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pcre2pattern(3)                                                pcre2pattern(3)




NAME

       PCRE2 - Perl-compatible regular expressions (revised API)


PCRE2 REGULAR EXPRESSION DETAILS


       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  one or both of the PCRE2_UTF or PCRE2_MATCH_INVALID_UTF
       options, or the pattern must start with the  special  sequence  (*UTF),
       which  is  equivalent  to setting the relevant PCRE2_UTF. 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.

   Unicode property support

       Another special sequence that may appear at the start of a  pattern  is
       (*UCP).   This  has the same effect as setting the PCRE2_UCP option: it
       causes sequences such as \d and \w to use Unicode properties to  deter-
       mine character types, instead of recognizing only characters with codes
       less than 256 via a lookup table. If  also  causes  upper/lower  casing
       operations  to  use  Unicode properties for characters with code points
       greater than 127, even when UTF is not set.

       Some applications that allow their users to supply patterns may wish to
       restrict  them  for  security reasons. If the PCRE2_NEVER_UCP option is
       passed to pcre2_compile(), (*UCP) 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

         (*LIMIT_HEAP=d)
         (*LIMIT_MATCH=d)
         (*LIMIT_DEPTH=d)

       where d is any number of decimal digits. However, the value of the set-
       ting must be less than the value set (or defaulted) by  the  caller  of
       pcre2_match()  for  it  to have any effect. In other words, the pattern
       writer can lower the limits set by the programmer, but not raise  them.
       If  there  is  more  than one setting of one of these limits, the lower
       value is used. The heap limit is specified in kibibytes (units of  1024
       bytes).

       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

       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. This effect can also be achieved by
       starting a pattern with (*BSR_ANYCRLF).  For  completeness,  (*BSR_UNI-
       CODE) is also recognized, corresponding to PCRE2_BSR_UNICODE.


EBCDIC CHARACTER CODES


       PCRE2  can be compiled to run in an environment that uses EBCDIC as its
       character code instead of ASCII or Unicode (typically a mainframe  sys-
       tem).  In  the  sections below, character code values are ASCII or Uni-
       code; in an EBCDIC environment these characters may have different code
       values, and there are no code points greater than 255.


CHARACTERS AND METACHARACTERS


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

         The quick brown fox

       matches a portion of a subject string that is identical to itself. When
       caseless  matching  is  specified  (the  PCRE2_CASELESS  option or (?i)
       within the pattern), letters are matched independently  of  case.  Note
       that  there  are  two  ASCII  characters, K and S, that, in addition to
       their lower case ASCII equivalents, are  case-equivalent  with  Unicode
       U+212A  (Kelvin  sign)  and  U+017F  (long  S) respectively when either
       PCRE2_UTF or PCRE2_UCP is set.

       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

       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 a # character as part of the  pattern.  If  the  PCRE2_EXTENDED_MORE
       option  is  set,  the same applies, but in addition unescaped space and
       horizontal tab characters are ignored inside a character  class.  Note:
       only  these  two  characters  are  ignored, not the full set of pattern
       white space characters that are  ignored  outside  a  character  class.
       Option  settings can be changed within a pattern; see the section enti-
       tled "Internal Option Setting" below.

       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
       meaning  that  character  may  have. This use of backslash as an escape
       character applies both inside and outside character classes.

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

       Only  ASCII  digits  and letters have any special meaning after a back-
       slash. All other characters (in particular, those whose code points are
       greater than 127) are treated as literals.

       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..
         \xhh        character with hex code hh
         \x{hhh..}   character with hex code hhh..
         \N{U+hhh..} character with Unicode hex code point hhh..

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

       Characters whose code points are less than 256 can be defined by either
       of the two syntaxes for \x or by an octal sequence. There is no differ-
       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 PCRE2 is  oper-
       ating  in  UTF  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 differ-
       ent 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
       generate  the  APC character. Unfortunately, there are several variants
       of EBCDIC. In most of them the APC character has  the  value  255  (hex
       FF),  but  in  the one Perl calls POSIX-BC its value is 95 (hex 5F). If
       certain other characters have POSIX-BC values, PCRE2 makes \c? generate
       95; otherwise it generates 255.

