pcre2pattern(3) — Linux manual page


PCRE2PATTERN(3)         Library Functions Manual         PCRE2PATTERN(3)

NAME         top

       PCRE2 - Perl-compatible regular expressions (revised API)


       The syntax and semantics of the regular expressions that are
       supported by PCRE2 are described in detail below. There is a
       quick-reference syntax summary in the pcre2syntax page. PCRE2
       tries to match Perl syntax and semantics as closely as it can.
       PCRE2 also supports some alternative regular expression syntax
       (which does not conflict with the Perl syntax) in order to
       provide some compatibility with regular expressions in Python,
       .NET, and 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
       supported 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 function, are discussed in the
       pcre2matching page.


       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-compatible, but are provided to make these options
       accessible to pattern writers who are not able to change the
       program that processes the pattern. 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 determine 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.  These behaviours can be changed
       within the pattern; see the section entitled "Internal Option
       Setting" below.

       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 pattern. 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 optimizations 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 optimizations 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
       provoked by patterns with huge matching trees. A common example
       is a pattern 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


       where d is any number of decimal digits. However, the value of
       the setting 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 (linefeed) character, the two-character sequence CRLF,
       any of the three preceding, 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
       pattern 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
       function. For example, on a Unix system where LF is the default
       newline sequence, the pattern


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

       The newline convention affects where the circumflex and dollar
       assertions are true. It also affects the interpretation of the
       dot metacharacter 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_UNICODE) is also recognized, corresponding to


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


       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,
       unless the PCRE2_EXTRA_CASELESS_RESTRICT option is in force
       (either passed to pcre2_compile() or set by (?r) within the

       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
       recognized 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
         {      potential start of min/max quantifier

       Brace characters { and } are also used to enclose data for
       constructions such as \g{2} or \k{name}. In almost all uses of
       braces, space and/or horizontal tab characters that follow { or
       precede } are allowed and are ignored. In the case of
       quantifiers, they may also appear before or after the comma. The
       exception to this is \u{...} which is an ECMAScript compatibility
       feature that is recognized only when the PCRE2_EXTRA_ALT_BSUX
       option is set. ECMAScript does not ignore such white space; it
       causes the item to be interpreted as literal.

       Part of a pattern that is in square brackets is called a
       "character class". In a character class the only metacharacters

         \      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,
       within a \Q...\E sequence, or 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 entitled "Internal Option Setting" below.

       The following sections describe the use of each of the

BACKSLASH         top

       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 backslash, you write \\.

       Only ASCII digits and letters have any special meaning after a
       backslash. 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. Note that this
       includes white space even when the PCRE2_EXTENDED option is set
       so that most other white space is ignored. The behaviour 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 character. 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 then not terminated by a closing square bracket.

   Non-printing characters

       A second use of backslash provides a way of encoding non-printing
       characters in patterns in a visible manner. There is no
       restriction on the appearance of non-printing characters 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 represents. In an ASCII or
       Unicode environment, these escapes are as follows:

         \a          alarm, that is, the BEL character (hex 07)
         \cx         "control-x", where x is a non-control ASCII
         \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
       hexadecimal digits are read (letters can be in upper or lower
       case). Any number of hexadecimal digits may appear between \x{
       and }. If a character other than a hexadecimal digit appears
       between \x{ and }, or if there is no terminating }, an error

       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 difference 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. Otherwise it is interpreted as a literal "x"
       character. In this mode, support 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 hexadecimal code point.  There may be any number of
       hexadecimal digits, but unlike other places that also use curly
       brackets, spaces are not allowed and would result in the string
       being interpreted as a literal. This syntax is from ECMAScript 6.

       The \N{U+hhh..} escape sequence is recognized only when PCRE2 is
       operating 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 different meaning, matching any character that
       is not a newline.

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

       An error occurs if \c is not followed by a character whose ASCII
       code point is in the range 32 to 126. The precise effect of \cx
       is as follows: if x is a lower case letter, it is converted to
       upper case. Then bit 6 of the character (hex 40) is inverted.
       Thus \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 code point 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 document. 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 numerical character code points, and \g{...}
       to specify backreferences. The following paragraphs describe the
       old, ambiguous syntax.

