It also affects the interpretation of the dot metachar-
       acter  when  PCRE2_DOTALL  is not set, and the behaviour of \N when not
       followed by an opening brace. 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
       next section and the description of \R in the section entitled "Newline
       sequences" below. A change of \R setting can be combined with a  change
       of newline convention.

   Specifying what \R matches

       values, and there are no code points greater than 255.

CHARACTERS AND METACHARACTERS

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

         The quick brown fox

       matches a portion of a subject string that is identical to itself. When
       caseless matching is specified (the PCRE2_CASELESS option), letters are
       matched independently of case.

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

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

         \      general escape character with several uses
         ^      assert start of string (or line, in multiline mode)
         $      assert end of string (or line, in multiline mode)
         .      match any character except newline (by default)
         [      start character class definition
         |      start of alternative branch
         (      start group or control verb
         )      end group or control verb
         *      0 or more quantifier
         +      1 or more quantifier; also "possessive quantifier"
         ?      0 or 1 quantifier; also quantifier minimizer
         {      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 (if followed by POSIX syntax)
         ]      terminates the character class

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

BACKSLASH

       The backslash character has several uses. Firstly, if it is followed by
       a  character that is not a digit or a letter, it takes away any special
       code points are greater than 127) are treated as literals.

       If a pattern is compiled with the  PCRE2_EXTENDED  option,  most  white
       space  in the pattern (other than in a character class), and characters
       between a # outside a character class and the next newline,  inclusive,
       are ignored. An escaping backslash can be used to include a white space
       or # character as part of the pattern.

       If you want to treat all characters in a sequence as literals, 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  PCRE2,
       whereas  in Perl, $ and @ cause variable interpolation. Also, Perl does
       "double-quotish backslash interpolation" on any backslashes between  \Q
       and  \E which, its documentation says, "may lead to confusing results".
       PCRE2 treats a backslash between \Q and \E just like any other  charac-
       ter. Note the following examples:

         Pattern            PCRE2 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
         \QA\B\E            A\B            A\B
         \Q\\E              \              \\E

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

   Non-printing characters

       A second use of backslash provides a way of encoding non-printing char-
       acters in patterns in a visible manner. There is no restriction on  the
       appearance  of non-printing characters in a pattern, but when a pattern
       is being prepared by text editing, it is often easier to use one of the
       following  escape  sequences  instead of the binary character it repre-
       sents. In an ASCII or Unicode environment, these escapes  are  as  fol-
       lows:

         \a          alarm, that is, the BEL character (hex 07)
         \cx         "control-x", where x is any printable ASCII character
         \e          escape (hex 1B)
         \f          form feed (hex 0C)
         \n          linefeed (hex 0A)
         \r          carriage return (hex 0D) (but see below)
         \t          tab (hex 09)
         \0dd        character with octal code 0dd
         \ddd        character with octal code ddd, or backreference
         \o{ddd..}   character with octal code ddd..

       ence in the way they are handled. For example, \xdc is exactly the same
       as \x{dc} or \334.  However, using the braced versions does  make  such
       sequences easier to read.

       Support  is  available  for  some  ECMAScript  (aka  JavaScript) escape
       sequences via two compile-time options. If PCRE2_ALT_BSUX is  set,  the
       sequence  \x followed by { is not recognized. Only if \x is followed by
       two hexadecimal digits is it recognized as a character  escape.  Other-
       wise  it  is interpreted as a literal "x" character. In this mode, sup-
       port for code points greater than 256 is provided by \u, which must  be
       followed  by  four hexadecimal digits; otherwise it is interpreted as a
       literal "u" character.

       PCRE2_EXTRA_ALT_BSUX has the same  effect  as  PCRE2_ALT_BSUX  and,  in
       addition,  \u{hhh..}  is recognized as the character specified by hexa-
       decimal code point.  There may be any  number  of  hexadecimal  digits.
       This syntax is from ECMAScript 6.

       The  \N{U+hhh..}  escape sequence is recognized only when the PCRE2_UTF
       option is set, that is, when PCRE2 is operating in a Unicode mode. Perl
       also  uses  \N{name}  to specify characters by Unicode name; PCRE2 does
       not support this.  Note that when \N is  not  followed  by  an  opening
       brace  (curly  bracket)  it has an entirely different meaning, matching
       any character that is not a newline.

       There are some legacy applications where  the  escape  sequence  \r  is
       expected to match a newline. If the PCRE2_EXTRA_ESCAPED_CR_IS_LF option
       is set, \r in a pattern is converted to \n so  that  it  matches  a  LF
       (linefeed) instead of a CR (carriage return) character.

       The  precise effect of \cx on ASCII characters is as follows: if x is a
       lower case letter, it is converted to upper case. Then  bit  6  of  the
       character (hex 40) is inverted. Thus \cA to \cZ become hex 01 to hex 1A
       (A is 41, Z is 5A), but \c{ becomes hex 3B ({ is 7B), and  \c;  becomes
       hex  7B  (; is 3B). If the code unit following \c has a value less than
       32 or greater than 126, a compile-time error occurs.

       When PCRE2 is compiled in EBCDIC mode, \N{U+hhh..}  is  not  supported.
       \a, \e, \f, \n, \r, and \t generate the appropriate EBCDIC code values.
       The \c escape is processed as specified for Perl in the perlebcdic doc-
       ument.  The  only characters that are allowed after \c are A-Z, a-z, or
       one of @, [, \, ], ^, _, or ?. Any other character provokes a  compile-
       time  error.  The  sequence  \c@ encodes character code 0; after \c the
       letters (in either case) encode characters 1-26 (hex 01 to hex 1A);  [,
       \,  ],  ^,  and  _  encode characters 27-31 (hex 1B to hex 1F), and \c?
       becomes either 255 (hex FF) or 95 (hex 5F).

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

       The  sequence  \c? generates DEL (127, hex 7F) in an ASCII environment,
       but because 127 is not a control character in  EBCDIC,  Perl  makes  it
       The  escape \o must be followed by a sequence of octal digits, enclosed
       in braces. An error occurs if this is not the case. This  escape  is  a
       recent  addition  to Perl; it provides way of specifying character code
       points as octal numbers greater than 0777, and  it  also  allows  octal
       numbers and backreferences to be unambiguously specified.

       For greater clarity and unambiguity, it is best to avoid following \ by
       a digit greater than zero. Instead, use \o{} or \x{} to specify numeri-
       cal character code points, and \g{} to specify backreferences. The fol-
       lowing paragraphs describe the old, ambiguous syntax.

       The handling of a backslash followed by a digit other than 0 is compli-
       cated, and Perl has changed over time, causing PCRE2 also to change.

       Outside a character class, PCRE2 reads the digit and any following dig-
       its as a decimal number. If the number is less than 10, begins with the
       digit  8  or  9,  or  if  there are at least that many previous capture
       groups in the expression, the entire sequence is taken as a  backrefer-
       ence.  A  description  of  how this works is given later, following the
       discussion of parenthesized groups.  Otherwise, up to three octal  dig-
       its are read to form a character code.

       Inside  a character class, PCRE2 handles \8 and \9 as the literal char-
       acters "8" and "9", and otherwise reads up to three octal  digits  fol-
       lowing the backslash, using them to generate a data character. Any sub-
       sequent digits stand for themselves. For example, outside  a  character
       class:

         \040   is another way of writing an ASCII space
         \40    is the same, provided there are fewer than 40
                   previous capture groups
         \7     is always a backreference
         \11    might be a backreference, 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 backreference, otherwise the
                   character with octal code 113
         \377   might be a backreference, otherwise
                   the value 255 (decimal)
         \81    is always a backreference

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

   Constraints on character values

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

         8-bit non-UTF mode    no greater than 0xff
         16-bit non-UTF mode   no greater than 0xffff
         32-bit non-UTF mode   no greater than 0xffffffff

       inside and outside character classes. In addition, inside  a  character
       class, \b is interpreted as the backspace character (hex 08).

       When not followed by an opening brace, \N is not allowed in a character
       class.  \B, \R, and \X are not special inside a character  class.  Like
       other  unrecognized  alphabetic  escape sequences, they cause an error.
       Outside a character class, these sequences have different meanings.

   Unsupported escape sequences

       In Perl, the sequences \F, \l, \L, \u, and \U  are  recognized  by  its
       string  handler and used to modify the case of following characters. By
       default, PCRE2 does not support these  escape  sequences  in  patterns.
       However,  if  either  of  the  PCRE2_ALT_BSUX  or  PCRE2_EXTRA_ALT_BSUX
       options is set, \U matches a "U" character,  and  \u  can  be  used  to
       define a character by code point, as described above.

   Absolute and relative backreferences

       The  sequence  \g  followed  by a signed or unsigned number, optionally
       enclosed in braces, is an absolute or relative backreference.  A  named
       backreference  can  be  coded as \g{name}. Backreferences are discussed
       later, following the discussion of parenthesized groups.

   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 capture group as a  subroutine.
       Details  are  discussed  later.   Note  that  \g{...} (Perl syntax) and
       \g<...> (Oniguruma syntax) are not synonymous. The former is a backref-
       erence; the latter is a subroutine call.

