# pcre2pattern

## NAME



           PCRE2 - Perl-compatible regular expressions (revised API)
    


## PCRE2 REGULAR EXPRESSION DETAILS



    
           The syntax and semantics of the regular expressions that are
           supported by PCRE2 are described in detail below. There is a
           quick-reference 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
           that does not conflict with the Perl syntax in order to provide
           some compatibility with regular expressions in Python, .NET, and
           Oniguruma. There are in addition some options that enable
           alternative syntax and semantics that are not the same as in Perl.
    
           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.
    


## EBCDIC CHARACTER CODES



    
           Most computers use ASCII or Unicode for encoding characters, and
           PCRE2 assumes this by default. However, it can be compiled to run
           in an environment that uses the EBCDIC code, which is the case for
           some IBM mainframe operating systems. 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. Differences in
           behaviour when PCRE2 is running in an EBCDIC environment are
           described in the section "EBCDIC environments" below, which you
           can ignore unless you really are in an EBCDIC environment.
    


## SPECIAL START-OF-PATTERN ITEMS



    
           A number of options that can be passed to **pcre2_compile()** can also
           be set by special items at the start of a pattern. These are not
           Perl-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, or calling
           **pcre2_set_optimize()** with a PCRE2_AUTO_POSSESS_OFF directive. 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, or calling
           **pcre2_set_optimize()** with a PCRE2_START_OPTIMIZE_OFF directive.
           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, or calling
           **pcre2_set_optimize()** with a PCRE2_DOTSTAR_ANCHOR_OFF directive.
           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
    
             (*LIMIT_HEAP=d)
             (*LIMIT_MATCH=d)
             (*LIMIT_DEPTH=d)
    
           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
    
             (*CR)a.b
    
           changes the convention to CR. That pattern matches "a\nb" because
           LF is no longer a newline. If more than one of these settings is
           present, the last one is used.
    
           The newline convention affects where the circumflex and dollar
           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_BSR_UNICODE.
    


## CHARACTERS AND METACHARACTERS



    
           A regular expression is a pattern that is matched against a
           subject string from left to right. Most characters stand for
           themselves in a pattern, and match the corresponding characters in
           the subject. As a trivial example, the pattern
    
             The quick brown fox
    
           matches a portion of a subject string that is identical to itself.
           When caseless matching is specified (the PCRE2_CASELESS option or
           (?i) within the pattern), letters are matched independently of
           case. Note that there are two ASCII characters, K and S, that, in
           addition to their lower case ASCII equivalents, are case-
           equivalent with Unicode U+212A (Kelvin sign) and U+017F (long S)
           respectively when either PCRE2_UTF or PCRE2_UCP is set, unless the
           PCRE2_EXTRA_CASELESS_RESTRICT option is in force (either passed to
           **pcre2_compile()** or set by (*CASELESS_RESTRICT) or (?r) within the
           pattern). If the PCRE2_EXTRA_TURKISH_CASING option is in force
           (either passed to **pcre2_compile()** or set by (*TURKISH_CASING)
           within the pattern), then the 'i' letters are matched according to
           Turkish and Azeri languages.
    
           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
           are:
    
             \      general escape character
             ^      negate the class, but only if the first character
             -      indicates character range
             [      POSIX character class (if followed by POSIX syntax)
             ]      terminates the character class
    
           If a pattern is compiled with the PCRE2_EXTENDED option, most
           white space in the pattern, other than in a character class,
           within a \Q...\E sequence, or between a # outside a character
           class and the next newline, inclusive, is 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
           metacharacters.
    


## BACKSLASH



    
           The backslash character has several uses. Firstly, if it is
           followed by a character that is not a digit or a letter, it takes
           away any special meaning that character may have. This use of
           backslash as an escape character applies both inside and outside
           character classes.
    
           For example, if you want to match a * character, you must write \*
           in the pattern. This escaping action applies whether or not the
           following character would otherwise be interpreted as a
           metacharacter, so it is always safe to precede a non-alphanumeric
           with backslash to specify that it stands for itself.  In
           particular, if you want to match a 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.
    
           Another difference from Perl is that any appearance of \Q or \E
           inside what might otherwise be a quantifier causes PCRE2 not to
           recognize the sequence as a quantifier. Perl recognizes a
           quantifier if (redundantly) either of the numbers is inside
           \Q...\E, but not if the separating comma is. When not recognized
           as a quantifier a sequence such as {\Q1\E,2} is treated as the
           literal string "{1,2}".
    
       **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
           character
             \e          escape (hex 1B)
             \f          form feed (hex 0C)
             \n          linefeed (hex 0A)
             \r          carriage return (hex 0D) (but see below)
             \t          tab (hex 09)
             \0dd        character with octal code 0dd
             \ddd        character with octal code ddd, or back reference
             \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..
    
           A description of how back references work is given later,
           following the discussion of parenthesized groups.
    
           By default, after \x that is not followed by {, one or two
           hexadecimal digits are read (letters can be in upper or lower
           case). If the character that follows \x is neither { nor a
           hexadecimal digit, an error occurs. This is different from Perl's
           default behaviour, which generates a NUL character, but is in line
           with the behaviour of Perl's 'strict' mode in re.
    
           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 occurs.
    
           Characters whose code points are less than 256 can be defined by
           either of the two syntaxes for \x or by an octal sequence. There
           is no 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.
    
           For differences in the way some escapes behave in EBCDIC
           environments, see section "EBCDIC environments" below.
    
       **Octal escapes and back references**
    
           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 provides a way of specifying character code points as octal
           numbers greater than 0777, and it also allows octal numbers and
           backreferences to be unambiguously distinguished.
    
           If braces are not used, after \0 up to two further octal digits
           are read.  However, if the PCRE2_EXTRA_NO_BS0 option is set, at
           least one more octal digit must follow \0 (use \00 to generate a
           NUL character). Make sure you supply two digits after the initial
           zero if the pattern character that follows is itself an octal
           digit.
    
           Inside a character class, when a backslash is followed by any
           octal digit, up to three octal digits are read to generate a code
           point. Any subsequent digits stand for themselves. The sequences
           \8 and \9 are treated as the literal characters "8" and "9".
    
           Outside a character class, Perl's handling of a backslash followed
           by a digit other than 0 is complicated by ambiguity, and Perl has
           changed over time, causing PCRE2 also to change. From PCRE2
           release 10.45 there is an option called PCRE2_EXTRA_PYTHON_OCTAL
           that causes PCRE2 to use Python's unambiguous rules. The next two
           subsections describe the two sets of rules.
    
           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.
    
       **Perl rules for non-class backslash 1-9**
    
           All the digits that follow the backslash are read 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 back reference.
           Otherwise, up to three octal digits are read to form a character
           code. For example:
    
             \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.
    
       **Python rules for non_class backslash 1-9**
    
           If there are at least three octal digits after the backslash,
           exactly three are read as an octal code point number, but the
           value must be no greater than \377, even in modes where higher
           code point values are supported. Any subsequent digits stand for
           themselves. If there are fewer than three octal digits, the
           sequence is taken as a decimal back reference. Thus, for example,
           \12 is always a back reference, independent of how many captures
           there are in the pattern. An error is generated for a reference to
           a non-existent capturing group.
    
       **Constraints on character values**
    
           Characters that are specified using octal or hexadecimal numbers
           are limited to certain values, as follows:
    
             8-bit non-UTF mode    no greater than 0xff
             16-bit non-UTF mode   no greater than 0xffff
             32-bit non-UTF mode   no greater than 0xffffffff
             All UTF modes         no greater than 0x10ffff and a valid code
           point
    
           Invalid Unicode code points are all those in the range 0xd800 to
           0xdfff (the so-called "surrogate" code points). The check for
           these can be disabled by the caller of **pcre2_compile()** by setting
           the option PCRE2_EXTRA_ALLOW_SURROGATE_ESCAPES. However, this is
           possible only in UTF-8 and UTF-32 modes, because these values are
           not representable in UTF-16.
    
       **Escape sequences in character classes**
    
           All the sequences that define a single character value can be used
           both inside and outside character classes. In addition, inside a
           character class, \b is interpreted as the backspace character (hex
           08).
    
           When not followed by an opening brace, \N is not allowed in a
           character class.  \B, \R, and \X are not special inside a
           character class. Like other unrecognized alphabetic escape
           sequences, they cause an error. Outside a character class, these
           sequences have different meanings.
    
       **Unsupported escape sequences**
    
           In Perl, the sequences \F, \l, \L, \u, and \U are recognized by
           its string handler and used to modify the case of following
           characters. By default, PCRE2 does not support these escape
           sequences in patterns. However, if either of the PCRE2_ALT_BSUX or
           PCRE2_EXTRA_ALT_BSUX options is set, \U matches a "U" character,
           and \u can be used to define a character by code point, as
           described above.
    