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

       Outside a character class, PCRE2 reads the digit and any
       following digits 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 backreference. A description of how this
       works is given later, following the discussion of parenthesized
       groups.  Otherwise, up to three octal digits are read to form a
       character code.

       Inside a character class, PCRE2 handles \8 and \9 as the literal
       characters "8" and "9", and otherwise reads up to three octal
       digits following the backslash, using them to generate a data
       character. Any subsequent 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

       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 backreference; the latter is a subroutine call.

   Generic character types

       Another use of backslash is for specifying generic character

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

       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 controlled 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 efficiency 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}, \p{N}, \p{Mn}, or \p{Pc}

       The addition of \p{Mn} (non-spacing mark) and the replacement of
       an explicit test for underscore with a test for \p{Pc} (connector
       punctuation) happened in PCRE2 release 10.43. This brings PCRE2
       into line with Perl.

       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 other character categories. 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 effect of PCRE2_UCP on any one of these escape sequences can
       be negated by the options PCRE2_EXTRA_ASCII_BSD,
       PCRE2_EXTRA_ASCII_BSS, and PCRE2_EXTRA_ASCII_BSW, respectively.
       These options can be set and reset within a pattern by means of
       an internal option setting (see below).

       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:


       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 (carriage 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 separator, U+2029).  Unicode support is not needed for
       these characters to be recognized.

       It is possible to restrict \R to match only CR, LF, or CRLF
       (instead of the complete set of Unicode line endings) by setting
       the option PCRE2_BSR_ANYCRLF at compile time. (BSR is an
       abbreviation for "backslash 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_UNICODE option. It is also
       possible to specify these settings by starting a pattern string
       with one of the following sequences:

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

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


       They can also be combined with the (*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
       additional 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 characters whose code points are less than
       U+0100 and U+10000, respectively. In 32-bit non-UTF mode, code
       points greater than 0x10ffff (the Unicode limit) may be
       encountered. These are all treated as being in the Unknown script
       and with an unassigned type.

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

       The extra escape sequences that provide property support are:

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

       The property names represented by xx above are not case-
       sensitive, and in accordance with Unicode's "loose matching"
       rules, spaces, hyphens, and underscores are ignored. There is
       support for Unicode script names, Unicode general category
       properties, "Any", which matches any character (including
       newline), Bidi_Class, a number of binary (yes/no) properties, and
       some special PCRE2 properties (described below).  Certain other
       Perl properties such as "InMusicalSymbols" are not supported by
       PCRE2. Note that \P{Any} does not match any characters, so always
       causes a match failure.

   Script properties for \p and \P

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

       Unassigned characters (and in non-UTF 32-bit mode, characters
       with code points greater than 0x10FFFF) are assigned the
       "Unknown" script. Others that are not part of an identified
       script are lumped together as "Common". The current list of
       recognized script names and their 4-character abbreviations can
       be obtained by running this command:

         pcre2test -LS

   The general category property for \p and \P

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

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


       The following general category property codes are supported:

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

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

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

         N     Number
         Nd    Decimal number
         Nl    Letter number
         No    Other number

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

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

         Z     Separator
         Zl    Line separator
         Zp    Paragraph separator
         Zs    Space separator

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

       The Cs (Surrogate) property applies only to characters whose code
       points are in the range U+D800 to U+DFFF. These characters are no
       different 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 validity 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)
       property.  Instead, this property is assumed for any code point
       that is not in the Unicode table.

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

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

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

         pcre2test -LP

   The Bidi_Class property for \p and \P

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

       The recognized classes are:

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

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

   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
       various emoji-specific properties. PCRE2 uses only the Extended
       Pictographic 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

       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 followed 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 characters.

       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
       supports 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
       (number) 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 those that match Mn (non-spacing mark) or Pc
       (connector punctuation, which includes underscore).