   Generic character types

       Another use of backslash is for specifying generic character types:

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

       The  \N  escape  sequence has the same meaning as the "." metacharacter
       when PCRE2_DOTALL is not set, but setting PCRE2_DOTALL does not  change
       the meaning of \N. Note that when \N is followed by an opening brace it
       has a different meaning. See the section entitled "Non-printing charac-
       ters"  above for details. Perl also uses \N{name} to specify characters
       locale. This list may vary if locale-specific matching is taking place.
       For  example, in some locales the "non-breaking space" character (\xA0)
       is recognized as white space, and in others the VT character is not.

       A "word" character is an underscore or any character that is  a  letter
       or  digit.   By  default,  the definition of letters and digits is con-
       trolled by PCRE2's low-valued character tables, and may vary if locale-
       specific matching is taking place (see "Locale support" in the pcre2api
       page). For example, in a French locale such  as  "fr_FR"  in  Unix-like
       systems,  or "french" in Windows, some character codes greater than 127
       are used for accented letters, and these are then matched  by  \w.  The
       use of locales with Unicode is discouraged.

       By  default,  characters  whose  code points are greater than 127 never
       match \d, \s, or \w, and always match \D, \S, and \W, although this may
       be  different  for characters in the range 128-255 when locale-specific
       matching is happening.  These escape sequences  retain  their  original
       meanings  from  before  Unicode support was available, mainly for effi-
       ciency reasons. If the  PCRE2_UCP  option  is  set,  the  behaviour  is
       changed  so  that  Unicode  properties  are used to determine character
       types, as follows:

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

       The upper case escapes match the inverse sets of characters. Note  that
       \d  matches  only decimal digits, whereas \w matches any Unicode digit,
       as well as any Unicode letter, and underscore. Note also that PCRE2_UCP
       affects  \b,  and  \B  because  they are defined in terms of \w and \W.
       Matching these sequences is noticeably slower when PCRE2_UCP is set.

       The sequences \h, \H, \v, and \V, in contrast to the  other  sequences,
       which  match  only ASCII characters by default, always match a specific
       list of code points, whether or not PCRE2_UCP is  set.  The  horizontal
       space characters are:

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

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

   Newline sequences

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

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

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

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

       It is possible to restrict \R to match only CR, LF, or CRLF (instead of
       the complete set  of  Unicode  line  endings)  by  setting  the  option
       PCRE2_BSR_ANYCRLF  at  compile  time. (BSR is an abbrevation for "back-
       slash R".) This can be made the default when PCRE2 is built; if this is
       the  case,  the other behaviour can be requested via the PCRE2_BSR_UNI-
       CODE 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 the compiling func-
       tion.  Note that these special settings, which are not Perl-compatible,
       are  recognized only at the very start of a pattern, and that they must
       be in upper case. If more than one of them is present, the last one  is
       used.  They  can  be  combined with a change of newline convention; for
       example, a pattern can start with:

         (*ANY)(*BSR_ANYCRLF)

       They can also be combined with the (*UTF) or (*UCP) special  sequences.
       Inside  a  character  class,  \R  is  treated as an unrecognized escape
       sequence, and causes an error.

   Unicode character properties

       When PCRE2 is built with Unicode support  (the  default),  three  addi-
       tional  escape sequences that match characters with specific properties
       are available. They can be used in any mode, though in 8-bit and 16-bit
       support  for Unicode script names, Unicode general category properties,
       "Any", which matches any character (including newline), and  some  spe-
       cial  PCRE2  properties  (described  in  the next section).  Other Perl
       properties such as "InMusicalSymbols" are not supported by PCRE2.  Note
       that  \P{Any}  does  not match any characters, so always causes a match
       failure.

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

         \p{Greek}
         \P{Han}

       Unassigned characters (and in non-UTF 32-bit mode, characters with code
       points greater than 0x10FFFF) are assigned the "Unknown" script. Others
       that are not part of an identified script are lumped together as  "Com-
       mon". The current list of scripts is:

       Adlam,  Ahom,  Anatolian_Hieroglyphs,  Arabic, Armenian, Avestan, Bali-
       nese, Bamum, Bassa_Vah, Batak, Bengali,  Bhaiksuki,  Bopomofo,  Brahmi,
       Braille,  Buginese, Buhid, Canadian_Aboriginal, Carian, Caucasian_Alba-
       nian, Chakma,  Cham,  Cherokee,  Common,  Coptic,  Cuneiform,  Cypriot,
       Cyrillic,  Deseret,  Devanagari, Dogra, Duployan, Egyptian_Hieroglyphs,
       Elbasan,  Ethiopic,  Georgian,  Glagolitic,  Gothic,  Grantha,   Greek,
       Gujarati,   Gunjala_Gondi,   Gurmukhi,  Han,  Hangul,  Hanifi_Rohingya,
       Hanunoo,  Hatran,  Hebrew,   Hiragana,   Imperial_Aramaic,   Inherited,
       Inscriptional_Pahlavi,  Inscriptional_Parthian,  Javanese, Kaithi, Kan-
       nada, Katakana, Kayah_Li, Kharoshthi, Khmer,  Khojki,  Khudawadi,  Lao,
       Latin,  Lepcha,  Limbu, Linear_A, Linear_B, Lisu, Lycian, Lydian, Maha-
       jani, Makasar, Malayalam, Mandaic, Manichaean, Marchen,  Masaram_Gondi,
       Medefaidrin,     Meetei_Mayek,     Mende_Kikakui,     Meroitic_Cursive,
       Meroitic_Hieroglyphs, Miao, Modi,  Mongolian,  Mro,  Multani,  Myanmar,
       Nabataean,  New_Tai_Lue, Newa, Nko, Nushu, Ogham, Ol_Chiki, Old_Hungar-
       ian, Old_Italic, Old_North_Arabian, Old_Permic,  Old_Persian,  Old_Sog-
       dian,    Old_South_Arabian,    Old_Turkic,   Oriya,   Osage,   Osmanya,
       Pahawh_Hmong,    Palmyrene,    Pau_Cin_Hau,    Phags_Pa,    Phoenician,
       Psalter_Pahlavi,  Rejang,  Runic,  Samaritan, Saurashtra, Sharada, Sha-
       vian, Siddham, SignWriting, Sinhala,  Sogdian,  Sora_Sompeng,  Soyombo,
       Sundanese,  Syloti_Nagri,  Syriac, Tagalog, Tagbanwa, Tai_Le, Tai_Tham,
       Tai_Viet, Takri, Tamil, Tangut, Telugu, Thaana,  Thai,  Tibetan,  Tifi-
       nagh,   Tirhuta,   Ugaritic,   Unknown,   Vai,  Warang_Citi,  Yi,  Zan-
       abazar_Square.

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

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

         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 whose code
       points are in the range U+D800 to U+DFFF. These characters are no  dif-
       ferent  to any other character when PCRE2 is not in UTF mode (using the
       16-bit or 32-bit library).  However, they  are  not  valid  in  Unicode
       strings and so cannot be tested by PCRE2 in UTF mode, unless UTF valid-
       ity  checking  has   been   turned   off   (see   the   discussion   of
       PCRE2_NO_UTF_CHECK in the pcre2api page).

       The  long  synonyms  for  property  names  that  Perl supports (such as
       \p{Letter}) are not supported by PCRE2, nor is it permitted  to  prefix
       any of these properties with "Is".

       them  do  so by setting the PCRE2_UCP option or by starting the pattern
       with (*UCP).

   Extended grapheme clusters

       The \X escape matches any number of Unicode  characters  that  form  an
       "extended grapheme cluster", and treats the sequence as an atomic group
       (see below).  Unicode supports various kinds of composite character  by
       giving  each  character  a grapheme breaking property, and having rules
       that use these properties to define the boundaries of extended grapheme
       clusters.  The rules are defined in Unicode Standard Annex 29, "Unicode
       Text Segmentation". Unicode 11.0.0 abandoned the use of  some  previous
       properties  that had been used for emojis.  Instead it introduced vari-
       ous emoji-specific properties. PCRE2  uses  only  the  Extended  Picto-
       graphic property.

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

       1. End at the end of the subject string.

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

       3. Do not break Hangul (a Korean  script)  syllable  sequences.  Hangul
       characters  are of five types: L, V, T, LV, and LVT. An L character may
       be followed by an L, V, LV, or LVT character; an LV or V character  may
       be followed by a V or T character; an LVT or T character may be follwed
       only by a T character.

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

       5. Do not end after prepend characters.

       6. Do not break within emoji modifier sequences or emoji zwj sequences.
       That is, do not break between characters with the Extended_Pictographic
       property.  Extend and ZWJ characters are allowed  between  the  charac-
       ters.

       7.  Do  not  break  within  emoji flag sequences. That is, do not break
       between regional indicator (RI) characters if there are an  odd  number
       of RI characters before the break point.

       8. Otherwise, end the cluster.

   PCRE2's additional properties

       As  well as the standard Unicode properties described above, PCRE2 sup-
       ports four more that make it possible  to  convert  traditional  escape
       sequences such as \w and \s to use Unicode properties. PCRE2 uses these
       non-standard, non-Perl properties internally  when  PCRE2_UCP  is  set.
       matches the same characters as Xan, plus underscore.

       There is another non-standard property, Xuc, which matches any  charac-
       ter  that  can  be represented by a Universal Character Name in C++ and
       other programming languages. These are the characters $,  @,  `  (grave
       accent),  and  all  characters with Unicode code points greater than or
       equal to U+00A0, except for the surrogates U+D800 to U+DFFF. Note  that
       most  base  (ASCII) characters are excluded. (Universal Character Names
       are of the form \uHHHH or \UHHHHHHHH where H is  a  hexadecimal  digit.
       Note that the Xuc property does not match these sequences but the char-
       acters that they represent.)