       **Absolute and relative backreferences**
    
           The sequence \g followed by a signed or unsigned number,
           optionally enclosed in braces, is an absolute or relative
           backreference. A named backreference can be coded as \g{name}.
           Backreferences are discussed later, following the discussion of
           parenthesized groups.
    
       **Absolute and relative subroutine calls**
    
           For compatibility with Oniguruma, the non-Perl syntax \g followed
           by a name or a number enclosed either in angle brackets or single
           quotes, is an alternative syntax for referencing a capture group
           as a subroutine. Details are discussed later.  Note that \g{...}
           (Perl syntax) and \g<...> (Oniguruma syntax) are _not_ synonymous.
           The former is a backreference; the latter is a subroutine call.
    
       **Generic character types**
    
           Another use of backslash is for specifying generic character
           types:
    
             \d     any decimal digit
             \D     any character that is not a decimal digit
             \h     any horizontal white space character
             \H     any character that is not a horizontal white space
           character
             \N     any character that is not a newline
             \s     any white space character
             \S     any character that is not a white space character
             \v     any vertical white space character
             \V     any character that is not a vertical white space
           character
             \w     any "word" character
             \W     any "non-word" character
    
           The \N escape sequence has the same meaning as the "."
           metacharacter when PCRE2_DOTALL is not set, but setting
           PCRE2_DOTALL does not change the meaning of \N. Note that when \N
           is followed by an opening brace it has a different meaning. See
           the section entitled "Non-printing 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:
    
             (?>\r\n|\n|\x0b|\f|\r|\x85)
    
           This is an example of an "atomic group", details of which are
           given below.  This particular group matches either the two-
           character sequence CR followed by LF, or one of the single
           characters LF (linefeed, U+000A), VT (vertical tab, U+000B), FF
           (form feed, U+000C), CR (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:
    
             (*ANY)(*BSR_ANYCRLF)
    
           They can also be combined with the (*UTF) or (*UCP) special
           sequences. Inside a character class, \R is treated as an
           unrecognized escape sequence, and causes an error.
    
       **Unicode character properties**
    
           When PCRE2 is built with Unicode support (the default), three
           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 or 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
    
           For compatibility with Perl, negation can be specified by
           including a circumflex between the opening brace and the property.
           For example, \p{^Lu} is the same as \P{Lu}.
    
           In accordance with Unicode's "loose matching" rules, ASCII white
           space characters, hyphens, and underscores are ignored in the
           properties represented by _xx_ above. As well as the space
           character, ASCII white space can be tab, linefeed, vertical tab,
           formfeed, or carriage return.
    
           Some properties are specified as a name only; others as a name and
           a value, separated by a colon or an equals sign. The names and
           values consist of ASCII letters and digits (with one Perl-specific
           exception, see below). They are not case sensitive. Note, however,
           that the escapes themselves, \p and \P, _are_ case sensitive. There
           are abbreviations for many names. The following examples are all
           equivalent:
    
             \p{bidiclass=al}
             \p{BC=al}
             \p{ Bidi_Class : AL }
             \p{ Bi-di class = Al }
             \P{ ^ Bi-di class = Al }
    
           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, as for all property
           specifications, an 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. If only one letter is
           specified with \p or \P, it includes all the general category
           properties that start with that letter. In this case, in the
           absence of negation, the curly brackets in the escape sequence are
           optional; these two examples have the same effect:
    
             \p{L}
             \pL
    
           The following general category property codes are supported:
    
             C     Other
             Cc    Control
             Cf    Format
             Cn    Unassigned
             Co    Private use
             Cs    Surrogate
    
             L     Letter
             Lc    Cased 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
    
           Perl originally used the name L& for the Lc property. This is
           still supported by Perl, but discouraged. PCRE2 also still
           supports it. This property matches any character that has the Lu,
           Ll, or Lt property, in other words, any letter that is not
           classified as a modifier or "other". From release 10.45 of PCRE2
           the properties Lu, Ll, and Lt are all treated as Lc when case-
           independent matching is set by the PCRE2_CASELESS option or (?i)
           within the pattern. The other properties are not affected by
           caseless matching.
    
           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.
    
       **Binary (yes/no) properties for \p and \P**
    
           Unicode defines a number of binary properties, that is, properties
           whose only values are true or false. You can obtain a list of
           those that are recognized by \p and \P, along with their
           abbreviations, by running this command:
    
             pcre2test -LP
    
       **The Bidi_Class property for \p and \P**
    
             \p{Bidi_Class:<class>}   matches a character with the given
           class
             \p{BC:<class>}           matches a character with the given
           class
    
           The recognized classes are:
    
             AL          Arabic letter
             AN          Arabic number
             B           paragraph separator
             BN          boundary neutral
             CS          common separator
             EN          European number
             ES          European separator
             ET          European terminator
             FSI         first strong isolate
             L           left-to-right
             LRE         left-to-right embedding
             LRI         left-to-right isolate
             LRO         left-to-right override
             NSM         non-spacing mark
             ON          other neutral
             PDF         pop directional format
             PDI         pop directional isolate
             R           right-to-left
             RLE         right-to-left embedding
             RLI         right-to-left isolate
             RLO         right-to-left override
             S           segment separator
             WS          white space
    
           As in all property specifications, an equals sign may be used
           instead of a colon and the class names are case-insensitive. Only
           the short names listed above are recognized; PCRE2 does not at
           present support any long alternatives.
    
       **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
           character.
    
           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 (ZWJ) character. Characters with the "mark"
           property always have the "extend" grapheme breaking property.
    
           5. Do not end after prepend characters.
    
           6. Do not end within emoji modifier sequences or emoji ZWJ (zero-
           width joiner) sequences. An emoji ZWJ sequence consists of a
           character with the Extended_Pictographic property, optionally
           followed by one or more characters with the Extend property,
           followed by the ZWJ character, followed by another
           Extended_Pictographic character.
    
           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 (this includes the
           space character). 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:
    
             foo\Kbar
    
           matches "foobar", but reports that it has matched "bar". \K does
           not interact with anchoring in any way. The pattern:
    
             ^foo\Kbar
    
           matches only when the subject begins with "foobar" (in single line
           mode), though it again reports the matched string as "bar". This
           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
    
             (foo)\Kbar
    
           matches "foobar", the first substring is still set to "foo".
    
           From version 5.32.0 Perl forbids the use of \K in lookaround
           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 pattern:
    
             (?<=\Kfoo)bar
    
           If the subject is "foobar", a call to **pcre2_match()** with a
           starting offset of 3 succeeds and reports the matching string as
           "foobar", that is, the start of the reported match is earlier than
           where the match started.
    
       **Simple assertions**
    
           The final use of backslash is for certain simple assertions. An
           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
           generated.
    
           A word boundary is a position in the subject string where the
           current character and the previous character do not both match \w
           or \W (i.e. one matches \w and the other matches \W), or the start
           or end of the string if the first or last character matches \w,
           respectively. When PCRE2 is built with Unicode support, the
           meanings of \w and \W can be changed by setting the PCRE2_UCP
           option. When this is done, it also affects \b and \B. Neither
           PCRE2 nor Perl has a separate "start of word" or "end of word"
           metasequence. However, whatever follows \b 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.
    


## CIRCUMFLEX AND DOLLAR



    
           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



    
           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 occurrences of CR of LF
           match dot. When all Unicode line endings are being recognized, dot
           does not match CR or LF or any of the other line ending
           characters.
    
           The behaviour of dot with regard to newlines can be changed. If
           the PCRE2_DOTALL option is set, a dot matches any one character,
           without exception.  If the two-character sequence CRLF is present
           in the subject string, it takes two dots to match it.
    
           The handling of dot is entirely independent of the handling of
           circumflex and dollar, the only relationship being that they both
           involve newlines. Dot has no special meaning in a character class.
    
           The escape sequence \N when not followed by an opening brace
           behaves like a dot, except that it is not affected by the
           PCRE2_DOTALL option. In other words, it matches any character
           except one that signifies the end of a line.
    
           When \N is followed by an opening brace it has a different
           meaning. See the section entitled "Non-printing characters" above
           for details. Perl also uses \N{name} to specify characters by
           Unicode name; PCRE2 does not support this.
    


## MATCHING A SINGLE CODE UNIT



    
           Outside a character class, the escape sequence \C matches any one
           code unit, whether or not a UTF mode is set. In the 8-bit library,
           one code unit is one byte; in the 16-bit library it is a 16-bit
           unit; in the 32-bit library it is a 32-bit unit. Unlike a dot, \C
           always matches line-ending characters. The feature is provided in
           Perl in order to match individual bytes in UTF-8 mode, but it is
           unclear how it can 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
           disabled.
    