       There is another non-standard property, Xuc, which matches any
       character 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 characters 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:


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


       matches only when the subject begins with "foobar" (in single
       line mode), though it again reports the matched string as "bar".
       This feature is similar to a lookbehind assertion (described
       below), but the part of the pattern that precedes \K is not
       constrained to match a limited number of characters, as is
       required for a lookbehind assertion. The use of \K does not
       interfere with the setting of captured substrings.  For example,
       when the pattern


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

       From version 5.32.0 Perl forbids the use of \K in lookaround
       assertions. From release 10.38 PCRE2 also forbids this by
       default. However, the PCRE2_EXTRA_ALLOW_LOOKAROUND_BSK option can
       be used when calling pcre2_compile() to re-enable the previous
       behaviour. When this option is set, \K is acted upon when it
       occurs inside positive assertions, but is ignored in negative
       assertions. Note that when a pattern such as (?=ab\K) matches,
       the reported start of the match can be greater than the end of
       the match. Using \K in a lookbehind assertion at the start of a
       pattern can also lead to odd effects. For example, consider this


       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
       assertion specifies a condition that has to be met at a
       particular point in a match, without consuming any characters
       from the subject string. The use of 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

       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 normally determines
       which it is. For example, the fragment \ba matches "a" at the
       start of a word.

       The \A, \Z, and \z assertions differ from the traditional
       circumflex and dollar (described in the next section) in that
       they only ever match at the very start and end of the subject
       string, whatever options are set. Thus, they are independent of
       multiline mode. These three assertions are not affected by the
       PCRE2_NOTBOL or PCRE2_NOTEOL options, which affect only the
       behaviour of the circumflex and dollar metacharacters. 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 startoffset 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.


       The circumflex and dollar metacharacters are zero-width
       assertions. That is, they test for a particular condition being
       true without consuming any characters from the subject string.
       These two metacharacters are concerned with matching the starts
       and ends of lines. If the newline 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 argument of pcre2_match() is non-zero, or if
       PCRE2_NOTBOL is set, circumflex 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 subject, 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 number of alternatives are
       involved, but it should be the last item in any branch in which
       it appears. Dollar has no special meaning in a character class.

       The meaning of dollar can be changed so that it matches only at
       the very end of the string, by setting the 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)
       recognizes the two-character sequence CRLF as a newline, this is
       preferred, even if the single characters CR and LF are also
       recognized as newlines. 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         top

       Outside a character class, a dot in the pattern matches any one
       character in the subject string except (by default) a character
       that signifies the end of a line. One or more characters may be
       specified as line terminators (see "Newline conventions" above).

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

       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 subject string, it takes two dots to match it.

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

       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.


       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 usefully 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 character 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

       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 characters 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) |

       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, respectively. The character's individual bytes
       are then captured by the appropriate number of \C groups.


       An opening square bracket introduces a character class,
       terminated by a closing square bracket. A closing square bracket
       on its own is not special by default.  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 consumes 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 versions, so for example, a caseless [aeiou] matches
       "A" as well as "a", and a caseless [^aeiou] does not match "A",
       whereas a caseful version would. 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 followed by
       an opening brace.

       The minus (hyphen) character can be used to specify a range of
       characters in a character class. For example, [d-m] matches any
       letter between d and m, inclusive. If a minus character is
       required in a class, it must be escaped with a backslash or
       appear in a position where it cannot be interpreted as indicating
       a range, 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 character, 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
       character of a range. A pattern such as [W-]46] is interpreted as
       a class of two characters ("W" and "-") followed by a literal
       string "46]", so it would match "W46]" or "-46]". However, if the
       "]" is escaped with a backslash it is interpreted as the end of
       range, so [W-\]46] is interpreted as a class containing a range
       followed by two other characters. The octal or hexadecimal
       representation of "]" can also be used to end a range.

       Ranges normally include all code points between the start and end
       characters, 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).
       However, 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 compatibility 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.


       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,


       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    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, as do "word" and \w.

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


       matches "1", "2", or any non-digit. 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 characters in the range 128-255 when locale-
       specific matching is happening. However, in UCP mode, unless
       certain options are set (see below), some of the classes are
       changed so that Unicode character properties are used. This is
       achieved by replacing POSIX classes with other sequences, as

         [: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. Four
       other POSIX classes are handled specially in UCP mode:

              This matches characters that have glyphs that mark the
              page when printed. In Unicode property terms, it matches
              all characters 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

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

              This matches all characters that have the Unicode P
              (punctuation) property, plus those characters with code
              points less than 256 that have the S (Symbol) property.

