pcre2pattern

PCRE2PATTERN(3)            Library Functions Manual            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  abbrevation  for  "back-
       slash R".) This can be made the default when PCRE2 is built; if this is
       the case, the other behaviour can be requested via  the  PCRE2_BSR_UNI-
       CODE  option. It is also possible to specify these settings by starting
       a pattern string with one of the following sequences:

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

       These override the default and the options given to the compiling func-
       tion.  Note that these special settings, which are not Perl-compatible,
       are recognized only at the very start of a pattern, and that they  must
       be  in upper case. If more than one of them is present, the last one is
       used. They can be combined with a change  of  newline  convention;  for
       example, a pattern can start with:

         (*ANY)(*BSR_ANYCRLF)

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

   Unicode character properties

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

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

         \p{Greek}
         \P{Han}

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

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

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

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

         \p{L}
         \pL

       The following general category property codes are supported:

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

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

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

         N     Number
         Nd    Decimal number
         Nl    Letter number
         No    Other number

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

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

         Z     Separator
         Zl    Line separator
         Zp    Paragraph separator
         Zs    Space separator

       The  special property L& is also supported: it matches a character that
       has the Lu, Ll, or Lt property, in other words, a letter  that  is  not
       classified as a modifier or "other".

       The  Cs  (Surrogate)  property  applies  only  to characters 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.

       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).

   Extended grapheme clusters

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

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

       1. End at the end of the subject string.

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

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

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

       5. Do not end after prepend characters.

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

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

       8. Otherwise, end the cluster.

   PCRE2's additional properties

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

       Perl  used  to document that the use of \K within lookaround assertions
       is "not well defined", but from version 5.32.0 Perl  does  not  support
       this  usage  at  all.  In PCRE2, \K is acted upon when it occurs inside
       positive assertions, but is ignored in negative assertions.  Note  that
       when  a  pattern  such  as  (?=ab\K) matches, the reported start of the
       match can be greater than the end of the match. Using \K in  a  lookbe-
       hind  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.

       When a line ending is defined as a single character, dot never  matches
       that  character; when the two-character sequence CRLF is used, dot does
       not match CR if it is immediately followed  by  LF,  but  otherwise  it
       matches  all characters (including isolated CRs and LFs). When any Uni-
       code line endings are being recognized, dot does not match CR or LF  or
       any of the other line ending characters.

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

   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-
       tifer  does not have the same effect as this example; although it would
       suppress backtracking  during  the  first  match  attempt,  the  second
       attempt  would  start at the second character instead of skipping on to
       "c".

       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
       University Computing Service
       Cambridge, England.

REVISION

       Last updated: 06 October 2020
       Copyright (c) 1997-2020 University of Cambridge.

PCRE2 10.35                     06 October 2020                PCRE2PATTERN(3)
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