           PCRE2 does not allow \C to appear in lookbehind assertions
           (described below) in UTF-8 or UTF-16 modes, because this would
           make it impossible to calculate the length of the lookbehind.
           Neither the alternative matching function **pcre2_dfa_match()** nor
           the JIT optimizer support \C in these UTF modes.  The former gives
           a match-time error; the latter fails to optimize and so the match
           is always run using the interpreter.
    
           In the 32-bit library, however, \C is always supported (when not
           explicitly locked out) because it always matches a single code
           unit, whether or not UTF-32 is specified.
    
           In general, the \C escape sequence is best avoided. However, one
           way of using it that avoids the problem of malformed UTF-8 or
           UTF-16 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) |
                 (?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C))
    
           In this example, a group that starts with (?| resets the capturing
           parentheses numbers in each alternative (see "Duplicate Group
           Numbers" below). The assertions at the start of each branch check
           the next UTF-8 character for values whose encoding uses 1, 2, 3,
           or 4 bytes, respectively. The character's individual bytes are
           then captured by the appropriate number of \C groups.
    


## SQUARE BRACKETS AND CHARACTER CLASSES



    
           An opening square bracket introduces a character class, terminated
           by a closing square bracket. A closing square bracket on its own
           is not 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
           English vowel, whereas [^aeiou] matches all other characters. 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 to match 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. If you do not want these ASCII/non-ASCII case
           equivalences, you can suppress them by setting
           PCRE2_EXTRA_CASELESS_RESTRICT, either as an option in a compile
           context, or by including (*CASELESS_RESTRICT) or (?r) within a
           pattern.
    
           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.
    
           There is some special treatment for alphabetic ranges in EBCDIC
           environments; see the section "EBCDIC environments" below.
    
           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 \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 a
           range, so [W-\]46] is interpreted as a class containing a range
           and 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.
    
           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 metacharacters that are recognized in character classes are
           backslash, hyphen (when it can be interpreted as specifying a
           range), circumflex (only at the start), and the terminating
           closing square bracket. An opening square bracket is also special
           when it can be interpreted as introducing a POSIX class (see
           "Posix character classes" below), or a special compatibility
           feature (see "Compatibility feature for word boundaries" below.
           Escaping any non-alphanumeric character in a class turns it into a
           literal, whether or not it would otherwise be a metacharacter.
    


## PERL EXTENDED CHARACTER CLASSES



    
           From release 10.45 PCRE2 supports Perl's (?[...]) extended
           character class syntax. This can be used to perform set operations
           such as intersection on character classes.
    
           The syntax permitted within (?[...]) is quite different to
           ordinary character classes. Inside the extended class, there is an
           expression syntax consisting of "atoms", operators, and ordinary
           parentheses "()" used for grouping. Such classes always have the
           Perl /xx modifier (PCRE2 option PCRE2_EXTENDED_MORE) turned on
           within them. This means that literal space and tab characters are
           ignored everywhere in the class.
    
           The allowed atoms are individual characters specified by escape
           sequences such as \n or \x{123}, character types such as \d, POSIX
           classes such as [:alpha:], and nested ordinary (non-extended)
           character classes. For example, in (?[\d & [...]]) the nested
           class [...] follows the usual rules for ordinary character
           classes, in which parentheses are not metacharacters, and
           character literals and ranges are permitted.
    
           Character literals and ranges may not appear outside a nested
           ordinary character class because they are not atoms in the
           extended syntax. The extended syntax does not introduce any
           additional escape sequences, so (?[\y]) is an unknown escape, as
           it would be in [\y].
    
           In the extended syntax, ^ does not negate a class (except within
           an ordinary class nested inside an extended class); it is instead
           a binary operator.
    
           The binary operators are "&" (intersection), "|" or "+" (union),
           "-" (subtraction) and "^" (symmetric difference). These are left-
           associative and "&" has higher (tighter) precedence, while the
           others have equal lower precedence. The one prefix unary operator
           is "!" (complement), with highest precedence.
    


## UTS#18 EXTENDED CHARACTER CLASSES



    
           The PCRE2_ALT_EXTENDED_CLASS option enables an alternative to
           Perl's (?[...])  syntax, allowing instead extended class behaviour
           inside ordinary [...]  character classes. This altered syntax for
           [...] classes is loosely described by the Unicode standard UTS#18.
           The PCRE2_ALT_EXTENDED_CLASS option does not prevent use of
           (?[...]) classes; it just changes the meaning of all [...] classes
           that are not nested inside a Perl (?[...]) class.
    
           Firstly, in ordinary Perl [...] syntax, an expression such as
           "[a[]" is a character class with two literal characters "a" and
           "[", but in UTS#18 extended classes the "[" character becomes an
           additional metacharacter within classes, denoting the start of a
           nested class, so a literal "[" must be escaped as "\[".
    
           Secondly, within the UTS#18 extended syntax, there are operators
           "||", "&&", "--" and "~~" which denote character class union,
           intersection, subtraction, and symmetric difference respectively.
           In standard Perl syntax, these would simply be needlessly-repeated
           literals (except for "--" which could be the start or end of a
           range). In UTS#18 extended classes these operators can be used in
           constructs such as [\p{L}--[QW]] for "Unicode letters, other than
           Q and W".  A literal "-" at the start or end of a range must be
           escaped, so while "[--1]" in Perl syntax is the range from hyphen
           to "1", it must be escaped as "[\--1]" in UTS#18 extended classes.
    
           Unlike Perl's (?[...]) extended classes, the PCRE2_EXTENDED_MORE
           option to ignore space and tab characters is not automatically
           enabled for UTS#18 extended classes, but it is honoured if set.
    
           Extended UTS#18 classes can be nested, and nested classes are
           themselves extended classes (unlike Perl, where nested classes
           must be simple classes).  For example,
           [\p{L}&&[\p{Thai}||\p{Greek}]] matches any letter that is in the
           Thai or Greek scripts. Note that this means that no special
           grouping characters (such as the parentheses used in Perl's
           (?[...]) class syntax) are needed.
    
           Individual class items (literal characters, literal ranges,
           properties such as \d or \p{...}, and nested classes) can be
           combined by juxtaposition or by an operator. Juxtaposition is the
           implicit union operator, and binds more tightly than any explicit
           operator. Thus a sequence of literals and/or ranges behaves as if
           it is enclosed in square brackets. For example, [A-Z0-9&&[^E8]] is
           the same as [[A-Z0-9]&&[^E8]], which matches any upper case
           alphanumeric character except "E" or "8".
    
           Precedence between the explicit operators is not defined, so
           mixing operators is a syntax error. For example, [A&&B--C] is an
           error, but [A&&[B--C]] is valid.
    
           This is an emerging syntax which is being adopted gradually across
           the regex ecosystem: for example JavaScript adopted the "/v" flag
           in ECMAScript 2024; Python's "re" module reserves the syntax for
           future use with a FutureWarning for unescaped use of "[" as a
           literal within character classes. Due to UTS#18 providing
           insufficient guidance, engines interpret the syntax differently.
           Rust's "regex" crate and Python's "regex" PyPi module both
           implement UTS#18 extended classes, but with slight
           incompatibilities ([A||B&&C] is parsed as [A||[B&&C]] in Python's
           "regex" but as [[A||B]&&C] in Rust's "regex").
    
           PCRE2's syntax adds syntax restrictions similar to ECMASCript's /v
           flag, so that all the UTS#18 extended classes accepted as valid by
           PCRE2 have the property that they are interpreted either with the
           same behaviour, or as invalid, by all other major engines. Please
           file an issue if you are aware of cross-engine differences in
           behaviour between PCRE2 and another major engine.
    


## POSIX CHARACTER CLASSES



    
           Perl supports the POSIX notation for character classes. This uses
           names enclosed by [: and :] within the enclosing square brackets.
           PCRE2 also supports this notation, in both ordinary and extended
           classes. For example,
    
             [01[:alpha:]%]
    
           matches "0", "1", any alphabetic character, or "%". The supported
           class names are:
    
             alnum    letters and digits
             alpha    letters
             ascii    character codes 0 - 127
             blank    space or tab only
             cntrl    control characters
             digit    decimal digits (same as \d)
             graph    printing characters, excluding space
             lower    lower case letters
             print    printing characters, including space
             punct    printing characters, excluding letters and digits and
           space
             space    white space (the same as \s from PCRE2 8.34)
             upper    upper case letters
             word     "word" characters (same as \w)
             xdigit   hexadecimal digits
    
           The default "space" characters are HT (9), LF (10), VT (11), FF
           (12), CR (13), and space (32). If locale-specific matching is
           taking place, the list of space characters may be different; there
           may be fewer or more of them. "Space" and \s match the same set of
           characters, 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,
    
             [12[:^digit:]]
    
           matches "1", "2", or any non-digit. PCRE2 (and Perl) also
           recognize the POSIX syntax [.ch.] and [=ch=] where "ch" is a
           "collating element", but these are not supported, and an error is
           given if they are encountered.
    