              In addition to the ASCII hexadecimal digits, this also
              matches the "fullwidth" versions of those characters,
              whose Unicode code points start at U+FF10. This is a
              change that was made in PCRE release 10.43 for Perl

       The other POSIX classes are unchanged by PCRE2_UCP, and match
       only characters with code points less than 256.

       There are two options that can be used to restrict the POSIX
       classes to ASCII characters when PCRE2_UCP is set. The option
       PCRE2_EXTRA_ASCII_DIGIT affects just [:digit:] and [:xdigit:].
       Within a pattern, this can be set and unset by (?aT) and (?-aT).
       The PCRE2_EXTRA_ASCII_POSIX option disables UCP processing for
       all POSIX classes, including [:digit:] and [:xdigit:]. Within a
       pattern, (?aP) and (?-aP) set and unset both these options for


       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

         [[:<:]]  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 assertions" 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 behaviour. Note also
       that the PCRE2_UCP option changes the meaning of \w (and
       therefore \b) by default, so it also affects these POSIX

VERTICAL BAR         top

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


       matches either "gilbert" or "sullivan". Any number of
       alternatives may appear, and an empty alternative is 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.


       The settings of several options can be changed within a pattern
       by a sequence of letters enclosed between "(?" and ")". The
       following are Perl-compatible, and are described in detail in the
       pcre2api documentation. The option letters 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
       possible to unset these options by preceding the relevant letters
       with a hyphen, for example (?-im). The two "extended" options are
       not independent; 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. Needless to say, it has no effect.

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

       Some PCRE2-specific options can be changed by the same mechanism
       using these pairs or individual letters:

         aD for PCRE2_EXTRA_ASCII_BSD
         aS for PCRE2_EXTRA_ASCII_BSS
         aW for PCRE2_EXTRA_ASCII_BSW
         J  for PCRE2_DUPNAMES
         U  for PCRE2_UNGREEDY

       However, except for 'r', these are not unset by (?^), which is
       equivalent to (?-imnrsx). If 'a' is not followed by any of the
       upper case letters shown above, it sets (or unsets) all the ASCII

       PCRE2_EXTRA_ASCII_DIGIT has no additional effect when
       PCRE2_EXTRA_ASCII_POSIX is set, but including it in (?aP) means
       that (?-aP) suppresses all ASCII restrictions for POSIX classes.

       When one of these option changes occurs at top level (that is,
       not inside group parentheses), the change applies until a
       subsequent change, or the end of the pattern. An option change
       within a group (see below for a description of groups) affects
       only that part of the group that follows it. At the end of the
       group these options are reset to the state they were before the
       group. For example,


       matches abc and aBc and no other strings (assuming PCRE2_CASELESS
       is not set externally). Any changes made in one alternative do
       carry on into subsequent branches within the same group. For


       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 letters may appear between the "?" and the ":". Thus the
       two patterns


       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 function 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,
       respectively. However, the application can set the
       PCRE2_NEVER_UTF or PCRE2_NEVER_UCP options, which lock out the
       use of the (*UTF) and (*UCP) sequences.

GROUPS         top

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

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


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

       2. It creates a "capture group". This means that, when the whole
       pattern 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 numbered 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 capturing. 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


       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 subsequent branches, so the above patterns
       match "SUNDAY" as well as "Saturday".


       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:


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


       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":


       A relative reference such as (?-1) is no different: it is just a
       convenient 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

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


       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 earlier, 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 different names.  Consider this pattern, where there are
       two capture groups, both numbered 1:


       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 provokes a compile-time error. However, there is
       still scope for confusion. Consider this pattern:


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


       The duplicate name constraint can be disabled by setting the
       PCRE2_DUPNAMES 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


       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":


       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         top

       Repetition is specified by quantifiers, which may follow any one
       of these items:

         a literal data character
         the dot metacharacter
         the \C escape sequence
         the \R escape sequence
         the \X escape sequence
         any escape sequence that matches a single character
         a character class
         a backreference
         a parenthesized group (including lookaround assertions)
         a subroutine call (recursive or otherwise)

       If a quantifier does not follow a repeatable item, an error
       occurs. The general repetition quantifier specifies a minimum and
       maximum number of permitted matches by giving two numbers in
       curly brackets (braces), separated by a comma. The numbers must
       be less than 65536, and the first must be less than or equal to
       the second. For example,


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


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


       matches exactly 8 digits. If the first number is omitted, the
       lower limit is taken as zero; in this case the upper limit must
       be present.