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

       The  escape \o must be followed by a sequence of octal digits, enclosed
       in braces. An error occurs if this is not the case. This  escape  is  a
       recent  addition  to Perl; it provides way of specifying character code
       points as octal numbers greater than 0777, and  it  also  allows  octal
       numbers and 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
         All UTF modes         no greater than 0x10ffff and a valid code point

       Invalid Unicode code points are all those in the range 0xd800 to 0xdfff
       (the so-called "surrogate" code points). The check  for  these  can  be
       disabled  by  the  caller  of  pcre2_compile()  by  setting  the option
       PCRE2_EXTRA_ALLOW_SURROGATE_ESCAPES. However, this is possible only  in
       UTF-8  and  UTF-32 modes, because these values are not representable in
       UTF-16.

   Escape sequences in character classes

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

       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
       by Unicode name; PCRE2 does not support this.

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

       The  default  \s  characters  are HT (9), LF (10), VT (11), FF (12), CR
       (13), and space (32), which are defined  as  white  space  in  the  "C"
       locale. This list may vary if locale-specific matching is 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
         U+205F     Medium mathematical space
         U+3000     Ideographic space

       The vertical space characters are:

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

       In 8-bit, non-UTF-8 mode, only the characters  with  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 abbreviation  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
       non-UTF modes these sequences are of course limited to testing  charac-
       ters  whose code points are less than U+0100 and U+10000, respectively.
       In 32-bit non-UTF mode, code points greater than 0x10ffff (the  Unicode
       limit)  may  be  encountered.  These  are  all  treated as being in the
       Unknown script and with an unassigned type.

       Matching characters by Unicode property is not fast, because PCRE2  has
       to  do  a  multistage table lookup in order to find a character's prop-
       erty. That is why the traditional escape sequences such as \d and \w do
       not  use  Unicode  properties  in PCRE2 by default, though you can make
       them do so by setting the PCRE2_UCP option or by starting  the  pattern
       with (*UCP).

       The extra escape sequences that provide property support are:

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

       The  property names represented by xx above are not case-sensitive, and
       in accordance with Unicode's "loose matching" rules,  spaces,  hyphens,
       and underscores are ignored. There is support for Unicode script names,
       Unicode general category properties, "Any", which matches any character
       (including  newline),  Bidi_Class,  a number of binary (yes/no) proper-
       ties, and some special PCRE2  properties  (described  below).   Certain
       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.

   Script properties for \p and \P

       There are three different syntax forms for matching a script. Each Uni-
       code character has a basic script and,  optionally,  a  list  of  other
       scripts ("Script Extensions") with which it is commonly used. Using the
       Adlam script as an example, \p{sc:Adlam} matches characters whose basic
       script is Adlam, whereas \p{scx:Adlam} matches, in addition, characters
       that have Adlam in their extensions list. The full names  "script"  and
       "script extensions" for the property types are recognized, and a equals
       sign is an alternative to the colon. If a script name is given  without
       a   property   type,   for   example,   \p{Adlam},  it  is  treated  as
       \p{scx:Adlam}. Perl changed to this interpretation at release 5.26  and
       PCRE2 changed at release 10.40.

       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 recognized script names and their 4-character
       abbreviations can be obtained by running this command:

         pcre2test -LS


   The general category property for \p and \P

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

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

         \p{L}
         \pL

       The following general category property codes are supported:

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

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

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

         N     Number
         Nd    Decimal number
         Nl    Letter number
         No    Other number

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

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

         Z     Separator
         Zl    Line separator
         Zp    Paragraph separator
         Zs    Space separator

       The special property LC, which has the synonym 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".