           By default, characters with values greater than 127 do not match
           any of the POSIX character classes, although this may be different
           for 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 follows:
    
             [:alnum:]  becomes  \p{Xan}
             [:alpha:]  becomes  \p{L}
             [:blank:]  becomes  \h
             [:cntrl:]  becomes  \p{Cc}
             [:digit:]  becomes  \p{Nd}
             [:lower:]  becomes  \p{Ll}
             [:space:]  becomes  \p{Xps}
             [:upper:]  becomes  \p{Lu}
             [:word:]   becomes  \p{Xwd}
    
           Negated versions, such as [:^alpha:] use \P instead of \p. Four
           other POSIX classes are handled specially in UCP mode:
    
           [:graph:]
                  This matches characters that have glyphs that mark the page
                  when printed. In Unicode property terms, it matches all
                  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
    
           [:print:]
                  This matches the same characters as [:graph:] plus space
                  characters that are not controls, that is, characters with
                  the Zs property.
    
           [:punct:]
                  This matches all characters that have the Unicode P
                  (punctuation) property, plus those characters with code
                  points less than 256 that have the S (Symbol) property.
    
           [:xdigit:]
                  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 PCRE2 release 10.43 for Perl compatibility.
    
           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
           consistency.
    


## COMPATIBILITY FEATURE FOR WORD BOUNDARIES



    
           In the POSIX.2 compliant library that was included in 4.4BSD Unix,
           the ugly syntax [[:<:]] and [[:>:]] is used for matching "start of
           word" and "end of word". PCRE2 treats these items as follows:
    
             [[:<:]]  is converted to  \b(?=\w)
             [[:>:]]  is converted to  \b(?<=\w)
    
           Only these exact character sequences are recognized. A sequence
           such as [a[:<:]b] provokes error for an unrecognized POSIX class
           name. This support is not compatible with Perl. It is provided to
           help migrations from other environments, and is best not used in
           any new patterns. Note that \b matches at the start and the end of
           a word (see "Simple 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 sequences.
    


## VERTICAL BAR



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


## INTERNAL OPTION SETTING



    
           The settings of 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
             aP for PCRE2_EXTRA_ASCII_POSIX and PCRE2_EXTRA_ASCII_DIGIT
             aT for PCRE2_EXTRA_ASCII_DIGIT
             r  for PCRE2_EXTRA_CASELESS_RESTRICT
             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
           options.
    
           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,
    
             (a(?i)b)c
    
           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
           example,
    
             (a(?i)b|c)
    
           matches "ab", "aB", "c", and "C", even though when matching "C"
           the first branch is abandoned before the option setting. This is
           because the effects of option settings happen at compile time.
           There would be some very weird behaviour otherwise.
    
           As a convenient shorthand, if any option settings are required at
           the start of a non-capturing group (see the next section), the
           option letters may appear between the "?" and the ":". Thus the
           two patterns
    
             (?i:saturday|sunday)
             (?:(?i)saturday|sunday)
    
           match exactly the same set of strings.
    
           **Note:** There are other PCRE2-specific options, applying to the
           whole pattern, which can be set by the application when the
           compiling 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



    
           Groups are delimited by parentheses (round brackets), which can be
           nested.  Turning part of a pattern into a group does two things:
    
           1. It localizes a set of alternatives. For example, the pattern
    
             cat(aract|erpillar|)
    
           matches "cataract", "caterpillar", or "cat". Without the
           parentheses, it would match "cataract", "erpillar" or an empty
           string.
    
           2. It creates a "capture group". This means that, when the whole
           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
    
             (?i:saturday|sunday)
             (?:(?i)saturday|sunday)
    
           match exactly the same set of strings. Because alternative
           branches are tried from left to right, and options are not reset
           until the end of the group is reached, an option setting in one
           branch does affect subsequent branches, so the above patterns
           match "SUNDAY" as well as "Saturday".
    


## DUPLICATE GROUP NUMBERS



    
           Perl 5.10 introduced a feature whereby each alternative in a group
           uses the same numbers for its capturing parentheses. Such a group
           starts with (?| and is itself a non-capturing group. For example,
           consider this pattern:
    
             (?|(Sat)ur|(Sun))day
    
           Because the two alternatives are inside a (?| group, both sets of
           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":
    
             /(?|(abc)|(def))\1/
    
           In contrast, a subroutine call to a capture group always refers to
           the first one in the pattern with the given number. The following
           pattern matches "abcabc" or "defabc":
    
             /(?|(abc)|(def))(?1)/
    
           A relative reference such as (?-1) is no different: it is just a
           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
           matched.
    
           An alternative approach to using this "branch reset" feature is to
           use duplicate named groups, as described in the next section.
    


## NAMED CAPTURE GROUPS



    
           Identifying capture groups by number is simple, but it can be very
           hard to keep track of the numbers in complicated patterns.
           Furthermore, if an expression is modified, the numbers may change.
           To help with this difficulty, PCRE2 supports the naming of capture
           groups. This feature was not added to Perl until release 5.10.
           Python had the feature 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 128 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:
    
             (?|(?<AA>aa)|(?<BB>bb))
    
           Perl allows this, with both names AA and BB as aliases of group 1.
           Thus, after a successful match, both names yield the same value
           (either "aa" or "bb").
    
           In an attempt to reduce confusion, PCRE2 does not allow the same
           group number to be associated with more than one name. The example
           above provokes a compile-time error. However, there is still scope
           for confusion. Consider this pattern:
    
             (?|(?<AA>aa)|(bb))
    
           Although the second group number 1 is not explicitly named, the
           name AA is still an alias for any group 1. Whether the pattern
           matches "aa" or "bb", a reference by name to group AA yields the
           matched string.
    
           By default, a name must be unique within a pattern, except that
           duplicate names are permitted for groups with the same number, for
           example:
    
             (?|(?<AA>aa)|(?<AA>bb))
    
           The duplicate name constraint can be disabled by setting the
           PCRE2_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 job:
    
             (?J)
             (?<DN>Mon|Fri|Sun)(?:day)?|
             (?<DN>Tue)(?:sday)?|
             (?<DN>Wed)(?:nesday)?|
             (?<DN>Thu)(?:rsday)?|
             (?<DN>Sat)(?:urday)?
    
           There are five capture groups, but only one is ever set after a
           match. The convenience functions for extracting the data by name
           returns the substring for the first (and in this example, the
           only) group of that name that matched. This saves searching to
           find which numbered group it was. (An alternative way of solving
           this problem is to use a "branch reset" group, as described in the
           previous section.)
    
           If you make a backreference to a non-unique named group from
           elsewhere in the pattern, the groups to which the name refers are
           checked in the order in which they appear in the overall pattern.
           The first one that is set is used for the reference. For example,
           this pattern matches both "foofoo" and "barbar" but not "foobar"
           or "barfoo":
    
             (?J)(?:(?<n>foo)|(?<n>bar))\k<n>
    
           If you make a subroutine call to a non-unique named group, the one
           that corresponds to the first occurrence of the name is used. In
           the absence of duplicate numbers this is the one with the lowest
           number.
    
           If you use a named reference in a condition test (see the section
           about conditions below), either to check whether a capture group
           has matched, or to check for recursion, all groups with the same
           name are tested. If the condition is true for any one of them, the
           overall condition is true. This is the same behaviour as testing
           by number. For further details of the interfaces for handling
           named capture groups, see the **pcre2api** documentation.
    


## REPETITION



    
           Repetition is specified by quantifiers, which 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,
    
             z{2,4}
    
           matches "zz", "zzz", or "zzzz". A closing brace on its own is not
           a special character. If the second number is omitted, but the
           comma is present, there is no upper limit; if the second number
           and the comma are both omitted, the quantifier specifies an exact
           number of required matches. Thus
    
             [aeiou]{3,}
    
           matches at least 3 successive vowels, but may match many more,
           whereas
    
             \d{8}
    
           matches exactly 8 digits. 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:
    
             (a?)*
    
           Earlier versions of Perl and PCRE1 used to give an error at
           compile time for such patterns. However, because there are cases
           where this can be useful, such patterns are now accepted, but
           whenever an iteration of such a group matches no characters,
           matching moves on to the next item in the pattern instead of
           repeatedly matching an empty string. This does not prevent
           backtracking into any of the iterations if a 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
    
             \d??\d
    
           which matches one digit by preference, but can match two if that
           is the only way the rest of the pattern matches.
    
           If the PCRE2_UNGREEDY option is set (an option that is not
           available in Perl), the quantifiers are not greedy by default, but
           individual ones can be made greedy by following them with a
           question mark. In other words, it inverts the default behaviour.
    
           When a parenthesized group is quantified with a minimum repeat
           count that is greater than 1 or with a limited maximum, more
           memory is required for the compiled pattern, in proportion to the
           size of the minimum or maximum.
    
