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    1 <html>
    2 <head>
    3 <title>pcrepattern specification</title>
    4 </head>
    5 <body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
    6 <h1>pcrepattern man page</h1>
    7 <p>
    8 Return to the <a href="index.html">PCRE index page</a>.
    9 </p>
   10 <p>
   11 This page is part of the PCRE HTML documentation. It was generated automatically
   12 from the original man page. If there is any nonsense in it, please consult the
   13 man page, in case the conversion went wrong.
   14 <br>
   15 <ul>
   16 <li><a name="TOC1" href="#SEC1">PCRE REGULAR EXPRESSION DETAILS</a>
   17 <li><a name="TOC2" href="#SEC2">SPECIAL START-OF-PATTERN ITEMS</a>
   18 <li><a name="TOC3" href="#SEC3">EBCDIC CHARACTER CODES</a>
   19 <li><a name="TOC4" href="#SEC4">CHARACTERS AND METACHARACTERS</a>
   20 <li><a name="TOC5" href="#SEC5">BACKSLASH</a>
   21 <li><a name="TOC6" href="#SEC6">CIRCUMFLEX AND DOLLAR</a>
   22 <li><a name="TOC7" href="#SEC7">FULL STOP (PERIOD, DOT) AND \N</a>
   23 <li><a name="TOC8" href="#SEC8">MATCHING A SINGLE DATA UNIT</a>
   24 <li><a name="TOC9" href="#SEC9">SQUARE BRACKETS AND CHARACTER CLASSES</a>
   25 <li><a name="TOC10" href="#SEC10">POSIX CHARACTER CLASSES</a>
   27 <li><a name="TOC12" href="#SEC12">VERTICAL BAR</a>
   28 <li><a name="TOC13" href="#SEC13">INTERNAL OPTION SETTING</a>
   29 <li><a name="TOC14" href="#SEC14">SUBPATTERNS</a>
   30 <li><a name="TOC15" href="#SEC15">DUPLICATE SUBPATTERN NUMBERS</a>
   31 <li><a name="TOC16" href="#SEC16">NAMED SUBPATTERNS</a>
   32 <li><a name="TOC17" href="#SEC17">REPETITION</a>
   34 <li><a name="TOC19" href="#SEC19">BACK REFERENCES</a>
   35 <li><a name="TOC20" href="#SEC20">ASSERTIONS</a>
   36 <li><a name="TOC21" href="#SEC21">CONDITIONAL SUBPATTERNS</a>
   37 <li><a name="TOC22" href="#SEC22">COMMENTS</a>
   38 <li><a name="TOC23" href="#SEC23">RECURSIVE PATTERNS</a>
   39 <li><a name="TOC24" href="#SEC24">SUBPATTERNS AS SUBROUTINES</a>
   40 <li><a name="TOC25" href="#SEC25">ONIGURUMA SUBROUTINE SYNTAX</a>
   41 <li><a name="TOC26" href="#SEC26">CALLOUTS</a>
   42 <li><a name="TOC27" href="#SEC27">BACKTRACKING CONTROL</a>
   43 <li><a name="TOC28" href="#SEC28">SEE ALSO</a>
   44 <li><a name="TOC29" href="#SEC29">AUTHOR</a>
   45 <li><a name="TOC30" href="#SEC30">REVISION</a>
   46 </ul>
   47 <br><a name="SEC1" href="#TOC1">PCRE REGULAR EXPRESSION DETAILS</a><br>
   48 <P>
   49 The syntax and semantics of the regular expressions that are supported by PCRE
   50 are described in detail below. There is a quick-reference syntax summary in the
   51 <a href="pcresyntax.html"><b>pcresyntax</b></a>
   52 page. PCRE tries to match Perl syntax and semantics as closely as it can. PCRE
   53 also supports some alternative regular expression syntax (which does not
   54 conflict with the Perl syntax) in order to provide some compatibility with
   55 regular expressions in Python, .NET, and Oniguruma.
   56 </P>
   57 <P>
   58 Perl's regular expressions are described in its own documentation, and
   59 regular expressions in general are covered in a number of books, some of which
   60 have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
   61 published by O'Reilly, covers regular expressions in great detail. This
   62 description of PCRE's regular expressions is intended as reference material.
   63 </P>
   64 <P>
   65 This document discusses the patterns that are supported by PCRE when one its
   66 main matching functions, <b>pcre_exec()</b> (8-bit) or <b>pcre[16|32]_exec()</b>
   67 (16- or 32-bit), is used. PCRE also has alternative matching functions,
   68 <b>pcre_dfa_exec()</b> and <b>pcre[16|32_dfa_exec()</b>, which match using a
   69 different algorithm that is not Perl-compatible. Some of the features discussed
   70 below are not available when DFA matching is used. The advantages and
   71 disadvantages of the alternative functions, and how they differ from the normal
   72 functions, are discussed in the
   73 <a href="pcrematching.html"><b>pcrematching</b></a>
   74 page.
   75 </P>
   76 <br><a name="SEC2" href="#TOC1">SPECIAL START-OF-PATTERN ITEMS</a><br>
   77 <P>
   78 A number of options that can be passed to <b>pcre_compile()</b> can also be set
   79 by special items at the start of a pattern. These are not Perl-compatible, but
   80 are provided to make these options accessible to pattern writers who are not
   81 able to change the program that processes the pattern. Any number of these
   82 items may appear, but they must all be together right at the start of the
   83 pattern string, and the letters must be in upper case.