   Resetting the match start

       In normal use, the escape sequence \K  causes  any  previously  matched
       characters  not  to  be  included in the final matched sequence that is
       returned. For example, the pattern:

         foo\Kbar

       matches "foobar", but reports that it has matched "bar".  \K  does  not
       interact with anchoring in any way. The pattern:

         ^foo\Kbar

       matches  only  when  the  subject  begins with "foobar" (in single line
       mode), though it again reports the matched string as "bar".  This  fea-
       ture  is similar to a lookbehind assertion (described below).  However,
       in this case, the part of the subject before the real  match  does  not
       have  to be of fixed length, as lookbehind assertions do. The use of \K
       does not interfere with the setting of captured substrings.  For  exam-
       ple, when the pattern

         (foo)\Kbar

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

       Perl  documents  that  the  use  of  \K  within assertions is "not well
       defined". In PCRE2, \K is acted upon when  it  occurs  inside  positive
       assertions,  but  is  ignored  in negative assertions. Note that when a
       pattern such as (?=ab\K) matches, the reported start of the  match  can
       be  greater  than the end of the match. Using \K in a lookbehind asser-
       tion at the start of a pattern can also lead to odd effects. For  exam-
       ple, consider this pattern:

         (?<=\Kfoo)bar

       If  the  subject  is  "foobar", a call to pcre2_match() with a starting
       offset of 3 succeeds and reports the matching string as "foobar",  that
       is,  the  start  of  the reported match is earlier than where the match
       started.

   Simple assertions

         \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, an "invalid escape sequence" error is generated.

       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. When
       PCRE2 is built with Unicode support, the meanings of \w and \W  can  be
       changed  by  setting  the  PCRE2_UCP option. When this is done, it also
       affects \b and \B. Neither PCRE2 nor Perl  has  a  separate  "start  of
       word"  or "end of word" metasequence. However, whatever follows \b nor-
       mally determines which it is. For example, the fragment \ba matches "a"
       at the start of a word.

       The  \A,  \Z,  and \z assertions differ from the traditional circumflex
       and dollar (described in the next section) in that they only ever match
       at  the  very start and end of the subject string, whatever options are
       set. Thus, they are independent of multiline mode. These  three  asser-
       tions  are  not  affected  by the PCRE2_NOTBOL or PCRE2_NOTEOL options,
       which affect only the behaviour of the circumflex and dollar  metachar-
       acters.  However,  if the startoffset argument of pcre2_match() 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 matching process, as specified by the  startoff-
       set  argument  of  pcre2_match().  It differs from \A when the value of
       startoffset is non-zero. By calling pcre2_match() 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 PCRE2's implementation of \G,  being  true  at  the
       starting  character  of  the matching process, is subtly different from
       Perl's, which defines it as true at the end of the previous  match.  In
       Perl,  these  can  be  different when the previously matched string was
       empty. Because PCRE2 does just one match at a time, it cannot reproduce
       this behaviour.

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

CIRCUMFLEX AND DOLLAR

       The  circumflex  and  dollar  metacharacters are zero-width assertions.
       That is, they test for a particular condition being true  without  con-
       suming any characters from the subject string. These two metacharacters
       are concerned with matching the starts and ends of lines. If  the  new-
       line  convention is set so that only the two-character sequence CRLF is
       of alternatives are involved, but it should be the first thing in  each
       alternative  in  which  it appears if the pattern is ever to match that
       branch. If all possible alternatives start with a circumflex, that  is,
       if  the  pattern  is constrained to match only at the start of the sub-
       ject, it is said to be an "anchored" pattern.  (There  are  also  other
       constructs that can cause a pattern to be anchored.)

       The  dollar  character is an assertion that is true only if the current
       matching point is at the end of  the  subject  string,  or  immediately
       before  a  newline  at  the  end  of  the  string  (by default), unless
       PCRE2_NOTEOL is set. Note, however, that it does not actually match the
       newline. Dollar need not be the last character of the pattern if a num-
       ber 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 charac-
       ter class.

       The meaning of dollar can be changed so that it  matches  only  at  the
       very  end  of the string, by setting the PCRE2_DOLLAR_ENDONLY option at
       compile time. This does not affect the \Z assertion.

       The meanings of the circumflex and dollar metacharacters are changed if
       the  PCRE2_MULTILINE  option  is  set.  When this is the case, a dollar
       character matches before any newlines in the string, as well as at  the
       very  end, and 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, for compatibility with Perl. However,
       this can be changed by setting the PCRE2_ALT_CIRCUMFLEX option.

       For example, the pattern /^abc$/ matches the subject string  "def\nabc"
       (where  \n  represents a newline) in multiline mode, but not otherwise.
       Consequently, patterns that are anchored in single  line  mode  because
       all  branches  start  with  ^ are not anchored in multiline mode, and a
       match for circumflex is  possible  when  the  startoffset  argument  of
       pcre2_match()  is  non-zero. The PCRE2_DOLLAR_ENDONLY option is ignored
       if PCRE2_MULTILINE is set.

       When the newline convention (see "Newline  conventions"  below)  recog-
       nizes  the two-character sequence CRLF as a newline, this is preferred,
       even if the single characters CR and LF are  also  recognized  as  new-
       lines.  For  example,  if  the newline convention is "any", a multiline
       mode circumflex matches before "xyz" in the string "abc\r\nxyz"  rather
       than  after  CR, even though CR on its own is a valid newline. (It also
       matches at the very start of the string, of course.)

       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 PCRE2_MULTILINE  is
       set.

FULL STOP (PERIOD, DOT) AND \N

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

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

       The  escape  sequence  \N when not followed by an opening brace behaves
       like a dot, except that it is not affected by the PCRE2_DOTALL  option.
       In  other words, it matches any character except one that signifies the
       end of a line.

       When \N is followed by an opening brace it has a different meaning. See
       the  section entitled "Non-printing characters" above for details. Perl
       also uses \N{name} to specify characters by Unicode  name;  PCRE2  does
       not support this.

MATCHING A SINGLE CODE UNIT

       Outside  a character class, the escape sequence \C matches any one code
       unit, whether or not a UTF mode is set. In the 8-bit library, one  code
       unit  is  one  byte;  in the 16-bit library it is a 16-bit unit; in the
       32-bit library it is a 32-bit unit. Unlike a  dot,  \C  always  matches
       line-ending  characters.  The  feature  is provided in Perl in order to
       match individual bytes in UTF-8 mode, but it is unclear how it can use-
       fully be used.

       Because  \C  breaks  up characters into individual code units, matching
       one unit with \C in UTF-8 or UTF-16 mode means that  the  rest  of  the
       string  may  start  with  a malformed UTF character. This has undefined
       results, because PCRE2 assumes that it is matching character by charac-
       ter  in  a  valid UTF string (by default it checks the subject string's
       validity at the  start  of  processing  unless  the  PCRE2_NO_UTF_CHECK
       option is used).

       An   application   can   lock   out  the  use  of  \C  by  setting  the
       PCRE2_NEVER_BACKSLASH_C option when compiling a  pattern.  It  is  also
       possible to build PCRE2 with the use of \C permanently disabled.

       PCRE2  does  not allow \C to appear in lookbehind assertions (described
       below) in UTF-8 or UTF-16 modes, because this would make it  impossible
       to  calculate  the  length  of  the lookbehind. Neither the alternative
       matching function pcre2_dfa_match() nor the JIT optimizer support \C in
       these UTF modes.  The former gives a match-time error; the latter fails
       to optimize and so the match is always run using the interpreter.

       In the 32-bit library,  however,  \C  is  always  supported  (when  not
       explicitly  locked  out)  because it always matches a single code unit,
       whether or not UTF-32 is specified.

       In general, the \C escape sequence is best avoided. However, one way of
       using  it  that avoids the problem of malformed UTF-8 or UTF-16 charac-
       ters is to use a lookahead to check the length of the  next  character,
       as  in  this  pattern,  which could be used with a UTF-8 string (ignore
       white space and line breaks):

SQUARE BRACKETS AND CHARACTER CLASSES

       An opening square bracket introduces a character class, terminated by a
       closing square bracket. A closing square bracket on its own is not spe-
       cial  by  default.  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. This
       means that, by default, an empty class cannot be defined.  However,  if
       the  PCRE2_ALLOW_EMPTY_CLASS option is set, a closing square bracket at
       the start does end the (empty) class.

       A character class matches a single character in the subject. A  matched
       character must be in the set of characters defined by the class, unless
       the first character in the class definition is a circumflex,  in  which
       case the subject character must not be in the set defined by the class.
       If a circumflex is actually required as a member of the  class,  ensure
       it is not the first character, or escape it with a backslash.

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

       Characters  in  a class may be specified by their code points using \o,
       \x, or \N{U+hh..} in the usual way. When caseless matching is set,  any
       letters  in a class represent both their upper case and lower case ver-
       sions, 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.

       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  PCRE2_DOTALL  and
       PCRE2_MULTILINE  options  is  used. A class such as [^a] always matches
       one of these characters.