           If a pattern starts with .* or .{0,} and the PCRE2_DOTALL option
           (equivalent to Perl's /s) is set, thus allowing the dot to match
           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
           example:
    
             (.*)abc\1
    
           If the subject is "xyz123abc123" the match point is the fourth
           character. For this reason, such a pattern is not implicitly
           anchored.
    
           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:
    
             (?>.*?a)b
    
           It matches "ab" in the subject "aab". The use of the backtracking
           control verbs (*PRUNE) and (*SKIP) also disable this optimization.
           To do so explicitly, either pass the compile option
           PCRE2_NO_DOTSTAR_ANCHOR, or call **pcre2_set_optimize()** with a
           PCRE2_DOTSTAR_ANCHOR_OFF directive.
    
           When a capture group is repeated, the value captured is the
           substring that matched the final iteration. For example, after
    
             (tweedle[dume]{3}\s*)+
    
           has matched "tweedledum tweedledee" the value of the captured
           substring is "tweedledee". However, if there are nested capture
           groups, the corresponding captured values may have been set in
           previous iterations. For example, after
    
             (a|(b))+
    
           matches "aba" the value of the second captured substring is "b".
    


## ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS



    
           With both maximizing ("greedy") and minimizing ("ungreedy" or
           "lazy") repetition, failure of what follows normally causes the
           repeated item to be re-evaluated to see if a different number of
           repeats allows the rest of the pattern to match. Sometimes it is
           useful to prevent this, either to change the nature of the match,
           or to cause it fail earlier than it otherwise might, when the
           author of the pattern knows there is no point in carrying on.
    
           Consider, for example, the pattern \d+foo when applied to the
           subject line
    
             123456bar
    
           After matching all 6 digits and then failing to match "foo", the
           normal action of the matcher is to try again with only 5 digits
           matching the \d+ item, and then with 4, and so on, before
           ultimately failing. "Atomic grouping" (a term taken from Jeffrey
           Friedl's book) provides the means for specifying that once a group
           has matched, it is not to be re-evaluated in this way.
    
           If we use atomic grouping for the previous example, the matcher
           gives up immediately on failing to match "foo" the first time. The
           notation is a kind of special parenthesis, starting with (?> as in
           this example:
    
             (?>\d+)foo
    
           Perl 5.28 introduced an experimental alphabetic form starting with
           (* which may be easier to remember:
    
             (*atomic:\d+)foo
    
           This kind of parenthesized group "locks up" the part of the
           pattern it contains once it has matched, and a failure further
           into the pattern is prevented from backtracking into it.
           Backtracking past it to previous items, however, works as normal.
    
           An alternative description is that a group of this type matches
           exactly the string of characters that an identical standalone
           pattern would match, if anchored at the current point in the
           subject string.
    
           Atomic groups are not capture groups. Simple cases such as the
           above example can be thought of as a maximizing repeat that must
           swallow everything it can.  So, while both \d+ and \d+? are
           prepared to adjust the number of digits they match in order to
           make the rest of the 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
    
             \d++foo
    
           Note that a possessive quantifier can be used with an entire
           group, for example:
    
             (abc|xyz){2,3}+
    
           Possessive quantifiers are always greedy; the setting of the
           PCRE2_UNGREEDY option is ignored. They are a convenient notation
           for the simpler forms of atomic group. However, there is no
           difference in the meaning of a possessive quantifier and the
           equivalent atomic group, though there may be a performance
           difference; possessive quantifiers should be slightly faster.
    
           The possessive quantifier syntax is an extension to the Perl 5.8
           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_AUTO_POSSESS option, by calling
           **pcre2_set_optimize()** with a PCRE2_AUTO_POSSESS_OFF directive, or
           by starting the pattern with (*NO_AUTO_POSSESS).
    
           When a pattern contains an unlimited repeat inside a group that
           can itself be repeated an unlimited number of times, the use of an
           atomic group is the only way to avoid some failing matches taking
           a very long time indeed. The pattern
    
             (\D+|<\d+>)*[!?]
    
           matches an unlimited number of substrings that either consist of
           non-digits, or digits enclosed in <>, followed by either ! or ?.
           When it matches, it runs quickly. However, if it is applied to
    
             aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
    
           it takes a long time before reporting failure. This is because the
           string can be divided between the internal \D+ repeat and the
           external * repeat in a large number of ways, and all have to be
           tried. (The example uses [!?] rather than a single character at
           the end, because both PCRE2 and Perl have an optimization that
           allows for fast failure when a single character is used. They
           remember the last single 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:
    
             ((?>\D+)|<\d+>)*[!?]
    
           sequences of non-digits cannot be broken, and failure happens
           quickly.
    


## BACKREFERENCES



    
           Outside a character class, a backslash followed by a digit greater
           than 0 (and possibly further digits) is a backreference to a
           capture group earlier (that is, to its left) in the pattern,
           provided there have been that many previous capture groups.
    
           However, if the decimal number following the backslash is less
           than 8, it is always taken as a backreference, and causes an error
           only if there are not that many capture groups in the entire
           pattern. In other words, the group that is referenced need not be
           to the left of the 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:
    
             (abc(def)ghi)\g{-1}
    
           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:
    
             (A)(\g{-2}B)
    
           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
           way.
    
           A backreference matches whatever actually most recently matched
           the capture group in the current subject string, rather than
           anything at all that matches the group (see "Groups as
           subroutines" below for a way of doing that). So the pattern
    
             (sens|respons)e and \1ibility
    
           matches "sense and sensibility" and "response and responsibility",
           but not "sense and responsibility". If caseful matching is in
           force at the time of the backreference, the case of letters is
           relevant. For example,
    
             ((?i)rah)\s+\1
    
           matches "rah rah" and "RAH RAH", but not "RAH rah", even though
           the original capture group is matched caselessly.
    
           There are several different ways of writing backreferences to
           named capture groups. The .NET syntax 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:
    
             (?<p1>(?i)rah)\s+\k<p1>
             (?'p1'(?i)rah)\s+\k{p1}
             (?P<p1>(?i)rah)\s+(?P=p1)
             (?<p1>(?i)rah)\s+\g{p1}
    
           A capture group that is referenced by name may appear in the
           pattern before or after the reference.
    
           There may be more than one backreference to the same group. If a
           group has not actually been used in a particular match,
           backreferences to it always fail by default. For example, the
           pattern
    
             (a|(bc))\2
    
           always fails if it starts to match "a" rather than "bc". However,
           if the PCRE2_MATCH_UNSET_BACKREF option is set at compile time, a
           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
    
             (a|b\1)+
    
           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



    
           An assertion is a test that does not consume any characters. The
           test must succeed for the match to continue. The simple assertions
           coded as \b, \B, \A, \G, \Z, \z, ^ and $ are described above.
    
           More complicated assertions are coded as parenthesized groups. If
           matching such a group succeeds, matching continues after it, but
           with the matching position in the subject string reset to what it
           was before the assertion was processed.
    
           A special kind of assertion, called a "scan substring" assertion,
           matches a subpattern against a previously captured substring. This
           is described in the section entitled "Scan substring assertions"
           below. It is a PCRE2 extension, not compatible with Perl.
    
           The other goup-based assertions are of 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).
    
           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 {3,4}.
    
       **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 synonyms:
    
             (*positive_lookahead:  or (*pla: is the same as (?=
             (*negative_lookahead:  or (*nla: is the same as (?!
             (*positive_lookbehind: or (*plb: is the same as (?<=
             (*negative_lookbehind: or (*nlb: is the same as (?<!
    
           For example, (*pla:foo) is the same assertion as (?=foo). In the
           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,
    
             \w+(?=;)
    
           matches a word followed by a semicolon, but does not include the
           semicolon in the match, and
    
             foo(?!bar)
    
           matches any occurrence of "foo" that is not followed by "bar".
           Note that the apparently similar pattern
    
             (?!foo)bar
    
           does not find an occurrence of "bar" that is preceded by something
           other than "foo"; it finds any occurrence of "bar" whatsoever,
           because the assertion (?!foo) is always true when the next three
           characters are "bar". A lookbehind assertion is needed to achieve
           the other effect.
    
           If you want to force a matching failure at some point in a
           pattern, the most convenient way to do it is with (?!) because an
           empty string always matches, so an assertion that requires there
           not to be an empty string must always fail.  The backtracking
           control verb (*FAIL) or (*F) is a synonym for (?!).
    
       **Lookbehind assertions**
    
           Lookbehind assertions start with (?<= for positive assertions and
           (?<! for negative assertions. For example,
    
             (?<!foo)bar
    
           does find an occurrence of "bar" that is not preceded by "foo".
           The contents of a lookbehind assertion are restricted such that
           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 example
    
             (?<=colour|color)
    
           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
    
             (?<=colou?r)
    
           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 supported.
    