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

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

   Binary (yes/no) properties for \p and \P

       Unicode defines a number of  binary  properties,  that  is,  properties
       whose  only  values  are  true or false. You can obtain a list of those
       that are recognized by \p and \P, along with  their  abbreviations,  by
       running this command:

         pcre2test -LP


   The Bidi_Class property for \p and \P

         \p{Bidi_Class:<class>}   matches a character with the given class
         \p{BC:<class>}           matches a character with the given class

       The recognized classes are:

         AL          Arabic letter
         AN          Arabic number
         B           paragraph separator
         BN          boundary neutral
         CS          common separator
         EN          European number
         ES          European separator
         ET          European terminator
         FSI         first strong isolate
         L           left-to-right
         LRE         left-to-right embedding
         LRI         left-to-right isolate
         LRO         left-to-right override
         NSM         non-spacing mark
         ON          other neutral
         PDF         pop directional format
         PDI         pop directional isolate
         R           right-to-left
         RLE         right-to-left embedding
         RLI         right-to-left isolate
         RLO         right-to-left override
         S           segment separator
         WS          which space

       An  equals  sign  may  be  used instead of a colon. The class names are
       case-insensitive; only the short names listed above are recognized.

   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 fol-
       lowed 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.
       However, they may also be used explicitly. These properties are:

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

       Xan  matches  characters that have either the L (letter) or the N (num-
       ber) property. Xps matches the characters tab, linefeed, vertical  tab,
       form  feed,  or carriage return, and any other character that has the Z
       (separator) property.  Xsp is the same as Xps;  in  PCRE1  it  used  to
       exclude  vertical  tab,  for  Perl compatibility, but Perl changed. Xwd
       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".

       From  version  5.32.0  Perl  forbids the use of \K in lookaround asser-
       tions. From release 10.38 PCRE2 also forbids this by default.  However,
       the  PCRE2_EXTRA_ALLOW_LOOKAROUND_BSK  option  can be used when calling
       pcre2_compile() to re-enable the previous behaviour. When  this  option
       is set, \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 assertion at  the  start
       of  a  pattern can also lead to odd effects. For example, 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

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

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

       Inside  a  character  class, \b has a different meaning; it matches the
       backspace character. If any other of  these  assertions  appears  in  a
       character class, 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
       recognized  as  a newline, isolated CR and LF characters are treated as
       ordinary data characters, and are not recognized as newlines.

       Outside a character class, in the default matching mode, the circumflex
       character  is  an  assertion  that is true only if the current matching
       point is at the start of the subject string. If the  startoffset  argu-
       ment  of  pcre2_match() is non-zero, or if PCRE2_NOTBOL is set, circum-
       flex can never match if the PCRE2_MULTILINE option is unset.  Inside  a
       character  class,  circumflex  has  an  entirely different meaning (see
       below).

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

       The dollar character is an assertion that is true only if  the  current
       matching  point  is  at  the  end of the subject string, or immediately
       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. One or more characters may be specified as line
       terminators (see "Newline conventions" above).

       Dot  never matches a single line-ending character. When the two-charac-
       ter sequence CRLF is the only line ending, dot does not match CR if  it
       is  immediately followed by LF, but otherwise it matches all characters
       (including isolated CRs and LFs). When ANYCRLF  is  selected  for  line
       endings,  no  occurences  of  CR of LF match dot. When all Unicode line
       endings are being recognized, dot does not match CR or LF or any of the
       other line ending characters.

       The  behaviour  of  dot  with regard to newlines can be changed. If the
       PCRE2_DOTALL option is set, a dot matches any  one  character,  without
       exception.   If  the two-character sequence CRLF is present in the sub-
       ject string, it takes two dots to match it.

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

       The escape sequence \N 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 or
       PCRE2_MATCH_INVALID_UTF 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):

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

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


SQUARE BRACKETS AND CHARACTER CLASSES


       An opening square bracket introduces a character class, terminated by a
       closing square bracket. A closing square bracket on its own is not spe-
       cial by default.  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.  Note  that  there  are  two ASCII characters, K and S, that, in
       addition to their lower case  ASCII  equivalents,  are  case-equivalent
       with Unicode U+212A (Kelvin sign) and U+017F (long S) respectively when
       either PCRE2_UTF or PCRE2_UCP is set.