   84 </P>
   85 <br><b>
   86 UTF support
   87 </b><br>
   88 <P>
   89 The original operation of PCRE was on strings of one-byte characters. However,
   90 there is now also support for UTF-8 strings in the original library, an
   91 extra library that supports 16-bit and UTF-16 character strings, and a
   92 third library that supports 32-bit and UTF-32 character strings. To use these
   93 features, PCRE must be built to include appropriate support. When using UTF
   94 strings you must either call the compiling function with the PCRE_UTF8,
   95 PCRE_UTF16, or PCRE_UTF32 option, or the pattern must start with one of
   96 these special sequences:
   97 <pre>
   98   (*UTF8)
   99   (*UTF16)
  100   (*UTF32)
  101   (*UTF)
  102 </pre>
  103 (*UTF) is a generic sequence that can be used with any of the libraries.
  104 Starting a pattern with such a sequence is equivalent to setting the relevant
  105 option. How setting a UTF mode affects pattern matching is mentioned in several
  106 places below. There is also a summary of features in the
  107 <a href="pcreunicode.html"><b>pcreunicode</b></a>
  108 page.
  109 </P>
  110 <P>
  111 Some applications that allow their users to supply patterns may wish to
  112 restrict them to non-UTF data for security reasons. If the PCRE_NEVER_UTF
  113 option is set at compile time, (*UTF) etc. are not allowed, and their
  114 appearance causes an error.
  115 </P>
  116 <br><b>
  117 Unicode property support
  118 </b><br>
  119 <P>
  120 Another special sequence that may appear at the start of a pattern is (*UCP).
  121 This has the same effect as setting the PCRE_UCP option: it causes sequences
  122 such as \d and \w to use Unicode properties to determine character types,
  123 instead of recognizing only characters with codes less than 128 via a lookup
  124 table.
  125 </P>
  126 <br><b>
  127 Disabling auto-possessification
  128 </b><br>
  129 <P>
  130 If a pattern starts with (*NO_AUTO_POSSESS), it has the same effect as setting
  131 the PCRE_NO_AUTO_POSSESS option at compile time. This stops PCRE from making
  132 quantifiers possessive when what follows cannot match the repeated item. For
  133 example, by default a+b is treated as a++b. For more details, see the
  134 <a href="pcreapi.html"><b>pcreapi</b></a>
  135 documentation.
  136 </P>
  137 <br><b>
  138 Disabling start-up optimizations
  139 </b><br>
  140 <P>
  141 If a pattern starts with (*NO_START_OPT), it has the same effect as setting the
  142 PCRE_NO_START_OPTIMIZE option either at compile or matching time. This disables
  143 several optimizations for quickly reaching "no match" results. For more
  144 details, see the
  145 <a href="pcreapi.html"><b>pcreapi</b></a>
  146 documentation.
  147 <a name="newlines"></a></P>
  148 <br><b>
  149 Newline conventions
  150 </b><br>
  151 <P>
  152 PCRE supports five different conventions for indicating line breaks in
  153 strings: a single CR (carriage return) character, a single LF (linefeed)
  154 character, the two-character sequence CRLF, any of the three preceding, or any
  155 Unicode newline sequence. The
  156 <a href="pcreapi.html"><b>pcreapi</b></a>
  157 page has
  158 <a href="pcreapi.html#newlines">further discussion</a>
  159 about newlines, and shows how to set the newline convention in the
  160 <i>options</i> arguments for the compiling and matching functions.
  161 </P>
  162 <P>
  163 It is also possible to specify a newline convention by starting a pattern
  164 string with one of the following five sequences:
  165 <pre>
  166   (*CR)        carriage return
  167   (*LF)        linefeed
  168   (*CRLF)      carriage return, followed by linefeed
  169   (*ANYCRLF)   any of the three above
  170   (*ANY)       all Unicode newline sequences
  171 </pre>
  172 These override the default and the options given to the compiling function. For
  173 example, on a Unix system where LF is the default newline sequence, the pattern
  174 <pre>
  175   (*CR)a.b
  176 </pre>
  177 changes the convention to CR. That pattern matches "a\nb" because LF is no
  178 longer a newline. If more than one of these settings is present, the last one
  179 is used.
  180 </P>
  181 <P>
  182 The newline convention affects where the circumflex and dollar assertions are
  183 true. It also affects the interpretation of the dot metacharacter when
  184 PCRE_DOTALL is not set, and the behaviour of \N. However, it does not affect
  185 what the \R escape sequence matches. By default, this is any Unicode newline
  186 sequence, for Perl compatibility. However, this can be changed; see the
  187 description of \R in the section entitled
  188 <a href="#newlineseq">"Newline sequences"</a>
  189 below. A change of \R setting can be combined with a change of newline
  190 convention.