           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:
    
              \b(\w)\w++(?<=\1)
    
           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
    
             abcd$
    
           when applied to a long string that does not match. Because
           matching proceeds from left to right, PCRE2 will look for each "a"
           in the subject and then see if what follows matches the rest of
           the pattern. If the pattern is specified as
    
             ^.*abcd$
    
           the initial .* matches the entire string at first, but when this
           fails (because there is no following "a"), it backtracks to match
           all but the last character, then all but the last two characters,
           and so on. Once again the search for "a" covers the entire string,
           from right to left, so we are no better off. However, if the
           pattern is written as
    
             ^.*+(?<=abcd)
    
           there can be no backtracking for the .*+ item because of the
           possessive quantifier; it can match only the entire string. The
           subsequent lookbehind assertion does a single test on the last
           four characters. If it fails, the match fails immediately. For
           long strings, this approach makes a significant difference to the
           processing time.
    
       **Using multiple assertions**
    
           Several assertions (of any sort) may occur in succession. For
           example,
    
             (?<=\d{3})(?<!999)foo
    
           matches "foo" preceded by three digits that are not "999". Notice
           that each of the assertions is applied independently at the same
           point in the subject string. First there is a check that the
           previous three characters are all digits, and then there is a
           check that the same three characters are not "999".  This pattern
           does _not_ match "foo" 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
    
             (?<=\d{3}...)(?<!999)foo
    
           This time the first assertion looks at the preceding six
           characters, checking that the first three are digits, and then the
           second assertion checks that the preceding three characters are
           not "999".
    
           Assertions can be nested in any combination. For example,
    
             (?<=(?<!foo)bar)baz
    
           matches an occurrence of "baz" that is preceded by "bar" which in
           turn is not preceded by "foo", while
    
             (?<=\d{3}(?!999)...)foo
    
           is another pattern that matches "foo" preceded by three digits and
           any three characters that are not "999".
    


## NON-ATOMIC ASSERTIONS



    
           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 lookahead (?= 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
           atomic.
    


## SCAN SUBSTRING ASSERTIONS



    
           A special kind of assertion, not compatible with Perl, makes it
           possible to check the contents of a captured substring by matching
           it with a subpattern.  Because this involves capturing, this
           feature is not supported by **pcre2_dfa_match()**.
    
           A scan substring assertion starts with the sequence
           (*scan_substring: or (*scs: which is followed by a list of
           substring numbers (absolute or relative) and/or substring names
           enclosed in single quotes or angle brackets, all within
           parentheses. The rest of the item is the subpattern that is
           applied to the substring, as shown in these examples:
    
             (*scan_substring:(1)...)
             (*scs:(-2)...)
             (*scs:('AB')...)
             (*scs:(1,'AB',-2)...)
    
           The list of groups is checked in the order they are given, and it
           is the contents of the first one that is found to be set that are
           scanned. When PCRE2_DUPNAMES is set and there are ambiguous group
           names, all groups with the same name are checked in numerical
           order. A scan substring assertion fails if none of the groups it
           references have been set.
    
           The pattern match on the substring is always anchored, that is, it
           must match from the start of the substring. There is no
           "bumpalong" if it does not match at the start. The end of the
           subject is temporarily reset to be the end of the substring, so
           \Z, \z, and $ will match there. However, the start of the subject
           is _not_ reset. This means that ^ matches only if the substring is
           actually at the start of the main subject, but it also means that
           lookbehind assertions into what precedes the substring are
           possible.
    
           Here is a very simple example: find a word that contains the rare
           (in English) sequence of letters "rh" not at the start:
    
             \b(\w++)(*scs:(1).+rh)
    
           The first group captures a word which is then scanned by the
           second group.  This example does not actually need this
           heavyweight feature; the same match can be achieved with:
    
             \b\w+?rh\w*\b
    
           When things are more complicated, however, scanning a captured
           substring can be a useful way to describe the required match. For
           exmple, there is a rather complicated pattern in the PCRE2 test
           data that checks an entire subject string for a palindrome, that
           is, the sequence of letters is the same in both directions.
           Suppose you want to search for individual words of two or more
           characters such as "level" that are palindromes:
    
             (\b\w{2,}+\b)(*scs:(1)...palindrome-matching-pattern...)
    
           Within a substring scanning subpattern, references to other groups
           work as normal. Capturing groups may appear, and will retain their
           values during ongoing matching if the assertion succeeds.
    


## SCRIPT RUNS



    
           In concept, a script run is a sequence of characters that are all
           from the same Unicode script such as Latin or Greek. However,
           because some scripts are commonly used together, and because some
           diacritical and other marks are used with multiple scripts, it is
           not that simple. There is a full description of the rules that
           PCRE2 uses in the section entitled "Script Runs" in the
           **pcre2unicode** documentation.
    
           If part of a pattern is enclosed between (*script_run: or (*sr:
           and a closing parenthesis, it fails if the sequence of characters
           that it matches are not a script run. After a failure, normal
           backtracking occurs. Script runs can be used to detect spoofing
           attacks using 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:
    
             \s+(*sr:\S+)
    
           To be sure that they are all from the Latin script (for example),
           a lookahead can be used:
    
             \s+(?=\p{Latin})(*sr:\S+)
    
           This works as long as the first character is expected to be a
           character in that script, and not (for example) punctuation, which
           is allowed with any script. If this is not the case, a more
           creative lookahead is needed. For example, if digits, underscore,
           and dots are permitted at the start:
    
             \s+(?=[0-9_.]*\p{Latin})(*sr:\S+)
    
           In many cases, backtracking into a script run pattern fragment is
           not desirable. The script run can employ an atomic group to
           prevent this. Because this is a common requirement, a shorthand
           notation is provided by (*atomic_script_run: or (*asr:
    
             (*asr:...) is the same as (*sr:(?>...))
    
           Note that the atomic group is inside the script run. Putting it
           outside would not prevent backtracking into the script run
           pattern.
    
           Support for script runs is not available if PCRE2 is compiled
           without Unicode support. A compile-time error is given if any of
           the above 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.
    


## CONDITIONAL GROUPS



    
           It is possible to cause the matching process to obey a pattern
           fragment conditionally or to choose between two alternative
           fragments, depending on the result of an assertion, or whether a
           specific capture group has already been matched. The two possible
           forms of conditional group are:
    
             (?(condition)yes-pattern)
             (?(condition)yes-pattern|no-pattern)
    
           If the condition is satisfied, the yes-pattern is used; otherwise
           the no-pattern (if present) is used. An absent no-pattern is
           equivalent to an empty string (it always matches). If there are
           more than two 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 error.
    
           Consider the following pattern, which contains non-significant
           white space to make it more readable (assume the PCRE2_EXTENDED
           option) and to divide it into three parts for ease of discussion:
    
             ( \( )?    [^()]+    (?(1) \) )
    
           The first part matches an optional opening parenthesis, and if
           that character is present, sets it as the first captured
           substring. The 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:
    
             ((?(R1)a+|(?1)b))
    
           However, in both cases, if there is a capture group with a
           matching name, the condition tests for its being set, as described
           in the 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 example:
    
             (?(R&name)...)
    
           the condition is true if the most recent recursion is into a group
           of that name (which must exist within the pattern).
    
           This condition does not check the entire recursion stack. It tests
           only the current level. If the name used in a condition of this
           kind is a duplicate, the test is applied to all groups of the same
           name, and is true if any one of them is the most recent recursion.
    
           At "top level", all these recursion test conditions are false.
    
       **Defining capture groups for use by reference only**
    
           If the condition is the string (DEFINE), the condition is always
           false, even if there is a group with the name DEFINE. In this
           case, there may be only one alternative in the rest of the
           conditional group. It is always skipped if control reaches this
           point in the pattern; the idea of DEFINE is that it can be used to
           define subroutines that can be referenced from elsewhere. (The use
           of subroutines is described below.) For example, a pattern to
           match an IPv4 address such as "192.168.23.245" could be written
           like this (ignore white space and line breaks):
    
             (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
             \b (?&byte) (\.(?&byte)){3} \b
    
           The first part of the pattern is a DEFINE group inside which
           another group named "byte" is defined. This matches an individual
           component of an IPv4 address (a number less than 256). When
           matching takes place, this part of the pattern is skipped because
           DEFINE acts like a false condition. The rest of the pattern uses
           references to the named group to match the four dot-separated
           components of an IPv4 address, 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:
    
             (?(VERSION>=10.4)yes|no)
    
           This pattern matches "yes" if the PCRE2 version is greater or
           equal to 10.4, or "no" otherwise. The fractional part of the
           version number may not contain more than two digits.
    
       **Assertion conditions**
    
           If the condition is not in any of the above formats, it must be a
           parenthesized assertion. This may be a positive or negative
           lookahead or lookbehind assertion. However, it must be a
           traditional atomic assertion, not one of the non-atomic
           assertions.
    