       The generic character type 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  modes, the PCRE2_UCP option
       affects the meanings of \d, \s, \w and their upper case partners,  just
       as  it does when they appear outside a character class, as described in
       the section  entitled  "Generic  character  types"  above.  The  escape
       sequence  \b  has  a  different  meaning  inside  a character class; it
       matches the backspace character. The sequences \B, \R, and \X  are  not
       special  inside  a  character class. Like any other unrecognized escape
       sequences, they cause an error. The same is true for \N when  not  fol-
       lowed by an opening brace.

       The  minus (hyphen) character can be used to specify a range of charac-
       ters in a character  class.  For  example,  [d-m]  matches  any  letter
       given in these cases.

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

       Ranges normally include all code points between the start and end char-
       acters,  inclusive.  They  can  also  be used for code points specified
       numerically, for example [\000-\037]. Ranges can include any characters
       that  are  valid  for  the current mode. In any UTF mode, the so-called
       "surrogate" characters (those whose code points lie between 0xd800  and
       0xdfff  inclusive)  may  not  be  specified  explicitly by default (the
       PCRE2_EXTRA_ALLOW_SURROGATE_ESCAPES option disables this  check).  How-
       ever, ranges such as [\x{d7ff}-\x{e000}], which include the surrogates,
       are always permitted.

       There is a special case in EBCDIC environments  for  ranges  whose  end
       points are both specified as literal letters in the same case. For com-
       patibility with Perl, EBCDIC code points within the range that are  not
       letters  are  omitted. For example, [h-k] matches only four characters,
       even though the codes for h and k are 0x88 and 0x92, a range of 11 code
       points.  However,  if  the range is specified numerically, for example,
       [\x88-\x92] or [h-\x92], all code points are included.

       If a range that includes letters is used when caseless matching is set,
       it matches the letters in either case. For example, [W-c] is equivalent
       to [][\\^_`wxyzabc], matched caselessly, and  in  a  non-UTF  mode,  if
       character  tables  for  a French locale are in use, [\xc8-\xcb] matches
       accented E characters in both cases.

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

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

POSIX CHARACTER CLASSES

       Perl supports the POSIX notation for character classes. This uses names
       enclosed by [: and :] within the enclosing square brackets. PCRE2  also
         digit    decimal digits (same as \d)
         graph    printing characters, excluding space
         lower    lower case letters
         print    printing characters, including space
         punct    printing characters, excluding letters and digits and space
         space    white space (the same as \s from PCRE2 8.34)
         upper    upper case letters
         word     "word" characters (same as \w)
         xdigit   hexadecimal digits

       The default "space" characters are HT (9), LF (10), VT (11),  FF  (12),
       CR  (13),  and space (32). If locale-specific matching is taking place,
       the list of space characters may be different; there may  be  fewer  or
       more of them. "Space" and \s match the same set of characters.

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

         [12[:^digit:]]

       matches "1", "2", or any non-digit. PCRE2 (and Perl) also recognize the
       POSIX syntax [.ch.] and [=ch=] where "ch" is a "collating element", but
       these are not supported, and an error is given if they are encountered.

       By default, characters with values greater than 127 do not match any of
       the POSIX character classes, although this may be different for charac-
       ters  in  the range 128-255 when locale-specific matching is happening.
       However, if the PCRE2_UCP option is passed to pcre2_compile(), some  of
       the  classes are changed so that Unicode character properties are used.
       This  is  achieved  by  replacing  certain  POSIX  classes  with  other
       sequences, as follows:

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

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

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

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


       In  the POSIX.2 compliant library that was included in 4.4BSD Unix, the
       ugly syntax [[:<:]] and [[:>:]] is used for matching  "start  of  word"
       and "end of word". PCRE2 treats these items as follows:

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

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

VERTICAL BAR

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

         gilbert|sullivan

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

INTERNAL OPTION SETTING

       The  settings  of  the  PCRE2_CASELESS,  PCRE2_MULTILINE, PCRE2_DOTALL,
       PCRE2_EXTENDED, PCRE2_EXTENDED_MORE, and PCRE2_NO_AUTO_CAPTURE  options
       can  be  changed  from  within  the  pattern  by  a sequence of letters
       enclosed between "(?"  and ")". These options are Perl-compatible,  and
       are  described in detail in the pcre2api documentation. The option let-
       ters are:

         i  for PCRE2_CASELESS
         m  for PCRE2_MULTILINE
         n  for PCRE2_NO_AUTO_CAPTURE
         s  for PCRE2_DOTALL
         x  for PCRE2_EXTENDED
         xx for PCRE2_EXTENDED_MORE

       For example, (?im) sets caseless, multiline matching. It is also possi-
       ble  to  unset  these  options by preceding the relevant letters with a
       hyphen, for example (?-im). The two "extended" options are not indepen-
       dent; unsetting either one cancels the effects of both of them.

       A   combined  setting  and  unsetting  such  as  (?im-sx),  which  sets
       changed in the same way as the Perl-compatible  options  by  using  the
       characters J and U respectively. However, these are not unset by (?^).

       When  one  of  these  option  changes occurs at top level (that is, not
       inside group parentheses), the change applies to the remainder  of  the
       pattern  that follows. An option change within a group (see below for a
       description of groups) affects only that part of the group that follows
       it, so

         (a(?i)b)c

       matches  abc  and  aBc and no other strings (assuming PCRE2_CASELESS is
       not used).  By this means, options can be made to have  different  set-
       tings in different parts of the pattern. Any changes made in one alter-
       native do carry on into subsequent branches within the same group.  For
       example,

         (a(?i)b|c)

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

       As a convenient shorthand, if any option settings are required  at  the
       start  of a non-capturing group (see the next section), the option let-
       ters may appear between the "?" and the ":". Thus the two patterns

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

       match exactly the same set of strings.

       Note: There are other PCRE2-specific options,  applying  to  the  whole
       pattern,  which  can be set by the application when the compiling func-
       tion is called. In addition, 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 (*UTF) and (*UCP) leading
       sequences that can be used to set UTF and Unicode property modes;  they
       are  equivalent to setting the PCRE2_UTF and PCRE2_UCP options, respec-
       tively. However,  the  application  can  set  the  PCRE2_NEVER_UTF  and
       PCRE2_NEVER_UCP  options,  which  lock  out  the  use of the (*UTF) and
       (*UCP) sequences.

GROUPS

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

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

         cat(aract|erpillar|)


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

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

       The  fact  that  plain  parentheses  fulfil two functions is not always
       helpful.  There are often times when grouping is required without  cap-
       turing.  If an opening parenthesis is followed by a question mark and a
       colon, the group does not do any capturing, and  is  not  counted  when
       computing  the number of any subsequent capture groups. 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 capture groups is 65535.

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

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

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

DUPLICATE GROUP NUMBERS

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

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

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

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

       A relative reference such as (?-1) is no different: it is just a conve-
       nient way of computing an absolute group number.

       If a condition test for a group's having matched refers to a non-unique
       number, the test is true if any group with that number has matched.

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

NAMED CAPTURE GROUPS

       Identifying capture groups by number is simple, but it can be very hard
       to keep track of the numbers in complicated patterns.  Furthermore,  if
       an  expression  is  modified, the numbers may change. To help with this
       difficulty, PCRE2 supports the naming of capture groups.  This  feature
       was  not  added to Perl until release 5.10. Python had the feature ear-
       lier, and PCRE1 introduced it at release 4.0, using the Python  syntax.
       PCRE2 supports both the Perl and the Python syntax.

       In  PCRE2,  a  capture  group  can  be  named  in  one  of  three ways:
       (?<name>...) or (?'name'...) as in Perl, or (?P<name>...) as in Python.
       Names  may be up to 32 code units long. When PCRE2_UTF is not set, they
       may contain only ASCII alphanumeric  characters  and  underscores,  but
       must start with a non-digit. When PCRE2_UTF is set, the syntax of group
       names is extended to allow any Unicode letter or Unicode decimal digit.
       In other words, group names must match one of these patterns:

         ^[_A-Za-z][_A-Za-z0-9]*\z   when PCRE2_UTF is not set
         ^[_\p{L}][_\p{L}\p{Nd}]*\z  when PCRE2_UTF is set

       References  to  capture groups from other parts of the pattern, such as
       backreferences, recursion, and conditions, can all be made by  name  as
       well as by number.

       Named capture groups are allocated numbers as well as names, exactly as
       if the names were not present. In both PCRE2 and Perl,  capture  groups
       are  primarily  identified  by  numbers; any names are just aliases for
       these numbers. The PCRE2 API provides function calls for extracting the
       complete  name-to-number  translation table from a compiled pattern, as
       well as convenience functions for  extracting  captured  substrings  by
       name.

       Warning:  When  more  than  one  capture  group has the same number, as
       described in the previous section, a name given to one of them  applies
       to all of them. Perl allows identically numbered groups to have differ-
       ent names.  Consider this pattern, where there are two capture  groups,
       both numbered 1:

         (?|(?<AA>aa)|(?<BB>bb))

       Perl  allows  this,  with  both  names AA and BB as aliases of group 1.
       Thus, after a successful match, both names yield the same value (either
       "aa" or "bb").

       cate names are permitted for groups with the same number, for example:

         (?|(?<AA>aa)|(?<AA>bb))

       The duplicate name constraint can be disabled by setting the PCRE2_DUP-
       NAMES option at compile time, or by the use of (?J) within the pattern.
       Duplicate  names  can be useful for patterns where only one instance of
       the named capture group can match. Suppose you want to match  the  name
       of  a  weekday,  either as a 3-letter abbreviation or as the full name,
       and in both cases you want to extract the  abbreviation.  This  pattern
       (ignoring the line breaks) does the job:

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

       There  are five capture groups, but only one is ever set after a match.
       The convenience functions for extracting the data by name  returns  the
       substring  for  the first (and in this example, the only) group of that
       name that matched. This saves searching to find which numbered group it
       was.  (An  alternative  way of solving this problem is to use a "branch
       reset" group, as described in the previous section.)