  191 </P>
  192 <br><b>
  193 Setting match and recursion limits
  194 </b><br>
  195 <P>
  196 The caller of <b>pcre_exec()</b> can set a limit on the number of times the
  197 internal <b>match()</b> function is called and on the maximum depth of
  198 recursive calls. These facilities are provided to catch runaway matches that
  199 are provoked by patterns with huge matching trees (a typical example is a
  200 pattern with nested unlimited repeats) and to avoid running out of system stack
  201 by too much recursion. When one of these limits is reached, <b>pcre_exec()</b>
  202 gives an error return. The limits can also be set by items at the start of the
  203 pattern of the form
  204 <pre>
  205   (*LIMIT_MATCH=d)
  206   (*LIMIT_RECURSION=d)
  207 </pre>
  208 where d is any number of decimal digits. However, the value of the setting must
  209 be less than the value set (or defaulted) by the caller of <b>pcre_exec()</b>
  210 for it to have any effect. In other words, the pattern writer can lower the
  211 limits set by the programmer, but not raise them. If there is more than one
  212 setting of one of these limits, the lower value is used.
  213 </P>
  214 <br><a name="SEC3" href="#TOC1">EBCDIC CHARACTER CODES</a><br>
  215 <P>
  216 PCRE can be compiled to run in an environment that uses EBCDIC as its character
  217 code rather than ASCII or Unicode (typically a mainframe system). In the
  218 sections below, character code values are ASCII or Unicode; in an EBCDIC
  219 environment these characters may have different code values, and there are no
  220 code points greater than 255.
  221 </P>
  222 <br><a name="SEC4" href="#TOC1">CHARACTERS AND METACHARACTERS</a><br>
  223 <P>
  224 A regular expression is a pattern that is matched against a subject string from
  225 left to right. Most characters stand for themselves in a pattern, and match the
  226 corresponding characters in the subject. As a trivial example, the pattern
  227 <pre>
  228   The quick brown fox
  229 </pre>
  230 matches a portion of a subject string that is identical to itself. When
  231 caseless matching is specified (the PCRE_CASELESS option), letters are matched
  232 independently of case. In a UTF mode, PCRE always understands the concept of
  233 case for characters whose values are less than 128, so caseless matching is
  234 always possible. For characters with higher values, the concept of case is
  235 supported if PCRE is compiled with Unicode property support, but not otherwise.
  236 If you want to use caseless matching for characters 128 and above, you must
  237 ensure that PCRE is compiled with Unicode property support as well as with
  238 UTF support.
  239 </P>
  240 <P>
  241 The power of regular expressions comes from the ability to include alternatives
  242 and repetitions in the pattern. These are encoded in the pattern by the use of
  243 <i>metacharacters</i>, which do not stand for themselves but instead are
  244 interpreted in some special way.
  245 </P>
  246 <P>
  247 There are two different sets of metacharacters: those that are recognized
  248 anywhere in the pattern except within square brackets, and those that are
  249 recognized within square brackets. Outside square brackets, the metacharacters
  250 are as follows:
  251 <pre>
  252   \      general escape character with several uses
  253   ^      assert start of string (or line, in multiline mode)
  254   $      assert end of string (or line, in multiline mode)
  255   .      match any character except newline (by default)
  256   [      start character class definition
  257   |      start of alternative branch
  258   (      start subpattern
  259   )      end subpattern
  260   ?      extends the meaning of (
  261          also 0 or 1 quantifier
  262          also quantifier minimizer
  263   *      0 or more quantifier
  264   +      1 or more quantifier
  265          also "possessive quantifier"
  266   {      start min/max quantifier
  267 </pre>
  268 Part of a pattern that is in square brackets is called a "character class". In
  269 a character class the only metacharacters are:
  270 <pre>
  271   \      general escape character
  272   ^      negate the class, but only if the first character
  273   -      indicates character range
  274   [      POSIX character class (only if followed by POSIX syntax)
  275   ]      terminates the character class
  276 </pre>
  277 The following sections describe the use of each of the metacharacters.
  278 </P>
  279 <br><a name="SEC5" href="#TOC1">BACKSLASH</a><br>
  280 <P>
  281 The backslash character has several uses. Firstly, if it is followed by a
  282 character that is not a number or a letter, it takes away any special meaning
  283 that character may have. This use of backslash as an escape character applies
  284 both inside and outside character classes.
  285 </P>
  286 <P>
  287 For example, if you want to match a * character, you write \* in the pattern.
  288 This escaping action applies whether or not the following character would
  289 otherwise be interpreted as a metacharacter, so it is always safe to precede a
  290 non-alphanumeric with backslash to specify that it stands for itself. In
  291 particular, if you want to match a backslash, you write \\.
  292 </P>
  293 <P>
  294 In a UTF mode, only ASCII numbers and letters have any special meaning after a
  295 backslash. All other characters (in particular, those whose codepoints are
  296 greater than 127) are treated as literals.
  297 </P>
  298 <P>
  299 If a pattern is compiled with the PCRE_EXTENDED option, most white space in the
  300 pattern (other than in a character class), and characters between a # outside a
  301 character class and the next newline, inclusive, are ignored. An escaping
  302 backslash can be used to include a white space or # character as part of the
  303 pattern.
  304 </P>
  305 <P>
  306 If you want to remove the special meaning from a sequence of characters, you
  307 can do so by putting them between \Q and \E. This is different from Perl in
  308 that $ and @ are handled as literals in \Q...\E sequences in PCRE, whereas in
  309 Perl, $ and @ cause variable interpolation. Note the following examples:
  310 <pre>
  311   Pattern            PCRE matches   Perl matches
  313   \Qabc$xyz\E        abc$xyz        abc followed by the contents of $xyz
  314   \Qabc\$xyz\E       abc\$xyz       abc\$xyz
  315   \Qabc\E\$\Qxyz\E   abc$xyz        abc$xyz
  316 </pre>
  317 The \Q...\E sequence is recognized both inside and outside character classes.
  318 An isolated \E that is not preceded by \Q is ignored. If \Q is not followed
  319 by \E later in the pattern, the literal interpretation continues to the end of
  320 the pattern (that is, \E is assumed at the end). If the isolated \Q is inside
  321 a character class, this causes an error, because the character class is not
  322 terminated.