         X{,4} is interpreted as X{0,4}

       This is a change in behaviour that happened in Perl 5.34.0 and
       PCRE2 10.43. In earlier versions such a sequence was not
       interpreted as a quantifier. Other regular expression engines may
       behave either way.

       If the characters that follow an opening brace do not match the
       syntax of a quantifier, the brace is taken as a literal
       character. In particular, this means that {,} is a literal string
       of three characters.

       Note that not every opening brace is potentially the start of a
       quantifier because braces are used in other items such as
       \N{U+345} or \k{name}.

       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. Similarly, \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 useful for capture groups that are referenced as
       subroutines from elsewhere in the pattern (but see also the
       section entitled "Defining capture 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-
       character abbreviations:

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

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


       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 subsequent item
       fails to match.

       By default, quantifiers are "greedy", that is, they match as much
       as possible (up to the maximum number of permitted repetitions),
       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 C comments. The meaning of the various
       quantifiers is not otherwise changed, just the preferred number
       of matches. Do not confuse this use of question mark with its use
       as a quantifier in its own right.  Because it has two uses, it
       can sometimes appear doubled, as in


       which matches one digit by preference, but can match two if that
       is the only way the rest of the pattern 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
       newlines, the pattern is implicitly anchored, because whatever
       follows will be tried against every character position in the
       subject string, so there is no point in retrying the overall
       match at any position after the first. 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
       newlines, it is worth setting PCRE2_DOTALL in order to obtain
       this optimization, 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


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

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


       It matches "ab" in the subject "aab". The use of the backtracking
       control 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


       has matched "tweedledum tweedledee" the value of the captured
       substring is "tweedledee". However, if there are nested capture
       groups, the corresponding captured values may have been set in
       previous iterations. For example, after


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


       With both maximizing ("greedy") and minimizing ("ungreedy" or
       "lazy") repetition, failure of what follows 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


       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:


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


       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 pattern 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 simpler notation, called a "possessive
       quantifier" can be used. This consists of an additional +
       character following a quantifier. Using this notation, the
       previous example can be rewritten as


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


       Possessive quantifiers are always greedy; the setting of the
       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
       syntax.  Jeffrey Friedl originated the idea (and the name) in the
       first edition of his book. Mike McCloskey liked it, so
       implemented it when he built Sun's Java package, and 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_AUTOPOSSESS 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


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


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


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


       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 reference 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 backreferencing 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
       ambiguity 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:


       The sequence \g{-1} is a reference to the capture group whose
       number is one less than the number of the next group to be
       started, so in this example (where the next group would be
       numbered 3) is it equivalent to \2, and \g{-2} would be
       equivalent to \1. Note that if this construct is inside a capture
       group, that group is included in the count, so in this example
       \g{-2} also refers to group 1:


       The use of relative references can be helpful in long patterns,
       and also in patterns that are created by joining together
       fragments that contain references within themselves.

       The sequence \g{+1} is a reference to the next capture group that
       is started after this item, and \g{+2} refers to the one after
       that, and so on. This kind of forward reference can be useful in
       patterns that repeat. Perl does not support the use of + in this

       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 example,


       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 is \k{name}, the Python
       syntax is (?=name), and the original Perl syntax is \k<name> or
       \k'name'. All of these are now supported by both Perl and PCRE2.
       Perl 5.10's unified backreference syntax, in which \g can be used
       for both numeric and named references, is also supported by
       PCRE2.  We could rewrite the above example in any of the
       following ways:


       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


       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
       backreference to an unset value matches an empty string.

       Because there may be many capture groups in a pattern, all digits
       following a backslash are taken as part of a potential
       backreference number. 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


       matches any number of "a"s and also "aba", "ababbaa" etc. At each
       iteration of the group, the backreference matches the character
       string corresponding to the previous iteration. In order for this
       to work, the pattern must be such that the first iteration does
       not need to match the backreference. This can be done using
       alternation, as in the example 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         top

       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 continues 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 backtracking 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
       capture 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 contains a matching branch, what
       happens depends on the type of assertion.