       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
       between  d  and  m,  inclusive.  If  a minus character is required in a
       class, it must be escaped with a backslash  or  appear  in  a  position
       where  it cannot be interpreted as indicating a range, typically as the
       first or last character in the class, or immediately after a range. For
       example,  [b-d-z] matches letters in the range b to d, a hyphen charac-
       ter, or z.

       Perl treats a hyphen as a literal if it appears before or after a POSIX
       class (see below) or before or after a character type escape such as as
       \d or \H.  However, unless the hyphen is  the  last  character  in  the
       class,  Perl  outputs  a  warning  in its warning mode, as this is most
       likely a user error. As PCRE2 has no facility for warning, an error  is
       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
       supports this notation. For example,

         [01[:alpha:]%]

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

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


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

       [:punct:] This matches all characters that have the Unicode P (punctua-
                 tion) property, plus those characters with code  points  less
                 than 256 that have the S (Symbol) property.

       The  other  POSIX classes are unchanged, and match only characters with
       code points less than 256.


COMPATIBILITY FEATURE FOR WORD BOUNDARIES


       In the POSIX.2 compliant library that was included in 4.4BSD Unix,  the
       ugly  syntax  [[:<:]]  and [[:>:]] is used for matching "start of word"
       and "end of word". 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
       PCRE2_CASELESS  and  PCRE2_MULTILINE  while  unsetting PCRE2_DOTALL and
       PCRE2_EXTENDED, is also permitted. Only one hyphen may  appear  in  the
       options  string.  If a letter appears both before and after the hyphen,
       the option is unset. An empty options setting "(?)" is  allowed.  Need-
       less to say, it has no effect.

       If  the  first character following (? is a circumflex, it causes all of
       the above options to be unset. Thus, (?^) is equivalent  to  (?-imnsx).
       Letters  may  follow  the  circumflex  to  cause some options to be re-
       instated, but a hyphen may not appear.

       The PCRE2-specific options PCRE2_DUPNAMES  and  PCRE2_UNGREEDY  can  be
       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|)

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

       2. It creates a "capture group". This means that, when the  whole  pat-
       tern  matches, the portion of the subject string that matched the group
       is passed back to the caller, separately from the portion that  matched
       the  whole  pattern.   (This  applies  only to the traditional matching
       function; the DFA matching function does not support capturing.)

       Opening parentheses are counted from left to right (starting from 1) to
       obtain  numbers for capture groups. For example, if the string "the red
       king" is matched against the pattern

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

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

       The  fact  that  plain  parentheses  fulfil two functions is not always
       helpful.  There are often times when 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  backreference  to a capture group uses the most recent value that is
       set for the group. The following pattern matches "abcabc" or "defdef":

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

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

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

       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").

       In  an attempt to reduce confusion, PCRE2 does not allow the same group
       number to be associated with more than one name. The example above pro-
       vokes  a  compile-time  error. However, there is still scope for confu-
       sion. Consider this pattern:

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

       Although the second group number 1 is not explicitly named, the name AA
       is  still an alias for any group 1. Whether the pattern matches "aa" or
       "bb", a reference by name to group AA yields the matched string.

       By default, a name must be unique within a pattern, except that  dupli-
       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,
       as described in the section entitled "Internal Option Setting" above.

       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:

         (?J)
         (?<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":

         (?J)(?:(?<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
         the \X escape sequence
         an escape such as \d or \pL that matches a single character
         a character class
         a backreference
         a parenthesized group (including lookaround assertions)
         a subroutine call (recursive or otherwise)

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

         z{2,4}

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

         [aeiou]{3,}

       matches at least 3 successive vowels, but may match many more, 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 whenever an iteration of
       such  a group matches no characters, matching moves on to the next item
       in the pattern instead of repeatedly matching  an  empty  string.  This
       does  not  prevent  backtracking into any of the iterations if a subse-
       quent item fails to match.