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

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

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

       If you use a named reference in a condition test (see the section about
       conditions below), either to check whether a capture group has matched,
       or to check for recursion, all groups 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 capture groups, see the
       pcre2api documentation.

REPETITION

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

         a literal data character
         the dot metacharacter
         the \C escape sequence
         the \R escape sequence
         z{2,4}

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

         [aeiou]{3,}

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

         \d{8}

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

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

       The quantifier {0} is permitted, causing the expression to behave as if
       the previous item and the quantifier were not present. This may be use-
       ful for capture groups that are referenced as  subroutines  from  else-
       where  in the pattern (but see also the section entitled "Defining cap-
       ture groups for use by reference only" below). Except for parenthesized
       groups,  items that have a {0} quantifier are omitted from the compiled
       pattern.

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

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

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

         (a?)*

       Earlier  versions  of  Perl  and PCRE1 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
       group does in fact match no characters, the loop is forcibly broken.

       By default, quantifiers are "greedy", that is, they match  as  much  as
       possible (up to the maximum number of permitted times), without causing
       fails, because it matches the entire string owing to the greediness  of
       the  .*  item. However, if a quantifier is followed by a question mark,
       it ceases to be greedy, and instead matches the minimum number of times
       possible, so the pattern

         /\*.*?\*/

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

         \d??\d

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

       If the PCRE2_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  group is quantified with a minimum repeat count
       that is greater than 1 or  with  a  limited  maximum,  more  memory  is
       required  for  the  compiled  pattern, in proportion to the size of the
       minimum or maximum.

       If a pattern starts with  .*  or  .{0,}  and  the  PCRE2_DOTALL  option
       (equivalent  to  Perl's /s) is set, thus allowing the dot to match new-
       lines, the pattern is implicitly  anchored,  because  whatever  follows
       will  be  tried against every character position in the subject string,
       so there is no point in retrying the  overall  match  at  any  position
       after the first. PCRE2 normally treats such a pattern as though it were
       preceded by \A.

       In cases where it is known that the subject  string  contains  no  new-
       lines,  it  is worth setting PCRE2_DOTALL in order to obtain this opti-
       mization, or alternatively, using ^ to indicate anchoring explicitly.

       However, there are some cases where the optimization  cannot  be  used.
       When  .*   is  inside  capturing  parentheses that are the subject of a
       backreference elsewhere in the pattern, a match at the start  may  fail
       where a later one succeeds. Consider, for example:

         (.*)abc\1

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

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

       is "tweedledee". However, if there are nested capture groups, the  cor-
       responding  captured  values  may have been set in previous iterations.
       For example, after

         (a|(b))+

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

ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS

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

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

         123456bar

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

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

         (?>\d+)foo

       Perl 5.28 introduced an experimental alphabetic form starting  with  (*
       which may be easier to remember:

         (*atomic:\d+)foo

       This kind of parenthesized group "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, however, works as normal.

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

       Atomic groups are not capture groups. 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 pat-
       example:

         (abc|xyz){2,3}+

       Possessive  quantifiers  are  always  greedy;  the   setting   of   the
       PCRE2_UNGREEDY  option  is  ignored. They are a convenient notation for
       the simpler forms of atomic group. However, there is no  difference  in
       the meaning of a possessive quantifier and the equivalent atomic group,
       though there may be a performance  difference;  possessive  quantifiers
       should be slightly faster.

       The  possessive  quantifier syntax is an extension to the Perl 5.8 syn-
       tax.  Jeffrey Friedl originated the idea (and the name)  in  the  first
       edition of his book. Mike McCloskey liked it, so implemented it when he
       built Sun's Java package, and PCRE1 copied it from there. It found  its
       way into Perl at release 5.10.

       PCRE2  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 follow.  This feature can be disabled by the PCRE2_NO_AUTO-
       POSSESS option, or starting the pattern with (*NO_AUTO_POSSESS).

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

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

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

         aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

       it takes a long time before reporting  failure.  This  is  because  the
       string  can be divided between the internal \D+ repeat and the external
       * repeat in a large number of ways, and all  have  to  be  tried.  (The
       example  uses  [!?]  rather than a single character at the end, because
       both PCRE2 and Perl have an optimization that allows for  fast  failure
       when  a single character is used. They remember the last single charac-
       ter that is required for a match, and fail early if it is  not  present
       in  the  string.)  If  the pattern is changed so that it uses an atomic
       group, like this:

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

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

BACKREFERENCES

       Outside a character class, a backslash followed by a digit greater than
       0  (and  possibly further digits) is a backreference to a capture group
       group whose number is 8 or more using this syntax  because  a  sequence
       such  as  \50  is  interpreted as a character defined in octal. See the
       subsection entitled "Non-printing characters" above for further details
       of  the  handling of digits following a backslash. Other forms of back-
       referencing do not suffer from this restriction. In  particular,  there
       is no problem when named capture groups are used (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 a signed or unsigned number, optionally enclosed in
       braces. These examples are all identical:

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

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

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

       The sequence \g{-1} is a reference to the most recently started capture
       group before \g, that is, is it equivalent to \2 in this example. Simi-
       larly, \g{-2} would be equivalent to \1. The use of relative references
       can be helpful in long patterns, and also in patterns that are  created
       by  joining  together  fragments  that  contain references within them-
       selves.

       The sequence \g{+1} is a reference to the next capture group. This kind
       of  forward  reference can be useful in patterns that repeat. Perl does
       not support the use of + in this way.

       A backreference matches whatever actually  most  recently  matched  the
       capture  group  in  the current subject string, rather than anything at
       all that matches the group (see "Groups as subroutines" below for a way
       of doing that). So the pattern

         (sens|respons)e and \1ibility

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

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

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

       There  are  several  different  ways of writing backreferences to named
       capture groups. The .NET syntax \k{name} and the Perl  syntax  \k<name>
       or  \k'name'  are  supported,  as  is the Python syntax (?P=name). Perl
       There  may be more than one backreference to the same group. If a group
       has not actually been used in a particular match, backreferences to  it
       always fail by default. For example, the pattern

         (a|(bc))\2

       always  fails  if  it starts to match "a" rather than "bc". However, if
       the PCRE2_MATCH_UNSET_BACKREF option is set at compile time, a backref-
       erence to an unset value matches an empty string.

       Because  there may be many capture groups in a pattern, all digits fol-
       lowing a backslash are taken as part of a potential backreference  num-
       ber.  If  the  pattern continues with a digit character, some delimiter
       must be used to terminate the backreference. If the  PCRE2_EXTENDED  or
       PCRE2_EXTENDED_MORE  option is set, this can be white space. Otherwise,
       the \g{} syntax or an empty comment (see "Comments" below) can be used.

   Recursive backreferences

       A backreference that occurs inside the group to which it  refers  fails
       when  the  group  is  first used, so, for example, (a\1) never matches.
       However, such references can be  useful  inside  repeated  groups.  For
       example, the pattern

         (a|b\1)+

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

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

ASSERTIONS

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

       More complicated assertions are coded as  parenthesized  groups.  There
       are  two  kinds:  those  that look ahead of the current position in the
       subject string, and those that look behind it,  and  in  each  case  an
       assertion  may be positive (must match for the assertion to be true) or
       negative (must not match for the assertion to be  true).  An  assertion
       group is matched in the normal way, and if it is true, matching contin-
       ues after it, but with the matching position in the subject  string  is
       was it was before the assertion was processed.

       were  captured  are  discarded (as happens with any pattern branch that
       fails to match). A  negative  assertion  is  true  only  when  all  its
       branches fail to match; this means that no captured substrings are ever
       retained after a successful negative assertion. When an assertion  con-
       tains a matching branch, what happens depends on the type of assertion.

       For  a  positive  assertion, internally captured substrings in the suc-
       cessful branch are retained, and matching continues with the next  pat-
       tern  item  after  the  assertion. For a negative assertion, a matching
       branch means that the assertion is not true. If such  an  assertion  is
       being  used as a condition in a conditional group (see below), captured
       substrings are retained,  because  matching  continues  with  the  "no"
       branch of the condition. For other failing negative assertions, control
       passes to the previous backtracking point, thus discarding any captured
       strings within the assertion.

       For  compatibility  with  Perl,  most assertion groups may be repeated;
       though it makes no sense to assert the same thing  several  times,  the
       side effect of capturing may occasionally be useful. However, an asser-
       tion that forms the condition for a conditional group may not be  quan-
       tified. In practice, for other assertions, there only three cases:

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

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

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

   Alphabetic assertion names

       Traditionally, symbolic sequences such as (?= and (?<= have  been  used
       to  specify lookaround assertions. Perl 5.28 introduced some experimen-
       tal alphabetic alternatives which might be easier to remember. They all
       start  with  (* instead of (? and must be written using lower case let-
       ters. PCRE2 supports the following synonyms:

         (*positive_lookahead:  or (*pla: is the same as (?=
         (*negative_lookahead:  or (*nla: is the same as (?!
         (*positive_lookbehind: or (*plb: is the same as (?<=
         (*negative_lookbehind: or (*nlb: is the same as (?<!