  323 <a name="digitsafterbackslash"></a></P>
  324 <br><b>
  325 Non-printing characters
  326 </b><br>
  327 <P>
  328 A second use of backslash provides a way of encoding non-printing characters
  329 in patterns in a visible manner. There is no restriction on the appearance of
  330 non-printing characters, apart from the binary zero that terminates a pattern,
  331 but when a pattern is being prepared by text editing, it is often easier to use
  332 one of the following escape sequences than the binary character it represents.
  333 In an ASCII or Unicode environment, these escapes are as follows:
  334 <pre>
  335   \a        alarm, that is, the BEL character (hex 07)
  336   \cx       "control-x", where x is any ASCII character
  337   \e        escape (hex 1B)
  338   \f        form feed (hex 0C)
  339   \n        linefeed (hex 0A)
  340   \r        carriage return (hex 0D)
  341   \t        tab (hex 09)
  342   \0dd      character with octal code 0dd
  343   \ddd      character with octal code ddd, or back reference
  344   \o{ddd..} character with octal code ddd..
  345   \xhh      character with hex code hh
  346   \x{hhh..} character with hex code hhh.. (non-JavaScript mode)
  347   \uhhhh    character with hex code hhhh (JavaScript mode only)
  348 </pre>
  349 The precise effect of \cx on ASCII characters is as follows: if x is a lower
  350 case letter, it is converted to upper case. Then bit 6 of the character (hex
  351 40) is inverted. Thus \cA to \cZ become hex 01 to hex 1A (A is 41, Z is 5A),
  352 but \c{ becomes hex 3B ({ is 7B), and \c; becomes hex 7B (; is 3B). If the
  353 data item (byte or 16-bit value) following \c has a value greater than 127, a
  354 compile-time error occurs. This locks out non-ASCII characters in all modes.
  355 </P>
  356 <P>
  357 When PCRE is compiled in EBCDIC mode, \a, \e, \f, \n, \r, and \t
  358 generate the appropriate EBCDIC code values. The \c escape is processed
  359 as specified for Perl in the <b>perlebcdic</b> document. The only characters
  360 that are allowed after \c are A-Z, a-z, or one of @, [, \, ], ^, _, or ?. Any
  361 other character provokes a compile-time error. The sequence \c@ encodes
  362 character code 0; after \c the letters (in either case) encode characters 1-26
  363 (hex 01 to hex 1A); [, \, ], ^, and _ encode characters 27-31 (hex 1B to hex
  364 1F), and \c? becomes either 255 (hex FF) or 95 (hex 5F).
  365 </P>
  366 <P>
  367 Thus, apart from \c?, these escapes generate the same character code values as
  368 they do in an ASCII environment, though the meanings of the values mostly
  369 differ. For example, \cG always generates code value 7, which is BEL in ASCII
  370 but DEL in EBCDIC.
  371 </P>
  372 <P>
  373 The sequence \c? generates DEL (127, hex 7F) in an ASCII environment, but
  374 because 127 is not a control character in EBCDIC, Perl makes it generate the
  375 APC character. Unfortunately, there are several variants of EBCDIC. In most of
  376 them the APC character has the value 255 (hex FF), but in the one Perl calls
  377 POSIX-BC its value is 95 (hex 5F). If certain other characters have POSIX-BC
  378 values, PCRE makes \c? generate 95; otherwise it generates 255.
  379 </P>
  380 <P>
  381 After \0 up to two further octal digits are read. If there are fewer than two
  382 digits, just those that are present are used. Thus the sequence \0\x\015
  383 specifies two binary zeros followed by a CR character (code value 13). Make
  384 sure you supply two digits after the initial zero if the pattern character that
  385 follows is itself an octal digit.
  386 </P>
  387 <P>
  388 The escape \o must be followed by a sequence of octal digits, enclosed in
  389 braces. An error occurs if this is not the case. This escape is a recent
  390 addition to Perl; it provides way of specifying character code points as octal
  391 numbers greater than 0777, and it also allows octal numbers and back references
  392 to be unambiguously specified.
  393 </P>
  394 <P>
  395 For greater clarity and unambiguity, it is best to avoid following \ by a
  396 digit greater than zero. Instead, use \o{} or \x{} to specify character
  397 numbers, and \g{} to specify back references. The following paragraphs
  398 describe the old, ambiguous syntax.
  399 </P>
  400 <P>
  401 The handling of a backslash followed by a digit other than 0 is complicated,
  402 and Perl has changed in recent releases, causing PCRE also to change. Outside a
  403 character class, PCRE reads the digit and any following digits as a decimal
  404 number. If the number is less than 8, or if there have been at least that many
  405 previous capturing left parentheses in the expression, the entire sequence is
  406 taken as a <i>back reference</i>. A description of how this works is given
  407 <a href="#backreferences">later,</a>
  408 following the discussion of
  409 <a href="#subpattern">parenthesized subpatterns.</a>
  410 </P>
  411 <P>
  412 Inside a character class, or if the decimal number following \ is greater than
  413 7 and there have not been that many capturing subpatterns, PCRE handles \8 and
  414 \9 as the literal characters "8" and "9", and otherwise re-reads up to three
  415 octal digits following the backslash, using them to generate a data character.