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

         /\*.*\*/

       to the string

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

       fails, because it matches the entire string owing to the greediness  of
       the  .*  item. However, if a quantifier is followed by a question mark,
       it ceases to be greedy, and instead matches the minimum number of times
       possible, so the pattern

         /\*.*?\*/

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

         \d??\d

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

       If the 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:

         (?>.*?a)b

       It matches "ab" in the subject "aab". The use of the backtracking  con-
       trol  verbs  (*PRUNE)  and  (*SKIP) also disable this optimization, and
       there is an option, PCRE2_NO_DOTSTAR_ANCHOR, to do so explicitly.

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

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

       has matched "tweedledum tweedledee" the value of the captured substring
       is "tweedledee". However, if there are nested 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-
       tern match, (?>\d+) can only match an entire sequence of digits.

       Atomic  groups in general can of course contain arbitrarily complicated
       expressions, and can be nested. However, when the contents of an atomic
       group  is  just a single repeated item, as in the example above, a sim-
       pler notation, called a "possessive quantifier" can be used. This  con-
       sists  of  an additional + character following a quantifier. Using this
       notation, the previous example can be rewritten as

         \d++foo

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

         (abc|xyz){2,3}+

       Possessive   quantifiers   are   always  greedy;  the  setting  of  the
       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
       earlier (that is, to its left) in the pattern, provided there have been
       that many previous capture groups.

       However,  if the decimal number following the backslash is less than 8,
       it is always taken as a backreference, and  causes  an  error  only  if
       there  are not that many capture groups in the entire pattern. In other
       words, the group that is referenced need not be to the left of the ref-
       erence  for numbers less than 8. A "forward backreference" of this type
       can make sense when a repetition is involved and the group to the right
       has participated in an earlier iteration.

       It  is  not  possible  to have a numerical "forward backreference" to a
       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
       5.10's unified backreference syntax, in which \g can be used  for  both
       numeric  and  named references, is also supported. We could rewrite the
       above example in any of the following ways:

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

       A capture group that is referenced by name may appear  in  the  pattern
       before or after the reference.

       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.

       For versions of PCRE2 less than 10.25, backreferences of this type used
       to cause the group that they reference  to  be  treated  as  an  atomic
       group.   This restriction no longer applies, and backtracking into such
       groups can occur as normal.


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
       reset to what it was before the assertion was processed.

       The Perl-compatible lookaround assertions are atomic. If  an  assertion
       is  true, but there is a subsequent matching failure, there is no back-
       tracking into the assertion. However, there are some cases  where  non-
       atomic  assertions  can  be  useful.  PCRE2 has some support for these,
       described in the section entitled "Non-atomic  assertions"  below,  but
       they are not Perl-compatible.

       A  lookaround  assertion  may  appear as the condition in a conditional
       group (see below). In this case, the result of matching  the  assertion
       determines which branch of the condition is followed.

       Assertion  groups are not capture groups. If an assertion contains cap-
       ture groups within it, these are counted for the purposes of  numbering
       the  capture  groups  in  the  whole  pattern. Within each branch of an
       assertion, locally captured substrings may be referenced in  the  usual
       way.  For  example,  a  sequence such as (.)\g{-1} can be used to check
       that two adjacent characters are the same.

       When a branch within an assertion fails to match, any  substrings  that
       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.

       Most assertion groups may be repeated; though  it  makes  no  sense  to
       assert  the  same  thing several times, the side effect of capturing in
       positive assertions may occasionally be useful. However,  an  assertion
       that forms the condition for a conditional group may not be quantified.
       PCRE2 used to restrict the repetition of assertions, but  from  release
       10.35  the  only restriction is that an unlimited maximum repetition is
       changed to be one more than the minimum. For example, {3,}  is  treated
       as {3,4}.