       For example, (*pla:foo) is the same assertion as (?=foo). In  the  fol-
       lowing  sections, the various assertions are described using the origi-
       nal symbolic forms.

   Lookahead assertions


         (?!foo)bar

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

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

   Lookbehind assertions

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

         (?<!foo)bar

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

         (?<=bullock|donkey)

       is permitted, but

         (?<!dogs?|cats?)

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

         (?<=ab(c|de))

       is not permitted, because its single top-level  branch  can  match  two
       different  lengths,  but  it is acceptable to PCRE2 if rewritten to use
       two top-level branches:

         (?<=abc|abde)

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

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

       them,   but   only    if    certain    conditions    are    met.    The
       PCRE2_MATCH_UNSET_BACKREF  option must not be set, there must be no use
       of (?| in the pattern (it creates duplicate group numbers), and if  the
       backreference  is by name, the name must be unique. Of course, the ref-
       erenced group must itself match a fixed length substring. The following
       pattern matches words containing at least two characters that begin and
       end with the same character:

          \b(\w)\w++(?<=\1)

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

         abcd$

       when applied to a long string that does  not  match.  Because  matching
       proceeds  from  left to right, PCRE2 will look for each "a" in the sub-
       ject and then see if what follows matches the rest of the  pattern.  If
       the pattern is specified as

         ^.*abcd$

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

         ^.*+(?<=abcd)

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

   Using multiple assertions

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

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

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

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


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

SCRIPT RUNS

       In concept, a script run is a sequence of characters that are all  from
       the  same  Unicode script such as Latin or Greek. However, because some
       scripts are commonly used together, and because  some  diacritical  and
       other  marks  are  used  with  multiple scripts, it is not that simple.
       There is a full description of the rules that PCRE2 uses in the section
       entitled "Script Runs" in the pcre2unicode documentation.

       If  part  of a pattern is enclosed between (*script_run: or (*sr: and a
       closing parenthesis, it fails if the sequence  of  characters  that  it
       matches  are  not  a  script  run. After a failure, normal backtracking
       occurs. Script runs can be used to detect spoofing attacks using  char-
       acters  that  look the same, but are from different scripts. The string
       "paypal.com" is an infamous example, where the letters could be a  mix-
       ture of Latin and Cyrillic. This pattern ensures that the matched char-
       acters in a sequence of non-spaces that follow white space are a script
       run:

         \s+(*sr:\S+)

       To  be  sure  that  they are all from the Latin script (for example), a
       lookahead can be used:

         \s+(?=\p{Latin})(*sr:\S+)

       This works as long as the first character is expected to be a character
       in  that  script,  and  not (for example) punctuation, which is allowed
       with any script. If this is not the case, a more creative lookahead  is
       needed.  For  example, if digits, underscore, and dots are permitted at
       the start:

         \s+(?=[0-9_.]*\p{Latin})(*sr:\S+)

       In many cases, backtracking into a script run pattern fragment  is  not
       desirable.  The  script run can employ an atomic group to prevent this.
       Because this is a common requirement, a shorthand notation is  provided
       by (*atomic_script_run: or (*asr:

         (*asr:...) is the same as (*sr:(?>...))

       Note that the atomic group is inside the script run. Putting it outside
       would not prevent backtracking into the script run pattern.

       Support for script runs is not available if PCRE2 is  compiled  without
       Unicode support. A compile-time error is given if any of the above con-
       structs is encountered. Script runs are not supported by the  alternate
       matching  function,  pcre2_dfa_match() because they use the same mecha-
       nism as capturing parentheses.

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

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

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

       There are five kinds of condition: references to capture groups, refer-
       ences  to  recursion,  two pseudo-conditions called DEFINE and VERSION,
       and assertions.

   Checking for a used capture group by number

       If the text between the parentheses consists of a sequence  of  digits,
       the  condition is true if a capture group of that number has previously
       matched. If there is more than one capture group with the  same  number
       (see  the earlier section about duplicate group numbers), the condition
       is true if any of them have matched. An alternative notation is to pre-
       cede the digits with a plus or minus sign. In this case, the group num-
       ber is relative rather than absolute. The most recently opened  capture
       group  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 capture group can be referenced as (?(+1), and so on.
       (The value zero in any of these forms is not used; it provokes  a  com-
       pile-time error.)

       Consider  the  following  pattern, which contains non-significant white
       space to make it more readable (assume the PCRE2_EXTENDED  option)  and
       to divide it into three parts for ease of discussion:

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

       The  first  part  matches  an optional opening parenthesis, and if that
       character is present, sets it as the first captured substring. The sec-
       ond  part  matches one or more characters that are not parentheses. The
       third part is a conditional group that tests whether or not  the  first
       capture  group  matched. If it 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 conditional group 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... ( \( )?    [^()]+    (?(-1) \) ) ...

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

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

   Checking for pattern recursion

       "Recursion"  in  this sense refers to any subroutine-like call from one
       part of the pattern to another, whether or not it  is  actually  recur-
       sive.  See  the  sections  entitled "Recursive patterns" and "Groups as
       subroutines" below for details of recursion and subroutine calls.

       If a condition is the string (R), and there is no  capture  group  with
       the  name R, the condition is true if matching is currently in a recur-
       sion or subroutine call to the whole pattern or any capture  group.  If
       digits  follow  the letter R, and there is no group with that name, the
       condition is true if the most recent call is  into  a  group  with  the
       given  number,  which must exist somewhere in the overall pattern. This
       is a contrived example that is equivalent to a+b:

         ((?(R1)a+|(?1)b))

       However, in both cases, if there is a capture  group  with  a  matching
       name,  the  condition tests for its being set, as described in the sec-
       tion above, instead of testing for recursion. For example,  creating  a
       group  with  the  name  R1  by adding (?<R1>) to the above pattern com-
       pletely changes its meaning.

       If a name preceded by ampersand follows the letter R, for example:

         (?(R&name)...)

       the condition is true if the most recent recursion is into a  group  of
       that name (which must exist within the pattern).

       This condition does not check the entire recursion stack. It tests only
       the current level. If the name used in a condition of this  kind  is  a
       duplicate,  the  test is applied to all groups 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.

   Defining capture groups for use by reference only

       If the condition is the string (DEFINE), the condition is always false,
       even  if there is a group with the name DEFINE. In this case, there may
       be only one alternative in the rest of the  conditional  group.  It  is
       always  skipped  if control reaches this point in the pattern; the idea
       of DEFINE is that it can be used to define subroutines that can be ref-
       erenced  from  elsewhere.  (The use of subroutines is described below.)
       For  example,  a  pattern  to   match   an   IPv4   address   such   as
       "192.168.23.245"  could  be  written  like this (ignore white space and
       line breaks):

   Checking the PCRE2 version

       Programs that link with a PCRE2 library can check the version by  call-
       ing  pcre2_config()  with  appropriate arguments. Users of applications
       that do not have access to the underlying code cannot do this.  A  spe-
       cial  "condition" called VERSION exists to allow such users to discover
       which version of PCRE2 they are dealing with by using this condition to
       match  a string such as "yesno". VERSION must be followed either by "="
       or ">=" and a version number.  For example:

         (?(VERSION>=10.4)yes|no)

       This pattern matches "yes" if the PCRE2 version is greater or equal  to
       10.4,  or "no" otherwise. The fractional part of the version number may
       not contain more than two digits.

   Assertion conditions

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

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

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

       When an assertion that is a condition contains capture groups, any cap-
       turing  that  occurs  in  a matching branch is retained afterwards, for
       both positive and negative assertions, because matching always  contin-
       ues  after  the  assertion, whether it succeeds or fails. (Compare non-
       conditional assertions, for which captures are retained only for  posi-
       tive assertions that succeed.)

COMMENTS

       There are two ways of including comments in patterns that are processed
       by PCRE2. 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 group name or number. The  characters  that
       make up a comment play no part in the pattern matching.

       The  sequence (?# marks the start of a comment that continues up to the
       next closing parenthesis. Nested parentheses are not permitted. If  the
       PCRE2_EXTENDED  or  PCRE2_EXTENDED_MORE  option  is set, an unescaped #
       character also introduces a comment, which in this  case  continues  to
       On  encountering  the # character, pcre2_compile() skips along, looking
       for a newline in the pattern. The sequence \n is still literal at  this
       stage,  so  it does not terminate the comment. Only an actual character
       with the code value 0x0a (the default newline) does so.

RECURSIVE PATTERNS

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

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

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

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

       Obviously,  PCRE2  cannot  support  the  interpolation  of  Perl  code.
       Instead, it supports special syntax for recursion of  the  entire  pat-
       tern, and also for individual capture group recursion. After its intro-
       duction in PCRE1 and Python, this kind of  recursion  was  subsequently
       introduced into Perl at release 5.10.