       For a positive assertion, internally captured substrings in the
       successful branch are retained, and matching continues with the
       next pattern 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

   Alphabetic assertion names

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

         (*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
       following sections, the various assertions are described using
       the original symbolic forms.

   Lookahead assertions

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


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


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


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

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

   Lookbehind assertions

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


       does find an occurrence of "bar" that is not preceded by "foo".
       The contents of a lookbehind assertion are restricted such that
       there must be a known maximum to the lengths of all the strings
       it matches. There are two cases:

       If every top-level alternative matches a fixed length, for


       there is a limit of 65535 characters to the lengths, which do not
       have to be the same, as this example demonstrates. This is the
       only kind of lookbehind supported by PCRE2 versions earlier than
       10.43 and by the alternative matching function pcre2_dfa_match().

       In PCRE2 10.43 and later, pcre2_match() supports lookbehind
       assertions in which one or more top-level alternatives can match
       more than one string length, for example


       The maximum matching length for any branch of the lookbehind is
       limited to a value set by the calling program (default 255
       characters). Unlimited repetition (for example \d*) is not
       supported. In some cases, the escape sequence \K (see above) can
       be used instead of a lookbehind assertion at the start of a
       pattern to get round the length limit restriction.

       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 numbers 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 limited-length string. However, recursion, that is, a
       "subroutine" call into a group that is already active, is not

       PCRE2 supports backreferences in lookbehinds, 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 referenced group must
       itself match a limited length substring. The following pattern
       matches words containing at least two characters that begin and
       end with the same character:


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


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


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


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

   Using multiple assertions

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


       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" preceded by six characters, the first of
       which are digits and the last three of which are not "999". For
       example, it doesn't match "123abcfoo". A pattern to do that is


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

       Assertions can be nested in any combination. For example,


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


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


       Traditional lookaround assertions are atomic. That is, if an
       assertion is true, but there is a subsequent matching failure,
       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 readability) 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 atomically. 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

SCRIPT RUNS         top

       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 characters that look the same, but are from
       different scripts. The string "paypal.com" is an infamous
       example, where the letters could be a mixture of Latin and
       Cyrillic. This pattern ensures that the matched characters in a
       sequence of non-spaces that follow white space are a script run:


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


       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:


       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

       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 constructs is encountered. Script runs are not
       supported by the alternate matching function, pcre2_dfa_match()
       because they use the same mechanism 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.


       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:


       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 alternatives 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,
       references 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, which is a PCRE2 extension, not
       supported by Perl, is to precede the digits with a plus or minus
       sign. In this case, the group number is relative rather than
       absolute. The most recently opened capture group (which could be
       enclosing this condition) 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 to
       be opened can be referenced as (?(+1), and so on. The value zero
       in any of these forms is not used; it provokes a compile-time

       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 second 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 section). 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
       recursive. 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 recursion 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:


       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 section above, instead of testing for recursion.
       For example, creating a group with the name R1 by adding (?<R1>)
       to the above pattern completely changes its meaning.

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


       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

       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 referenced from elsewhere. (The
       use of subroutines is described below.) For example, a pattern to
       match an IPv4 address such as "" could be written
       like this (ignore white space and line breaks):

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

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

   Checking the PCRE2 version

       Programs that link with a PCRE2 library can check the version by
       calling pcre2_config() with appropriate arguments. Users of
       applications that do not have access to the underlying code
       cannot do this. A special "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:


       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 non-atomic

       Consider this pattern, again containing non-significant white
       space, and with the two alternatives on the second line:

         \d{2}-[a-z]{3}-\d{2}  |  \d{2}-\d{2}-\d{2} )

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

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

COMMENTS         top

       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 "Newline
       conventions" above. Note that the end of this type of comment is
       a literal newline sequence in the pattern; escape sequences that
       happen to represent a newline do not count. For example, consider
       this pattern when PCRE2_EXTENDED is set, and the default newline
       convention (a single 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.