   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

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

         \w+(?=;)

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

         foo(?!bar)

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

         (?!foo)bar

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

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

   Lookbehind assertions

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

         (?<!foo)bar

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

         (?<=bullock|donkey)

       is permitted, but

         (?<!dogs?|cats?)

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

         (?<=ab(c|de))

       is  not  permitted,  because  its single top-level branch can match two
       different lengths, but it is acceptable to 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.

       In  UTF-8  and  UTF-16 modes, PCRE2 does not allow the \C escape (which
       matches a single code unit even in a UTF mode) to appear in  lookbehind
       assertions,  because  it makes it impossible to calculate the length of
       the lookbehind. The \X and \R escapes, which can match  different  num-
       bers of code units, are never permitted in lookbehinds.

       "Subroutine"  calls  (see below) such as (?2) or (?&X) are permitted in
       lookbehinds, as long as the called capture group matches a fixed-length
       string.  However,  recursion, that is, a "subroutine" call into a group
       that is already active, is not supported.

       Perl does not support backreferences in lookbehinds. PCRE2 does support
       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

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

       Assertions can be nested in any combination. For example,

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

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

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

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


NON-ATOMIC ASSERTIONS


       The  traditional Perl-compatible lookaround assertions are atomic. That
       is, if an assertion is true, but there is a subsequent  matching  fail-
       ure,  there  is  no backtracking into the assertion. However, there are
       some cases where non-atomic positive assertions can  be  useful.  PCRE2
       provides these using the following syntax:

         (*non_atomic_positive_lookahead:  or (*napla: or (?*
         (*non_atomic_positive_lookbehind: or (*naplb: or (?<*

       Consider  the  problem  of finding the right-most word in a string that
       also appears earlier in the string, that is, it must  appear  at  least
       twice  in  total.  This pattern returns the required result as captured
       substring 1:

         ^(?x)(*napla: .* \b(\w++)) (?> .*? \b\1\b ){2}

       For a subject such as "word1 word2 word3 word2 word3 word4" the  result
       is  "word3".  How does it work? At the start, ^(?x) anchors the pattern
       and sets the "x" option, which causes white space (introduced for read-
       ability)  to  be  ignored. Inside the assertion, the greedy .* at first
       consumes the entire string, but then has to backtrack until the rest of
       the  assertion can match a word, which is captured by group 1. In other
       words, when the assertion first succeeds, it  captures  the  right-most
       word in the string.

       The  current  matching point is then reset to the start of the subject,
       and the rest of the pattern match checks for  two  occurrences  of  the
       captured  word,  using  an  ungreedy .*? to scan from the left. If this
       succeeds, we are done, but if the last word  in  the  string  does  not
       occur  twice,  this  part of the pattern fails. If a traditional atomic
       lookhead (?= or (*pla: had been used, the assertion could  not  be  re-
       entered, and the whole match would fail. The pattern would succeed only
       if the very last word in the subject was found twice.

       Using a non-atomic lookahead, however, means that when  the  last  word
       does  not  occur  twice  in the string, the lookahead can backtrack and
       find the second-last word, and so on, until either the match  succeeds,
       or all words have been tested.

       Two conditions must be met for a non-atomic assertion to be useful: the
       contents of one or more capturing groups must change after a  backtrack
       into  the  assertion,  and  there  must be a backreference to a changed
       group later in the pattern. If this is not the case, the  rest  of  the
       pattern  match  fails exactly as before because nothing has changed, so
       using a non-atomic assertion just wastes resources.

       There is one exception to backtracking into a non-atomic assertion.  If
       an  (*ACCEPT)  control verb is triggered, the assertion succeeds atomi-
       cally. That is, a subsequent match failure cannot  backtrack  into  the
       assertion.