           Consider this pattern, again containing non-significant white
           space, and with the two alternatives on the second line:
    
             (?(?=[^a-z]*[a-z])
             \d{2}-[a-z]{3}-\d{2}  |  \d{2}-\d{2}-\d{2} )
    
           The condition is a positive lookahead assertion that matches an
           optional sequence of non-letters followed by a letter. In other
           words, it tests for the presence of at least one letter in the
           subject. If a letter is found, the subject is matched against the
           first alternative; otherwise it is matched against the second.
           This pattern matches strings in one of the two forms dd-aaa-dd or
           dd-dd-dd, where aaa are letters and dd are digits.
    
           When an assertion that is a condition contains capture groups, any
           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



    
           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 or a Unicode property name. 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.
    


## RECURSIVE PATTERNS



    
           Consider the problem of matching a string in parentheses, allowing
           for unlimited nested parentheses. Without the use of recursion,
           the best that can be done is to use a pattern that matches up to
           some fixed depth of nesting. It is not possible to handle an
           arbitrary nesting depth.
    
           For some time, Perl has provided a facility that allows regular
           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
    
             (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
    
           it yields "no match" quickly. However, if a possessive quantifier
           is not used, the match runs for a very long time indeed because
           there are so many different ways the + and * repeats can carve up
           the subject, and all have to be tested before failure can be
           reported.
    
           At the end of a match, the values of capturing parentheses are
           those from the outermost level. If you want to obtain intermediate
           values, a callout function can be used (see below and the
           **pcre2callout** documentation). If the pattern above is matched
           against
    
             (ab(cd)ef)
    
           the value for the inner capturing parentheses (numbered 2) is
           "ef", which is the last value taken on at the top level. If a
           capture group is not matched at the top level, its final captured
           value is unset, even if it was (temporarily) set at a deeper level
           during the matching process.
    
           Do not confuse the (?R) item with the condition (R), which tests
           for recursion.  Consider this pattern, which matches text in angle
           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:
    
             ^((.)(?1)\2|.?)$
    
           The second branch in the group matches a single central character
           in the palindrome when there are an odd number of characters, or
           nothing when there are an even number of characters, but in order
           to work it has to be able to try the second case when the rest of
           the pattern match fails. If you want to match typical palindromic
           phrases, the pattern has to ignore all non-word characters, which
           can be done like this:
    
             ^\W*+((.)\W*+(?1)\W*+\2|\W*+.?)\W*+$
    
           If run with the PCRE2_CASELESS option, this pattern matches
           phrases such as "A man, a plan, a canal: Panama!". Note the use of
           the 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:
    
             ^(.)(\1|a(?2))
    
           This pattern matches "bab". The first capturing parentheses match
           "b", then in the second group, when the backreference \1 fails to
           match "b", the second alternative matches "a" and then recurses.
           In the recursion, \1 does now match "b" and so the whole match
           succeeds. This match used to fail in Perl, but in later versions
           (I tried 5.024) it now works.
    
       **Groups as subroutines**
    
           If the syntax for a recursive group call (either by number or by
           name) is used outside the parentheses to which it refers, it
           operates a bit like a subroutine in a programming language. More
           accurately, PCRE2 treats the referenced group as an independent
           subpattern which it tries to match at the current matching
           position. The called group may be defined before or after the
           reference. A numbered reference can be absolute or relative, as in
           these examples:
    
             (...(absolute)...)...(?2)...
             (...(relative)...)...(?-1)...
             (...(?+1)...(relative)...
    
           An earlier example pointed out that the pattern
    
             (sens|respons)e and \1ibility
    
           matches "sense and sensibility" and "response and responsibility",
           but not "sense and responsibility". If instead the pattern
    
             (sens|respons)e and (?1)ibility
    
           is used, it does match "sense and responsibility" as well as the
           other two strings. Another example is given in the discussion of
           DEFINE above.
    
           Like recursions, subroutine calls used to be treated as atomic,
           but this changed at PCRE2 release 10.30, so backtracking into
           subroutine calls can now occur. However, any capturing parentheses
           that are set during the subroutine call revert to their previous
           values afterwards.
    
           Processing options such as case-independence are fixed when a
           group is defined, so if it is used as a subroutine, such options
           cannot be changed for different calls. For example, consider this
           pattern:
    
             (abc)(?i:(?-1))
    
           It matches "abcabc". It does not match "abcABC" because the change
           of processing option does not affect the called group.
    
           The behaviour of backtracking control verbs in groups when called
           as subroutines is described in the section entitled "Backtracking
           verbs in subroutines" below.
    
       **Recursion and subroutines with returned capture groups**
    
           Since PCRE2 10.46, recursion and subroutine calls may also specify
           a list of capture groups to return. This is a PCRE2 syntax
           extension not supported by Perl. The pattern matching recurses
           into the referenced expression as described above, however, when
           the recursion returns to the calling expression the subgroups
           captured during the recursion can be retained when the calling
           expression's context is restored.
    
           When used as a subroutine, this allows the subroutine's capture
           groups to be used as return values.
    
           Only the specific capture groups listed by the caller will be
           retained, using the following syntax:
    
             (?R(grouplist))       recurse whole pattern, returning capture
           groups
             (?n(grouplist))       )
             (?+n(grouplist))      )
             (?-n(grouplist))      ) call subroutine, returning capture
           groups
             (?&name(grouplist))   )
             (?P>name(grouplist))  )
    
           The list of capture groups "grouplist" is a comma-separated list
           of (absolute or relative) group numbers, and group names enclosed
           in single quotes or angle brackets.
    
           Here is an example which first uses the DEFINE condition to create
           a re-usable routine for matching a weekday, then calls that
           subroutine and retains the groups it captures for use later:
    
             (?x: # ignore whitespace for clarity
               # Define the routine "weekendday" which matches Saturday or
               # Sunday, and returns the Sat/Sun prefix as \k<short>.
               (?(DEFINE) (?<weekendday>
                   (?|(?<short>Sat)urday|(?<short>Sun)day) ) )
               # Call the routine. Matches "Saturday,Sat" or "Sunday,Sun".
               (?&weekendday(<short>)),\k<short> )
    
           This feature is not available using the Oniguruma syntax \g<...>
           or \g'...'  below.
    
       **Oniguruma subroutine syntax**
    
           For compatibility with Oniguruma, the non-Perl syntax \g followed
           by a name or a number enclosed either in angle brackets or single
           quotes, is an alternative syntax for calling a group as a
           subroutine, possibly recursively. Here are two of the examples
           used above, rewritten using this syntax:
    
             (?<pn> \( ( (?>[^()]+) | \g<pn> )* \) )
             (sens|respons)e and \g'1'ibility
    
           PCRE2 supports an extension to Oniguruma: if a number is preceded
           by a plus or a minus sign it is taken as a relative reference. For
           example:
    
             (abc)(?i:\g<-1>)
    
           Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are
           _not_ synonymous. The former is a backreference; the latter is a
           subroutine call.
    


## CALLOUTS



    
           Perl has a feature whereby using the sequence (?{...}) causes
           arbitrary Perl code to be obeyed in the middle of matching a
           regular expression. This makes it possible, amongst other things,
           to extract different 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, callout points will be passed over silently during
           matching. To disallow callouts in the pattern syntax, you may use
           the PCRE2_EXTRA_NEVER_CALLOUT option.
    
           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** documentation.
    
       **Callouts with numerical arguments**
    
           If you just want to have a means of identifying different callout
           points, put a number less than 256 after the letter C. For
           example, this pattern has two callout points:
    
             (?C1)abc(?C2)def
    
           If the PCRE2_AUTO_CALLOUT flag is passed to **pcre2_compile()** ,
           numerical callouts are automatically installed before each item in
           the pattern. They are all numbered 255. If there is a conditional
           group in the 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
           example:
    
             (?(?C9)(?=a)abc|def)
    
           Note that this applies only to assertion conditions, not to other
           types of condition.
    
       **Callouts with string arguments**
    
           A delimited string may be used instead of a number as a callout
           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.
    


## BACKTRACKING CONTROL



    
           There are a number of special "Backtracking Control Verbs" (to use
           Perl's terminology) that modify the behaviour of backtracking
           during matching. They are generally of the form (*VERB) or
           (*VERB:NAME). Some verbs take either form, and may behave
           differently depending on whether or not a name argument is
           present. The names are not required to be unique within the
           pattern.
    
           By default, for compatibility with Perl, a name is any sequence of
           characters that does not include a closing parenthesis. The name
           is not processed in any way, and it is not possible to include a
           closing parenthesis in the name.  This can be changed by setting
           the PCRE2_ALT_VERBNAMES option, but the result is no longer Perl-
           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 faulted.
    