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

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

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

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

       Obviously, PCRE2 cannot support the interpolation of Perl code.
       Instead, it supports special syntax for recursion of the entire
       pattern, and also for individual capture group recursion. After
       its introduction 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 parenthesized 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, relative 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 supported. 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
       unlimited repeats, and so the use of a possessive quantifier for
       matching strings of non-parentheses is important when applying
       the pattern to strings that do not match. For example, when this
       pattern is applied to


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

       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 documentation). If the pattern above is
       matched against


       the value for the inner capturing parentheses (numbered 2) is
       "ef", which is the last value taken on at the top level. If a
       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 brackets, allowing for arbitrary nesting. Only digits are
       allowed in nested brackets (that is, when recursing), whereas any
       characters are permitted at the outer level.

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

       In this pattern, (?(R) is the start of a conditional 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 alternatives 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:


       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 pattern has to ignore all non-word characters, which
       can be done like this:


       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 possessive 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:


       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.


       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:


       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


       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.


       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:


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

CALLOUTS         top

       Perl has a feature whereby using the sequence (?{...}) causes
       arbitrary Perl code to be obeyed in the middle of matching a
       regular expression. This makes it possible, amongst other things,
       to extract different substrings that match the same pair of
       parentheses when there is a repetition.

       PCRE2 provides a similar feature, but of course it cannot obey
       arbitrary 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 disabled.

       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
       function 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

   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:


       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 pattern 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


       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
       argument. The starting delimiter must be one of ` ' " ^ % # $ {
       and the ending delimiter is the same as the start, except for {,
       where the ending 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.


       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

       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-compatible.

       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. Character type escapes such as \d are

       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 pattern. 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 traditional 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

       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_compile(), 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 positive assertion, the assertion
       succeeds; in a negative assertion, the assertion fails.

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


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

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


       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 captured. Whereas (*COMMIT) (see below) means "fail on
       backtrack", a repeated (*ACCEPT) of this type means "succeed on

       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 feature, as for example in this


       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
       backtracking 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 section entitled "Other information about the match" in the
       pcre2api documentation. 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
          0: XZ
         MK: B

       The (*MARK) name is tagged with "MK:" in this output, and in this
       example it indicates which of the two alternatives matched. This
       is a more efficient way of obtaining this information than
       putting each alternative 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-
       encountered. 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 continues with what follows, but if there is a
       subsequent match failure, causing a backtrack to the verb, a
       failure is forced. That is, backtracking cannot pass to the left
       of the verb. However, when one of these verbs appears inside an
       atomic group 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. Backtracking
       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
       backtracking reaches them. The behaviour described below is what
       happens when the verb is not in a subroutine or an assertion.
       Subsequent sections 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 pattern 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 committed to finding a match at the
       current starting point, or not at all. For example:


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

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


       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 encountered. (*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:


       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 quantifier 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.


       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 corresponds 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 backtracking. Compare the following pcre2test

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

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

         (*THEN) or (*THEN:NAME)

       This verb causes a skip to the next innermost alternative when
       backtracking 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 subsequently BAZ fails, there are no more
       alternatives, so there is a backtrack 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
       failure 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
       alternatives, 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 pattern, where A, B, etc. are complex pattern fragments:


       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:


       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:


       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

       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 assertion, and therefore verbs such as
       (*COMMIT) can be triggered by backtracks from later in the

       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

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

   Backtracking verbs in subroutines

       These behaviours occur whether or not the group is called

       (*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 treatment 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 subroutine. 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         top

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

AUTHOR         top

       Philip Hazel
       Retired from University Computing Service
       Cambridge, England.

REVISION         top

       Last updated: 12 October 2023
       Copyright (c) 1997-2023 University of Cambridge.

COLOPHON         top

       This page is part of the PCRE (Perl Compatible Regular
       Expressions) project.  Information about the project can be found
       at ⟨http://www.pcre.org/⟩.  If you have a bug report for this
       manual page, see
       ⟨http://bugs.exim.org/enter_bug.cgi?product=PCRE⟩.  This page was
       obtained from the tarball fetched from
       ⟨https://github.com/PhilipHazel/pcre2.git⟩ on 2023-12-22.  If you
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       information in this COLOPHON (which is not part of the original
       manual page), send a mail to man-pages@man7.org

PCRE2 10.43                  12 October 2023             PCRE2PATTERN(3)

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