       Non-atomic  assertions  are  not  supported by the alternative matching
       function pcre2_dfa_match(). They are supported by JIT, but only if they
       do not contain any control verbs such as (*ACCEPT). (This may change in
       future). Note that assertions that appear as conditions for conditional
       groups (see below) must be atomic.


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.

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


CONDITIONAL GROUPS


       It is possible to cause the matching process to obey a pattern fragment
       conditionally or to choose between two alternative fragments, depending
       on  the result of an assertion, or whether a specific capture group has
       already been matched. The two possible forms of conditional group are:

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

       If the condition is satisfied, the yes-pattern is used;  otherwise  the
       no-pattern  (if present) is used. 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... ( \( )?    [^()]+    (?(-1) \) ) ...

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

   Checking for a used capture group by name

       Perl  uses  the  syntax  (?(<name>)...) or (?('name')...) to test for a
       used capture group by name. For compatibility with earlier versions  of
       PCRE1,  which had this facility before Perl, the syntax (?(name)...) is
       also recognized.  Note, however, that undelimited names  consisting  of
       the  letter  R followed by digits are ambiguous (see the following sec-
       tion). Rewriting the above example to use a named group gives this:

         (?<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):

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

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

   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.  However,  it  must  be a traditional atomic
       assertion, not one of the PCRE2-specific non-atomic assertions.

       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
       immediately after the next newline character or character  sequence  in
       the pattern. Which characters are interpreted as newlines is controlled
       by an option passed to the compiling function or by a special  sequence
       at the start of the pattern, as described in the section entitled "New-
       line conventions" above. Note that the end of this type of comment is a
       literal  newline  sequence in the pattern; escape sequences that happen
       to represent a newline do not count. For example, consider this pattern
       when  PCRE2_EXTENDED is set, and the default newline convention (a sin-
       gle linefeed character) is in force:

         abc #comment \n still comment

       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:

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

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

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

       Be aware however, that if duplicate capture group numbers are  in  use,
       relative  references  refer  to the earliest group with the appropriate
       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
       brackets  (that is, when recursing), whereas any characters are permit-
       ted at the outer level.

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

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

   Differences in recursion processing between PCRE2 and Perl

       Some former differences between PCRE2 and Perl no longer exist.

       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


       If  the syntax for a recursive group call (either by number or by name)
       is used outside the parentheses to which it refers, it operates  a  bit
       like  a  subroutine  in  a programming language. More accurately, PCRE2
       treats the referenced group as an independent subpattern which it tries
       to  match  at  the  current  matching position. The called group may be
       defined before or after the reference.  A  numbered  reference  can  be
       absolute or relative, as in these examples:

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

       An earlier example pointed out that the pattern

         (sens|respons)e and \1ibility

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

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

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

       Like 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
       Perl  code to be obeyed in the middle of matching a regular expression.
       This makes it possible, amongst other things, to extract different sub-
       strings that match the same pair of parentheses when there is a repeti-
       tion.

       PCRE2 provides a similar feature, but of course it  cannot  obey  arbi-
       trary  Perl  code. The feature is called "callout". The caller of PCRE2
       provides an external function by putting its entry  point  in  a  match
       context  using  the function pcre2_set_callout(), and then passing that
       context to pcre2_match() or pcre2_dfa_match(). If no match  context  is
       passed, or if the callout entry point is set to NULL, callouts are dis-
       abled.

       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
       function.


BACKTRACKING CONTROL


       There  are  a  number  of  special "Backtracking Control Verbs" (to use
       Perl's terminology) that modify the behaviour  of  backtracking  during
       matching.  They are generally of the form (*VERB) or (*VERB:NAME). Some
       verbs take either form, and may behave differently depending on whether
       or  not  a  name  argument is present. The names are not required to be
       unique within the pattern.

       By default, for compatibility with Perl, a  name  is  any  sequence  of
       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. Except for (*ACCEPT), they may not be quantified.

       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

       The following verbs act as soon as they are encountered.