           A closing parenthesis can be included in a name either as \) or
           between \Q and \E. In addition to backslash processing, if the
           PCRE2_EXTENDED or PCRE2_EXTENDED_MORE option is also set,
           unescaped white space 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 or JIT, because they use
           backtracking algorithms. With the exception of (*FAIL), which
           behaves like a failing negative assertion, the backtracking
           control verbs cause an error if encountered by the DFA matching
           function.
    
           The behaviour of these verbs in repeated groups, assertions, and
           in capture groups called as subroutines (whether or not
           recursively) is documented below.
    
       **Optimizations that affect backtracking verbs**
    
           PCRE2 contains some optimizations that are used to speed up
           matching by running some checks at the start of each match
           attempt. For example, it may know the minimum length of matching
           subject, or that a particular character must be present. When one
           of these optimizations bypasses the running of a match, any
           included backtracking verbs will not, of course, be processed. You
           can suppress the start-of-match optimizations by setting the
           PCRE2_NO_START_OPTIMIZE option when calling **pcre2_compile()** , by
           calling **pcre2_set_optimize()** with a PCRE2_START_OPTIMIZE_OFF
           directive, 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:
    
             A((?:A|B(*ACCEPT)|C)D)
    
           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,
    
             (A(*ACCEPT)??B)C
    
           where A, B, and C may be complex expressions. After matching "A",
           the matcher processes "BC"; if that fails, causing a backtrack,
           (*ACCEPT) is triggered and the match succeeds. In both cases, all
           but C is captured. Whereas (*COMMIT) (see below) means "fail on
           backtrack", a repeated (*ACCEPT) of this type means "succeed on
           backtrack".
    
           **Warning:**(*ACCEPT) should not be used within a script run group,
           because it causes an immediate exit from the group, bypassing the
           script run checking.
    
             (*FAIL) or (*FAIL:NAME)
    
           This verb causes a matching failure, forcing backtracking to
           occur. It may be abbreviated to (*F). It is equivalent to (?!) but
           easier to read. The Perl documentation notes that it is probably
           useful only when combined with (?{}) or (??{}). Those are, of
           course, Perl features that are not present in PCRE2. The nearest
           equivalent is the callout feature, as for example in this pattern:
    
             a+(?C)(*FAIL)
    
           A match with the string "aaaa" always fails, but the callout is
           taken before each backtrack happens (in this example, 10 times).
    
           (*ACCEPT:NAME) and (*FAIL:NAME) behave the same as
           (*MARK:NAME)(*ACCEPT) and (*MARK:NAME)(*FAIL), respectively, that
           is, a (*MARK) is recorded just before the verb acts.
    
       **Recording which path was taken**
    
           There is one verb whose main purpose is to track how a match was
           arrived at, though it also has a secondary use in conjunction with
           advancing the match starting point (see (*SKIP) below).
    
             (*MARK:NAME) or (*:NAME)
    
           A name is always required with this verb. For all the other
           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
             XZ
              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 either set the PCRE2_NO_START_OPTIMIZE option or
           call **pcre2_set_optimize()** with a PCRE2_START_OPTIMIZE_OFF
           directive (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 an atomic 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 atomic assertion or 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:
    
             a+(*COMMIT)b
    
           This matches "xxaab" but not "aacaab". It can be thought of as a
           kind of dynamic anchor, or "I've started, so I must finish."
    
           The behaviour of (*COMMIT:NAME) is not the same as
           (*MARK:NAME)(*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
             data>
             re> /(*COMMIT)abc/no_start_optimize
             data> xyzabc
             No match
    
           For the first pattern, PCRE2 knows that any match must start with
           "a", so the optimization skips along the subject to "a" before
           applying the pattern to the first set of data. The match attempt
           then succeeds. The second pattern disables the optimization that
           skips along to the first character. The pattern is now applied
           starting at "x", and so the (*COMMIT) causes the match to fail
           without trying any other starting points.
    
             (*PRUNE) or (*PRUNE:NAME)
    
           This verb causes the match to fail at the current starting
           position in the subject if there is a later matching failure that
           causes 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.
    
             (*SKIP)
    
           This verb, when given without a name, is like (*PRUNE), except
           that if the pattern is unanchored, the "bumpalong" advance is not
           to the next character, but to the position in the subject where
           (*SKIP) was 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:
    
             a+(*SKIP)b
    
           If the subject is "aaaac...", after the first match attempt fails
           (starting at the first character in the string), the starting
           point skips on to start the next attempt at "c". Note that a
           possessive 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.
    
             (*SKIP:NAME)
    
           When (*SKIP) has an associated name, its behaviour is modified.
           When such a (*SKIP) is triggered, the previous path through the
           pattern is searched for the most recent (*MARK) that has the same
           name. If one is found, the "bumpalong" advance is to the subject
           position that 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** examples:
    
               re> /a(?>(*MARK:X))(*SKIP:X)(*F)|(.)/
             data: abc
              0: a
              1: a
             data:
               re> /a(?:(*MARK:X))(*SKIP:X)(*F)|(.)/
             data: abc
              0: b
              1: b
    
           In the first example, the (*MARK) setting is in an atomic group,
           so it is not seen when (*SKIP:X) triggers, causing the (*SKIP) to
           be ignored. This allows the second branch of the pattern to be
           tried at the first character position.  In the second example, the
           (*MARK) setting is not in an atomic group. This allows (*SKIP:X)
           to find the (*MARK) when it backtracks, and this causes a new
           matching attempt to start at the 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 pattern.
    
           Note that (*SKIP:NAME) searches only for names set by
           (*MARK:NAME). It ignores names that are set by other backtracking
           verbs.
    
             (*THEN) or (*THEN:NAME)
    
           This verb causes a skip to the next innermost alternative when
           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:
    
             (A(*COMMIT)B(*THEN)C|ABD)
    
           If A matches but B fails, the backtrack to (*COMMIT) causes the
           entire match to fail. However, if A and B match, but C fails, the
           backtrack to (*THEN) causes the next alternative (ABD) to be
           tried. This behaviour is consistent, but is not always the same as
           Perl's. It means that if two or more backtracking verbs appear in
           succession, all but the last of them has no effect. Consider this
           example:
    
             ...(*COMMIT)(*PRUNE)...
    
           If there is a matching failure to the right, backtracking onto
           (*PRUNE) causes it to be triggered, and its action is taken. There
           can never be a backtrack onto (*COMMIT).
    
       **Backtracking verbs in repeated groups**
    
           PCRE2 sometimes differs from Perl in its handling of backtracking
           verbs in repeated groups. For example, consider:
    
             /(a(*COMMIT)b)+ac/
    
           If the subject is "abac", Perl matches unless its optimizations
           are disabled, but PCRE2 always fails because the (*COMMIT) in the
           second repeat of the group acts.
    
       **Backtracking verbs in assertions**
    
           (*FAIL) in any assertion has its normal effect: it forces an
           immediate backtrack. The behaviour of the other backtracking verbs
           depends on whether or not the assertion is standalone or acting as
           the condition in a conditional group.
    
           (*ACCEPT) in a standalone positive assertion causes the assertion
           to succeed without any further processing; captured strings and a
           mark name (if set) are retained. In a standalone negative
           assertion, (*ACCEPT) causes the assertion to fail without any
           further processing; captured substrings and any mark name are
           discarded.
    
           If the assertion is a condition, (*ACCEPT) causes the condition to
           be true for a positive assertion and false for a negative one;
           captured substrings are retained in both cases.
    
           The remaining verbs act only when a later failure causes a
           backtrack to reach them. This means that, for the Perl-compatible
           assertions, their effect is confined to the assertion, because
           Perl lookaround assertions are atomic. A backtrack that occurs
           after such an assertion is complete does not jump back into the
           assertion. Note in particular that a (*MARK) name that is set in
           an assertion is not "seen" by an instance of (*SKIP:NAME) later in
           the pattern.
    
           PCRE2 now supports non-atomic positive assertions and also "scan
           substring" assertions, as described in the sections entitled "Non-
           atomic assertions" and "Scan substring 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
           pattern.
    
           The effect of (*THEN) is not allowed to escape beyond an
           assertion. If there are no more branches to try, (*THEN) causes a
           positive assertion to be false, and a negative assertion to be
           true. This behaviour differs from Perl when the assertion has only
           one branch.
    
           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
           branches.
    
       **Backtracking verbs in subroutines**
    
           These behaviours occur whether or not the group is called
           recursively.
    
           (*ACCEPT) in a group called as a subroutine causes the subroutine
           match to succeed without any further processing. Matching then
           continues after the subroutine call. Perl documents this
           behaviour. Perl's 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.
    


## EBCDIC ENVIRONMENTS



    
           Differences in the way PCRE behaves when it is running in an
           EBCDIC environment are covered in this section.
    
       **Escape sequences**
    
           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 or Unicode 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.
    
       **Character classes**
    
           In character classes 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 EBCDIC 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.
    


## REVISION



    
           Last updated: 27 November 2024
           Copyright (c) 1997-2024 University of Cambridge.
    


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