  416 Any subsequent digits stand for themselves. For example:
  417 <pre>
  418   \040   is another way of writing an ASCII space
  419   \40    is the same, provided there are fewer than 40 previous capturing subpatterns
  420   \7     is always a back reference
  421   \11    might be a back reference, or another way of writing a tab
  422   \011   is always a tab
  423   \0113  is a tab followed by the character "3"
  424   \113   might be a back reference, otherwise the character with octal code 113
  425   \377   might be a back reference, otherwise the value 255 (decimal)
  426   \81    is either a back reference, or the two characters "8" and "1"
  427 </pre>
  428 Note that octal values of 100 or greater that are specified using this syntax
  429 must not be introduced by a leading zero, because no more than three octal
  430 digits are ever read.
  431 </P>
  432 <P>
  433 By default, after \x that is not followed by {, from zero to two hexadecimal
  434 digits are read (letters can be in upper or lower case). Any number of
  435 hexadecimal digits may appear between \x{ and }. If a character other than
  436 a hexadecimal digit appears between \x{ and }, or if there is no terminating
  437 }, an error occurs.
  438 </P>
  439 <P>
  440 If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \x is
  441 as just described only when it is followed by two hexadecimal digits.
  442 Otherwise, it matches a literal "x" character. In JavaScript mode, support for
  443 code points greater than 256 is provided by \u, which must be followed by
  444 four hexadecimal digits; otherwise it matches a literal "u" character.
  445 </P>
  446 <P>
  447 Characters whose value is less than 256 can be defined by either of the two
  448 syntaxes for \x (or by \u in JavaScript mode). There is no difference in the
  449 way they are handled. For example, \xdc is exactly the same as \x{dc} (or
  450 \u00dc in JavaScript mode).
  451 </P>
  452 <br><b>
  453 Constraints on character values
  454 </b><br>
  455 <P>
  456 Characters that are specified using octal or hexadecimal numbers are
  457 limited to certain values, as follows:
  458 <pre>
  459   8-bit non-UTF mode    less than 0x100
  460   8-bit UTF-8 mode      less than 0x10ffff and a valid codepoint
  461   16-bit non-UTF mode   less than 0x10000
  462   16-bit UTF-16 mode    less than 0x10ffff and a valid codepoint
  463   32-bit non-UTF mode   less than 0x100000000
  464   32-bit UTF-32 mode    less than 0x10ffff and a valid codepoint
  465 </pre>
  466 Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-called
  467 "surrogate" codepoints), and 0xffef.
  468 </P>
  469 <br><b>
  470 Escape sequences in character classes
  471 </b><br>
  472 <P>
  473 All the sequences that define a single character value can be used both inside
  474 and outside character classes. In addition, inside a character class, \b is
  475 interpreted as the backspace character (hex 08).
  476 </P>
  477 <P>
  478 \N is not allowed in a character class. \B, \R, and \X are not special
  479 inside a character class. Like other unrecognized escape sequences, they are
  480 treated as the literal characters "B", "R", and "X" by default, but cause an
  481 error if the PCRE_EXTRA option is set. Outside a character class, these
  482 sequences have different meanings.
  483 </P>
  484 <br><b>
  485 Unsupported escape sequences
  486 </b><br>
  487 <P>
  488 In Perl, the sequences \l, \L, \u, and \U are recognized by its string
  489 handler and used to modify the case of following characters. By default, PCRE
  490 does not support these escape sequences. However, if the PCRE_JAVASCRIPT_COMPAT
  491 option is set, \U matches a "U" character, and \u can be used to define a
  492 character by code point, as described in the previous section.
  493 </P>
  494 <br><b>
  495 Absolute and relative back references
  496 </b><br>
  497 <P>
  498 The sequence \g followed by an unsigned or a negative number, optionally
  499 enclosed in braces, is an absolute or relative back reference. A named back
  500 reference can be coded as \g{name}. Back references are discussed
  501 <a href="#backreferences">later,</a>
  502 following the discussion of
  503 <a href="#subpattern">parenthesized subpatterns.</a>
  504 </P>
  505 <br><b>
  506 Absolute and relative subroutine calls
  507 </b><br>
  508 <P>
  509 For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or
  510 a number enclosed either in angle brackets or single quotes, is an alternative
  511 syntax for referencing a subpattern as a "subroutine". Details are discussed
  512 <a href="#onigurumasubroutines">later.</a>
  513 Note that \g{...} (Perl syntax) and \g&#60;...&#62; (Oniguruma syntax) are <i>not</i>
  514 synonymous. The former is a back reference; the latter is a
  515 <a href="#subpatternsassubroutines">subroutine</a>
  516 call.