          (*ACCEPT) or (*ACCEPT:NAME)

       This  verb causes the match to end successfully, skipping the remainder
       of the pattern. However, when it is inside  a  capture  group  that  is
       called as a subroutine, only that group is ended successfully. Matching
       then continues at the outer level. If (*ACCEPT) in triggered in a posi-
       tive  assertion,  the  assertion succeeds; in a negative assertion, the
       assertion fails.

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

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

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

       (*ACCEPT) is the only backtracking verb that is allowed to  be  quanti-
       fied  because  an  ungreedy  quantification with a minimum of zero acts
       only when a backtrack happens. Consider, for example,

         (A(*ACCEPT)??B)C

       where A, B, and C may be complex expressions. After matching  "A",  the
       matcher  processes  "BC"; if that fails, causing a backtrack, (*ACCEPT)
       is triggered and the match succeeds. In both cases, all but C  is  cap-
       tured.  Whereas  (*COMMIT)  (see  below)  means  "fail on backtrack", a
       repeated (*ACCEPT) of this type means "succeed on backtrack".

       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)    behave     the     same     as
       (*MARK:NAME)(*ACCEPT) and (*MARK:NAME)(*FAIL), respectively, that is, a
       (*MARK) is recorded just before the verb acts.

   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
          0: XY
         MK: A
         XZ
          0: XZ
         MK: B

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

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

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

           re> /X(*MARK:A)Y|X(*MARK:B)Z/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-
       ing back to the caller. However, (*SKIP:NAME) searches only  for  names
       that are set with (*MARK), ignoring those set by any of the other back-
       tracking verbs.

       If there is more than one backtracking verb in a pattern,  a  different
       one  that  follows  (*COMMIT) may be triggered first, so merely passing
       (*COMMIT) during a match does not always guarantee that a match must be
       at this starting point.

       Note  that  (*COMMIT)  at  the start of a pattern is not the same as an
       anchor, unless PCRE2's start-of-match optimizations are turned off,  as
       shown in this output from pcre2test:

           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-
       tifier 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".

       If  (*SKIP) is used to specify a new starting position that is the same
       as the starting position of the current match, or (by  being  inside  a
       lookbehind)  earlier, the position specified by (*SKIP) is ignored, and
       instead the normal "bumpalong" occurs.

         (*SKIP:NAME)

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

       The  search  for a (*MARK) name uses the normal backtracking mechanism,
       which means that it does not  see  (*MARK)  settings  that  are  inside
       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:

         A (B(*THEN)C) | D

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

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

       The effect of (*THEN) is now confined to the inner group. After a fail-
       ure in C, matching moves to (*FAIL), which causes the  whole  group  to
       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/

       If the subject is "abac", Perl matches  unless  its  optimizations  are
       disabled,  but  PCRE2  always fails because the (*COMMIT) in the second
       repeat of the group acts.

   Backtracking verbs in assertions

       (*FAIL) in any assertion has its normal effect: it forces an  immediate
       backtrack.  The  behaviour  of  the other backtracking verbs depends on
       whether or not the assertion is standalone or acting as  the  condition
       in a conditional group.

       (*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, for the Perl-compatible assertions,  their
       effect is confined to the assertion, because Perl lookaround assertions
       are atomic. A backtrack that occurs after such 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) later in the pattern.

       PCRE2  now supports non-atomic positive assertions, as described in the
       section entitled "Non-atomic assertions" above. These  assertions  must
       be  standalone  (not used as conditions). They are not Perl-compatible.
       For these assertions, a later backtrack does jump back into the  asser-
       tion,  and  therefore verbs such as (*COMMIT) can be triggered by back-
       tracks from later 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).


AUTHOR


       Philip Hazel
       Retired from University Computing Service
       Cambridge, England.


REVISION


       Last updated: 12 January 2022
       Copyright (c) 1997-2022 University of Cambridge.



PCRE2 10.40                     12 January 2022                pcre2pattern(3)

pcre2 10.40 - Generated Tue May 10 18:54:21 CDT 2022
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