       A  special  item  that consists of (? followed by a number greater than
       zero and a closing parenthesis is a recursive subroutine  call  of  the
       capture  group of the given number, provided that it occurs inside that
       group. (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 PCRE2 pattern solves the nested parentheses  problem  (assume  the
       PCRE2_EXTENDED option is set so that white space is ignored):

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

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

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

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

       number. Consider, for example:

         (?|(a)|(b)) (c) (?-2)

       The first two capture groups (a) and (b) are both numbered 1, and group
       (c) is number 2. When the reference (?-2) is  encountered,  the  second
       most  recently opened parentheses has the number 1, but it is the first
       such group (the (a) group) to which the recursion refers. This would be
       the  same if an absolute reference (?1) was used. In other words, rela-
       tive references are just a shorthand for computing a group number.

       It is also possible to refer to subsequent capture groups,  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.  The  Perl  syntax
       for  this  is  (?&name);  PCRE1's earlier syntax (?P>name) is also sup-
       ported. We could rewrite the above example as follows:

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

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

       The example pattern that we have been looking at contains nested unlim-
       ited 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

         (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()

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

       At  the  end  of a match, the values of capturing parentheses are those
       from the outermost level. If you want to obtain intermediate values,  a
       callout function can be used (see below and the pcre2callout documenta-
       tion). If the pattern above is matched against

         (ab(cd)ef)

       the value for the inner capturing parentheses  (numbered  2)  is  "ef",
       which  is  the last value taken on at the top level. If a capture group
       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.

       Do not confuse the (?R) item with the condition (R),  which  tests  for
       recursion.   Consider  this pattern, which matches text in angle brack-
       ets, allowing for arbitrary nesting. Only digits are allowed in  nested

       Before release 10.30, recursion processing in PCRE2 differed from  Perl
       in  that  a  recursive  subroutine call was always treated as an atomic
       group. That is, once it had matched some of the subject string, it  was
       never  re-entered,  even if it contained untried alternatives and there
       was a subsequent matching failure. (Historical note:  PCRE  implemented
       recursion before Perl did.)

       Starting  with  release 10.30, recursive subroutine calls are no longer
       treated as atomic. That is, they can be re-entered to try unused alter-
       natives  if  there  is a matching failure later in the pattern. This is
       now compatible with the way Perl works. If you want a  subroutine  call
       to be atomic, you must explicitly enclose it in an atomic group.

       Supporting  backtracking  into  recursions  simplifies certain types of
       recursive  pattern.  For  example,  this  pattern  matches  palindromic
       strings:

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

       The  second  branch  in the group matches a single central character in
       the palindrome when there are an odd number of characters,  or  nothing
       when  there  are  an even number of characters, but in order to work it
       has to be able to try the second case when  the  rest  of  the  pattern
       match fails. If you want to match typical palindromic phrases, the pat-
       tern has to ignore all non-word characters,  which  can  be  done  like
       this:

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

       If  run  with  the  PCRE2_CASELESS option, this pattern matches phrases
       such as "A man, a plan, a canal: Panama!". Note the use of the  posses-
       sive  quantifier  *+  to  avoid backtracking into sequences of non-word
       characters. Without this, PCRE2 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.

       Another way in which PCRE2 and Perl used to differ in  their  recursion
       processing  is  in  the  handling of captured values. Formerly in Perl,
       when a group was called recursively or as a subroutine  (see  the  next
       section), it had no access to any values that were captured outside the
       recursion, whereas in PCRE2 these values can  be  referenced.  Consider
       this pattern:

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

       This  pattern matches "bab". The first capturing parentheses match "b",
       then in the second group, when the backreference \1 fails to match "b",
       the second alternative matches "a" and then recurses. In the recursion,
       \1 does now match "b" and so the whole match succeeds. This match  used
       to fail in Perl, but in later versions (I tried 5.024) it now works.

GROUPS AS SUBROUTINES

       An earlier example pointed out that the pattern

         (sens|respons)e and \1ibility

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

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

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

       Like recursions, subroutine calls used to be  treated  as  atomic,  but
       this  changed  at  PCRE2 release 10.30, so backtracking into subroutine
       calls can now occur. However, 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 group is
       defined, so if it is used as  a  subroutine,  such  options  cannot  be
       changed for different calls. For example, consider this pattern:

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

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

       The behaviour of backtracking control verbs in groups  when  called  as
       subroutines is described in the section entitled "Backtracking verbs in
       subroutines" below.

ONIGURUMA SUBROUTINE SYNTAX

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

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

       PCRE2 supports an extension to Oniguruma: if a number is preceded by  a
       plus or a minus sign it is taken as a relative reference. For example:

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

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

CALLOUTS

       Perl has a feature whereby using the sequence (?{...}) causes arbitrary

       Within  a  regular expression, (?C<arg>) indicates a point at which the
       external function is to be called. There  are  two  kinds  of  callout:
       those  with a numerical argument and those with a string argument. (?C)
       on its own with no argument is treated as (?C0). A  numerical  argument
       allows  the  application  to  distinguish  between  different callouts.
       String arguments were added for release 10.20 to make it  possible  for
       script  languages that use PCRE2 to embed short scripts within patterns
       in a similar way to Perl.

       During matching, when PCRE2 reaches a callout point, the external func-
       tion  is  called.  It is provided with the number or string argument of
       the callout, the position in the pattern, and one item of data that  is
       also set in the match block. The callout function may cause matching to
       proceed, to backtrack, or to fail.

       By default, PCRE2 implements a  number  of  optimizations  at  matching
       time,  and  one  side-effect is that sometimes callouts are skipped. If
       you need all possible callouts to happen, you need to set options  that
       disable  the relevant optimizations. More details, including a complete
       description of the programming interface to the callout  function,  are
       given in the pcre2callout documentation.

   Callouts with numerical arguments

       If  you  just  want  to  have  a means of identifying different callout
       points, put a number less than 256 after the  letter  C.  For  example,
       this pattern has two callout points:

         (?C1)abc(?C2)def

       If  the PCRE2_AUTO_CALLOUT flag is passed to pcre2_compile(), numerical
       callouts are automatically installed before each item in  the  pattern.
       They  are all numbered 255. If there is a conditional group in the pat-
       tern whose condition is an assertion, an additional callout is inserted
       just  before the condition. An explicit callout may also be set at this
       position, as in this example:

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

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

   Callouts with string arguments

       A  delimited  string may be used instead of a number as a callout argu-
       ment. The starting delimiter must be one of ` ' " ^ % #  $  {  and  the
       ending delimiter is the same as the start, except for {, where the end-
       ing delimiter is }. If  the  ending  delimiter  is  needed  within  the
       string, it must be doubled. For example:

         (?C'ab ''c'' d')xyz(?C{any text})pqr

       The  doubling  is  removed  before  the string is passed to the callout
       characters that does not include a closing parenthesis. The name is not
       processed  in  any  way,  and  it  is not possible to include a closing
       parenthesis  in  the  name.   This  can  be  changed  by  setting   the
       PCRE2_ALT_VERBNAMES  option,  but the result is no longer Perl-compati-
       ble.

       When PCRE2_ALT_VERBNAMES is set, backslash  processing  is  applied  to
       verb  names  and  only  an unescaped closing parenthesis terminates the
       name. However, the only backslash items that are permitted are \Q,  \E,
       and  sequences such as \x{100} that define character code points. Char-
       acter type escapes such as \d are faulted.

       A closing parenthesis can be included in a name either as \) or between
       \Q  and  \E. In addition to backslash processing, if the PCRE2_EXTENDED
       or PCRE2_EXTENDED_MORE option is also set, unescaped whitespace in verb
       names is skipped, and #-comments are recognized, exactly as in the rest
       of the pattern.  PCRE2_EXTENDED and PCRE2_EXTENDED_MORE do  not  affect
       verb names unless PCRE2_ALT_VERBNAMES is also set.

       The  maximum  length of a name is 255 in the 8-bit library and 65535 in
       the 16-bit and 32-bit libraries. If the name is empty, that is, if  the
       closing  parenthesis immediately follows the colon, the effect is as if
       the colon were not there. Any number of these verbs may occur in a pat-
       tern.

       Since  these  verbs  are  specifically related to backtracking, most of
       them can be used only when the pattern is to be matched using the  tra-
       ditional matching function, because that uses a backtracking algorithm.
       With the exception of (*FAIL), which behaves like  a  failing  negative
       assertion, the backtracking control verbs cause an error if encountered
       by the DFA matching function.

       The behaviour of these verbs in repeated  groups,  assertions,  and  in
       capture  groups  called  as subroutines (whether or not recursively) is
       documented below.

   Optimizations that affect backtracking verbs

       PCRE2 contains some optimizations that are used to speed up matching by
       running some checks at the start of each match attempt. For example, it
       may know the minimum length of matching subject, or that  a  particular
       character must be present. When one of these optimizations bypasses the
       running of a match,  any  included  backtracking  verbs  will  not,  of
       course, be processed. You can suppress the start-of-match optimizations
       by setting the PCRE2_NO_START_OPTIMIZE option when  calling  pcre2_com-
       pile(),  or by starting the pattern with (*NO_START_OPT). There is more
       discussion of this option in the section entitled "Compiling a pattern"
       in the pcre2api documentation.

       Experiments  with  Perl  suggest that it too has similar optimizations,
       and like PCRE2, turning them off can change the result of a match.

   Verbs that act immediately

       tured. For example:

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

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

       Warning:  (*ACCEPT)  should  not  be  used  within  a script run group,
       because it causes an immediate  exit  from  the  group,  bypassing  the
       script run checking.