  517 <a name="genericchartypes"></a></P>
  518 <br><b>
  519 Generic character types
  520 </b><br>
  521 <P>
  522 Another use of backslash is for specifying generic character types:
  523 <pre>
  524   \d     any decimal digit
  525   \D     any character that is not a decimal digit
  526   \h     any horizontal white space character
  527   \H     any character that is not a horizontal white space character
  528   \s     any white space character
  529   \S     any character that is not a white space character
  530   \v     any vertical white space character
  531   \V     any character that is not a vertical white space character
  532   \w     any "word" character
  533   \W     any "non-word" character
  534 </pre>
  535 There is also the single sequence \N, which matches a non-newline character.
  536 This is the same as
  537 <a href="#fullstopdot">the "." metacharacter</a>
  538 when PCRE_DOTALL is not set. Perl also uses \N to match characters by name;
  539 PCRE does not support this.
  540 </P>
  541 <P>
  542 Each pair of lower and upper case escape sequences partitions the complete set
  543 of characters into two disjoint sets. Any given character matches one, and only
  544 one, of each pair. The sequences can appear both inside and outside character
  545 classes. They each match one character of the appropriate type. If the current
  546 matching point is at the end of the subject string, all of them fail, because
  547 there is no character to match.
  548 </P>
  549 <P>
  550 For compatibility with Perl, \s did not used to match the VT character (code
  551 11), which made it different from the the POSIX "space" class. However, Perl
  552 added VT at release 5.18, and PCRE followed suit at release 8.34. The default
  553 \s characters are now HT (9), LF (10), VT (11), FF (12), CR (13), and space
  554 (32), which are defined as white space in the "C" locale. This list may vary if
  555 locale-specific matching is taking place. For example, in some locales the
  556 "non-breaking space" character (\xA0) is recognized as white space, and in
  557 others the VT character is not.
  558 </P>
  559 <P>
  560 A "word" character is an underscore or any character that is a letter or digit.
  561 By default, the definition of letters and digits is controlled by PCRE's
  562 low-valued character tables, and may vary if locale-specific matching is taking
  563 place (see
  564 <a href="pcreapi.html#localesupport">"Locale support"</a>
  565 in the
  566 <a href="pcreapi.html"><b>pcreapi</b></a>
  567 page). For example, in a French locale such as "fr_FR" in Unix-like systems,
  568 or "french" in Windows, some character codes greater than 127 are used for
  569 accented letters, and these are then matched by \w. The use of locales with
  570 Unicode is discouraged.
  571 </P>
  572 <P>
  573 By default, characters whose code points are greater than 127 never match \d,
  574 \s, or \w, and always match \D, \S, and \W, although this may vary for
  575 characters in the range 128-255 when locale-specific matching is happening.
  576 These escape sequences retain their original meanings from before Unicode
  577 support was available, mainly for efficiency reasons. If PCRE is compiled with
  578 Unicode property support, and the PCRE_UCP option is set, the behaviour is
  579 changed so that Unicode properties are used to determine character types, as
  580 follows:
  581 <pre>
  582   \d  any character that matches \p{Nd} (decimal digit)
  583   \s  any character that matches \p{Z} or \h or \v
  584   \w  any character that matches \p{L} or \p{N}, plus underscore
  585 </pre>
  586 The upper case escapes match the inverse sets of characters. Note that \d
  587 matches only decimal digits, whereas \w matches any Unicode digit, as well as
  588 any Unicode letter, and underscore. Note also that PCRE_UCP affects \b, and
  589 \B because they are defined in terms of \w and \W. Matching these sequences
  590 is noticeably slower when PCRE_UCP is set.
  591 </P>
  592 <P>
  593 The sequences \h, \H, \v, and \V are features that were added to Perl at
  594 release 5.10. In contrast to the other sequences, which match only ASCII
  595 characters by default, these always match certain high-valued code points,
  596 whether or not PCRE_UCP is set. The horizontal space characters are:
  597 <pre>
  598   U+0009     Horizontal tab (HT)
  599   U+0020     Space
  600   U+00A0     Non-break space
  601   U+1680     Ogham space mark
  602   U+180E     Mongolian vowel separator
  603   U+2000     En quad
  604   U+2001     Em quad
  605   U+2002     En space
  606   U+2003     Em space
  607   U+2004     Three-per-em space
  608   U+2005     Four-per-em space
  609   U+2006     Six-per-em space
  610   U+2007     Figure space
  611   U+2008     Punctuation space
  612   U+2009     Thin space
  613   U+200A     Hair space
  614   U+202F     Narrow no-break space
  615   U+205F     Medium mathematical space
  616   U+3000     Ideographic space
  617 </pre>
  618 The vertical space characters are:
  619 <pre>
  620   U+000A     Linefeed (LF)
  621   U+000B     Vertical tab (VT)
  622   U+000C     Form feed (FF)
  623   U+000D     Carriage return (CR)
  624   U+0085     Next line (NEL)
  625   U+2028     Line separator
  626   U+2029     Paragraph separator
  627 </pre>
  628 In 8-bit, non-UTF-8 mode, only the characters with codepoints less than 256 are
  629 relevant.
  630 <a name="newlineseq"></a></P>
  631 <br><b>
  632 Newline sequences
  633 </b><br>
  634 <P>
  635 Outside a character class, by default, the escape sequence \R matches any
  636 Unicode newline sequence. In 8-bit non-UTF-8 mode \R is equivalent to the
  637 following:
  638 <pre>
  639   (?&#62;\r\n|\n|\x0b|\f|\r|\x85)
  640 </pre>
  641 This is an example of an "atomic group", details of which are given
  642 <a href="#atomicgroup">below.</a>
  643 This particular group matches either the two-character sequence CR followed by
  644 LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
  645 U+000B), FF (form feed, U+000C), CR (carriage return, U+000D), or NEL (next
  646 line, U+0085). The two-character sequence is treated as a single unit that
  647 cannot be split.