         (*FAIL) or (*FAIL:NAME)

       This  verb causes a matching failure, forcing backtracking to occur. It
       may be abbreviated to (*F). 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 PCRE2. The nearest equivalent is the callout fea-
       ture, as for example in this pattern:

         a+(?C)(*FAIL)

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

       (*ACCEPT:NAME)  and  (*FAIL:NAME)  are treated as (*MARK:NAME)(*ACCEPT)
       and (*MARK:NAME)(*FAIL), respectively.

   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. For all the other  backtrack-
       ing control verbs, a NAME argument is optional.

       When  a  match  succeeds, the name of the last-encountered mark name on
       the matching path is passed back to the caller as described in the sec-
       tion entitled "Other information about the match" in the pcre2api docu-
       mentation. This applies to all instances of (*MARK)  and  other  verbs,
       including those inside assertions and atomic groups. However, there are
       differences in those cases when (*MARK) is  used  in  conjunction  with
       (*SKIP) as described below.

       The  mark name that was last encountered on the matching path is passed
       back. A verb without a NAME argument is ignored for this purpose.  Here
       is  an  example of pcre2test output, where the "mark" modifier requests
       the retrieval and outputting of (*MARK) data:

           re> /X(*MARK:A)Y|X(*MARK:B)Z/mark
         data> XY

       true, the name is recorded and passed back if it  is  the  last-encoun-
       tered. This does not happen for negative assertions or failing positive
       assertions.

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

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

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

       If you are interested in  (*MARK)  values  after  failed  matches,  you
       should  probably  set the PCRE2_NO_START_OPTIMIZE option (see above) to
       ensure that the match is always attempted.

   Verbs that act after backtracking

       The following verbs do nothing when they are encountered. Matching con-
       tinues  with  what follows, but if there is a subsequent match failure,
       causing a backtrack to the verb, a failure is forced.  That  is,  back-
       tracking  cannot  pass  to  the  left of the verb. However, when one of
       these verbs appears inside an atomic group or in a lookaround assertion
       that  is  true,  its effect is confined to that group, because once the
       group has been matched, there is never any backtracking into it.  Back-
       tracking from beyond an assertion or an atomic group ignores the entire
       group, and seeks a preceding backtracking point.

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

         (*COMMIT) or (*COMMIT:NAME)

       This  verb  causes the whole match to fail outright if there is a later
       matching failure that causes backtracking to reach it. Even if the pat-
       tern  is  unanchored,  no further attempts to find a match by advancing
       the starting point take place. If (*COMMIT) is  the  only  backtracking
       verb that is encountered, once it has been passed pcre2_match() is com-
       mitted to finding a match at the current starting point, or not at all.
       For example:

         a+(*COMMIT)b

       This  matches  "xxaab" but not "aacaab". It can be thought of as a kind
       of dynamic anchor, or "I've started, so I must finish."

       The behaviour of (*COMMIT:NAME) is not the same  as  (*MARK:NAME)(*COM-
       MIT).  It is like (*MARK:NAME) in that the name is remembered for pass-

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

       For  the first pattern, PCRE2 knows that any match must start with "a",
       so the optimization skips along the subject to "a" before applying  the
       pattern  to the first set of data. The match attempt then succeeds. The
       second pattern disables the optimization that skips along to the  first
       character.  The  pattern  is  now  applied  starting at "x", and so the
       (*COMMIT) causes the match to fail without trying  any  other  starting
       points.

         (*PRUNE) or (*PRUNE:NAME)

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

       The behaviour of (*PRUNE:NAME) is not the same as (*MARK:NAME)(*PRUNE).
       It is like (*MARK:NAME) in that the name is remembered for passing back
       to  the  caller. However, (*SKIP:NAME) searches only for names set with
       (*MARK), ignoring those set by other backtracking verbs.

         (*SKIP)

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

         a+(*SKIP)b

       If  the  subject  is  "aaaac...",  after  the first match attempt fails
       (starting at the first character in the  string),  the  starting  point
       skips on to start the next attempt at "c". Note that a possessive quan-
       tifer does not have the same effect as this example; although it  would
       suppress  backtracking  during  the  first  match  attempt,  the second
       attempt would start at the second character instead of skipping  on  to
       "c".
       atomic groups or assertions, because they are never re-entered by back-
       tracking. Compare the following pcre2test examples:

           re> /a(?>(*MARK:X))(*SKIP:X)(*F)|(.)/
         data: abc
          0: a
          1: a
         data:
           re> /a(?:(*MARK:X))(*SKIP:X)(*F)|(.)/
         data: abc
          0: b
          1: b

       In  the first example, the (*MARK) setting is in an atomic group, so it
       is not seen when (*SKIP:X) triggers, causing the (*SKIP) to be ignored.
       This  allows  the second branch of the pattern to be tried at the first
       character position.  In the second example, the (*MARK) setting is  not
       in  an  atomic group. This allows (*SKIP:X) to find the (*MARK) when it
       backtracks, and this causes a new matching attempt to start at the sec-
       ond  character.  This  time, the (*MARK) is never seen because "a" does
       not match "b", so the matcher immediately jumps to the second branch of
       the pattern.

       Note  that (*SKIP:NAME) searches only for names set by (*MARK:NAME). It
       ignores names that are set by other backtracking verbs.

         (*THEN) or (*THEN:NAME)

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

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

       If  the COND1 pattern matches, FOO is tried (and possibly further items
       after the end of the group if FOO succeeds); on  failure,  the  matcher
       skips  to  the second alternative and tries COND2, without backtracking
       into COND1. If that succeeds and BAR fails, COND3 is tried.  If  subse-
       quently  BAZ fails, there are no more alternatives, so there is a back-
       track to whatever came before the  entire  group.  If  (*THEN)  is  not
       inside an alternation, it acts like (*PRUNE).

       The  behaviour  of (*THEN:NAME) is not the same as (*MARK:NAME)(*THEN).
       It is like (*MARK:NAME) in that the name is remembered for passing back
       to  the  caller. However, (*SKIP:NAME) searches only for names set with
       (*MARK), ignoring those set by other backtracking verbs.

       A group 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 group  to  the
       enclosing  alternative.   Consider  this  pattern, where A, B, etc. are
       complex pattern fragments that do not contain any | characters at  this
       level:
       fail because there are no more  alternatives  to  try.  In  this  case,
       matching does backtrack into A.

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

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

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

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

   More than one backtracking verb

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

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

       If  A matches but B fails, the backtrack to (*COMMIT) causes the entire
       match to fail. However, if A and B match, but C fails, the backtrack to
       (*THEN)  causes  the next alternative (ABD) to be tried. This behaviour
       is consistent, but is not always the same as Perl's. It means  that  if
       two  or  more backtracking verbs appear in succession, all the the last
       of them has no effect. Consider this example:

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

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

   Backtracking verbs in repeated groups

       PCRE2 sometimes differs from Perl in its handling of backtracking verbs
       in repeated groups. For example, consider:

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

       (*ACCEPT) in a standalone positive assertion causes  the  assertion  to
       succeed  without  any  further  processing; captured strings and a mark
       name (if  set)  are  retained.  In  a  standalone  negative  assertion,
       (*ACCEPT)  causes the assertion to fail without any further processing;
       captured substrings and any mark name are discarded.

       If the assertion is a condition, (*ACCEPT) causes the condition  to  be
       true  for  a  positive assertion and false for a negative one; captured
       substrings are retained in both cases.

       The remaining verbs act only when a later failure causes a backtrack to
       reach  them. This means that their effect is confined to the assertion,
       because lookaround assertions are atomic. A backtrack that occurs after
       an assertion is complete does not jump back into the assertion. Note in
       particular that a (*MARK) name that is  set  in  an  assertion  is  not
       "seen" by an instance of (*SKIP:NAME) latter in the pattern.

       The  effect of (*THEN) is not allowed to escape beyond an assertion. If
       there are no more branches to try, (*THEN) causes a positive  assertion
       to be false, and a negative assertion to be true.

       The  other  backtracking verbs are not treated specially if they appear
       in a standalone positive assertion. In a  conditional  positive  asser-
       tion, backtracking (from within the assertion) into (*COMMIT), (*SKIP),
       or (*PRUNE) causes the condition to be false. However, for both  stand-
       alone and conditional negative assertions, backtracking into (*COMMIT),
       (*SKIP), or (*PRUNE) causes the assertion to be true, without consider-
       ing any further alternative branches.

   Backtracking verbs in subroutines

       These behaviours occur whether or not the group is called recursively.

       (*ACCEPT) in a group called as a subroutine causes the subroutine match
       to succeed without any  further  processing.  Matching  then  continues
       after the subroutine call. Perl documents this behaviour. Perl's treat-
       ment of the other verbs in subroutines is different in some cases.

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

       (*COMMIT),  (*SKIP),  and  (*PRUNE)  cause the subroutine match to fail
       when triggered by being backtracked to in a group called as  a  subrou-
       tine. There is then a backtrack at the outer level.

       (*THEN), when triggered, skips to the next alternative in the innermost
       enclosing group that has alternatives (its normal behaviour).  However,
       if there is no such group within the subroutine's group, the subroutine
       match fails and there is a backtrack at the outer level.

SEE ALSO

       pcre2api(3),   pcre2callout(3),    pcre2matching(3),    pcre2syntax(3),
       pcre2(3).

PCRE2 10.33                    12 February 2019                PCRE2PATTERN(3)
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