  648 </P>
  649 <P>
  650 In other modes, two additional characters whose codepoints are greater than 255
  651 are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
  652 Unicode character property support is not needed for these characters to be
  653 recognized.
  654 </P>
  655 <P>
  656 It is possible to restrict \R to match only CR, LF, or CRLF (instead of the
  657 complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
  658 either at compile time or when the pattern is matched. (BSR is an abbrevation
  659 for "backslash R".) This can be made the default when PCRE is built; if this is
  660 the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
  661 It is also possible to specify these settings by starting a pattern string with
  662 one of the following sequences:
  663 <pre>
  664   (*BSR_ANYCRLF)   CR, LF, or CRLF only
  665   (*BSR_UNICODE)   any Unicode newline sequence
  666 </pre>
  667 These override the default and the options given to the compiling function, but
  668 they can themselves be overridden by options given to a matching function. Note
  669 that these special settings, which are not Perl-compatible, are recognized only
  670 at the very start of a pattern, and that they must be in upper case. If more
  671 than one of them is present, the last one is used. They can be combined with a
  672 change of newline convention; for example, a pattern can start with:
  673 <pre>
  674   (*ANY)(*BSR_ANYCRLF)
  675 </pre>
  676 They can also be combined with the (*UTF8), (*UTF16), (*UTF32), (*UTF) or
  677 (*UCP) special sequences. Inside a character class, \R is treated as an
  678 unrecognized escape sequence, and so matches the letter "R" by default, but
  679 causes an error if PCRE_EXTRA is set.
  680 <a name="uniextseq"></a></P>
  681 <br><b>
  682 Unicode character properties
  683 </b><br>
  684 <P>
  685 When PCRE is built with Unicode character property support, three additional
  686 escape sequences that match characters with specific properties are available.
  687 When in 8-bit non-UTF-8 mode, these sequences are of course limited to testing
  688 characters whose codepoints are less than 256, but they do work in this mode.
  689 The extra escape sequences are:
  690 <pre>
  691   \p{<i>xx</i>}   a character with the <i>xx</i> property
  692   \P{<i>xx</i>}   a character without the <i>xx</i> property
  693   \X       a Unicode extended grapheme cluster
  694 </pre>
  695 The property names represented by <i>xx</i> above are limited to the Unicode
  696 script names, the general category properties, "Any", which matches any
  697 character (including newline), and some special PCRE properties (described
  698 in the
  699 <a href="#extraprops">next section).</a>
  700 Other Perl properties such as "InMusicalSymbols" are not currently supported by
  701 PCRE. Note that \P{Any} does not match any characters, so always causes a
  702 match failure.
  703 </P>
  704 <P>
  705 Sets of Unicode characters are defined as belonging to certain scripts. A
  706 character from one of these sets can be matched using a script name. For
  707 example:
  708 <pre>
  709   \p{Greek}
  710   \P{Han}
  711 </pre>
  712 Those that are not part of an identified script are lumped together as
  713 "Common". The current list of scripts is:
  714 </P>
  715 <P>
  716 Arabic,
  717 Armenian,
  718 Avestan,
  719 Balinese,
  720 Bamum,
  721 Bassa_Vah,
  722 Batak,
  723 Bengali,
  724 Bopomofo,
  725 Brahmi,
  726 Braille,
  727 Buginese,
  728 Buhid,
  729 Canadian_Aboriginal,
  730 Carian,
  731 Caucasian_Albanian,
  732 Chakma,
  733 Cham,
  734 Cherokee,
  735 Common,
  736 Coptic,
  737 Cuneiform,
  738 Cypriot,
  739 Cyrillic,
  740 Deseret,
  741 Devanagari,
  742 Duployan,
  743 Egyptian_Hieroglyphs,
  744 Elbasan,
  745 Ethiopic,
  746 Georgian,
  747 Glagolitic,
  748 Gothic,
  749 Grantha,
  750 Greek,
  751 Gujarati,
  752 Gurmukhi,
  753 Han,
  754 Hangul,
  755 Hanunoo,
  756 Hebrew,
  757 Hiragana,
  758 Imperial_Aramaic,
  759 Inherited,
  760 Inscriptional_Pahlavi,
  761 Inscriptional_Parthian,
  762 Javanese,
  763 Kaithi,
  764 Kannada,
  765 Katakana,
  766 Kayah_Li,
  767 Kharoshthi,
  768 Khmer,
  769 Khojki,
  770 Khudawadi,
  771 Lao,
  772 Latin,
  773 Lepcha,
  774 Limbu,
  775 Linear_A,
  776 Linear_B,
  777 Lisu,
  778 Lycian,
  779 Lydian,
  780 Mahajani,
  781 Malayalam,
  782 Mandaic,
  783 Manichaean,
  784 Meetei_Mayek,
  785 Mende_Kikakui,
  786 Meroitic_Cursive,
  787 Meroitic_Hieroglyphs,
  788 Miao,
  789 Modi,
  790 Mongolian,
  791 Mro,
  792 Myanmar,
  793 Nabataean,
  794 New_Tai_Lue,
  795 Nko,
  796 Ogham,
  797 Ol_Chiki,
  798 Old_Italic,
  799 Old_North_Arabian,
  800 Old_Permic,
  801 Old_Persian,
  802 Old_South_Arabian,
  803 Old_Turkic,
  804 Oriya,
  805 Osmanya,
  806 Pahawh_Hmong,
  807 Palmyrene,
  808 Pau_Cin_Hau,
  809 Phags_Pa,
  810 Phoenician,
  811 Psalter_Pahlavi,
  812 Rejang,
  813 Runic,
  814 Samaritan,
  815 Saurashtra,
  816 Sharada,
  817 Shavian,
  818 Siddham,
  819 Sinhala,
  820 Sora_Sompeng,
  821 Sundanese,
  822 Syloti_Nagri,
  823 Syriac,
  824 Tagalog,
  825 Tagbanwa,
  826 Tai_Le,
  827 Tai_Tham,
  828 Tai_Viet,
  829 Takri,
  830 Tamil,
  831 Telugu,
  832 Thaana,
  833 Thai,
  834 Tibetan,
  835 Tifinagh,
  836 Tirhuta,
  837 Ugaritic,
  838 Vai,
  839 Warang_Citi,
  840 Yi.
  841 </P>
  842 <P>
  843 Each character has exactly one Unicode general category property, specified by
  844 a two-letter abbreviation. For compatibility with Perl, negation can be
  845 specified by including a circumflex between the opening brace and the property
  846 name. For example, \p{^Lu} is the same as \P{Lu}.
  847 </P>
  848 <P>
  849 If only one letter is specified with \p or \P, it includes all the general
  850 category properties that start with that letter. In this case, in the absence
  851 of negation, the curly brackets in the escape sequence are optional; these two
  852 examples have the same effect:
  853 <pre>
  854   \p{L}
  855   \pL
  856 </pre>
  857 The following general category property codes are supported:
  858 <pre>
  859   C     Other
  860   Cc    Control
  861   Cf    Format
  862   Cn    Unassigned
  863   Co    Private use
  864   Cs    Surrogate
  866   L     Letter
  867   Ll    Lower case letter
  868   Lm    Modifier letter
  869   Lo    Other letter
  870   Lt    Title case letter
  871   Lu    Upper case letter
  873   M     Mark
  874   Mc    Spacing mark
  875   Me    Enclosing mark
  876   Mn    Non-spacing mark
  878   N     Number
  879   Nd    Decimal number
  880   Nl    Letter number
  881   No    Other number
  883   P     Punctuation
  884   Pc    Connector punctuation
  885   Pd    Dash punctuation
  886   Pe    Close punctuation
  887   Pf    Final punctuation
  888   Pi    Initial punctuation
  889   Po    Other punctuation
  890   Ps    Open punctuation
  892   S     Symbol
  893   Sc    Currency symbol
  894   Sk    Modifier symbol
  895   Sm    Mathematical symbol
  896   So    Other symbol
  898   Z     Separator
  899   Zl    Line separator
  900   Zp    Paragraph separator
  901   Zs    Space separator
  902 </pre>
  903 The special property L& is also supported: it matches a character that has
  904 the Lu, Ll, or Lt property, in other words, a letter that is not classified as
  905 a modifier or "other".
  906 </P>
  907 <P>
  908 The Cs (Surrogate) property applies only to characters in the range U+D800 to
  909 U+DFFF. Such characters are not valid in Unicode strings and so
  910 cannot be tested by PCRE, unless UTF validity checking has been turned off
  911 (see the discussion of PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK and
  912 PCRE_NO_UTF32_CHECK in the
  913 <a href="pcreapi.html"><b>pcreapi</b></a>
  914 page). Perl does not support the Cs property.
  915 </P>
  916 <P>
  917 The long synonyms for property names that Perl supports (such as \p{Letter})
  918 are not supported by PCRE, nor is it permitted to prefix any of these
  919 properties with "Is".
  920 </P>
  921 <P>
  922 No character that is in the Unicode table has the Cn (unassigned) property.
  923 Instead, this property is assumed for any code point that is not in the
  924 Unicode table.
  925 </P>
  926 <P>
  927 Specifying caseless matching does not affect these escape sequences. For
  928 example, \p{Lu} always matches only upper case letters. This is different from
  929 the behaviour of current versions of Perl.
  930 </P>
  931 <P>
  932 Matching characters by Unicode property is not fast, because PCRE has to do a
  933 multistage table lookup in order to find a character's property. That is why
  934 the traditional escape sequences such as \d and \w do not use Unicode
  935 properties in PCRE by default, though you can make them do so by setting the
  936 PCRE_UCP option or by starting the pattern with (*UCP).
  937 </P>
  938 <br><b>
  939 Extended grapheme clusters
  940 </b><br>
  941 <P>
  942 The \X escape matches any number of Unicode characters that form an "extended
  943 grapheme cluster", and treats the sequence as an atomic group
  944 <a href="#atomicgroup">(see below).</a>
  945 Up to and including release 8.31, PCRE matched an earlier, simpler definition
  946 that was equivalent to
  947 <pre>
  948   (?&#62;\PM\pM*)
  949 </pre>
  950 That is, it matched a character without the "mark" property, followed by zero
  951 or more characters with the "mark" property. Characters with the "mark"
  952 property are typically non-spacing accents that affect the preceding character.
  953 </P>
  954 <P>
  955 This simple definition was extended in Unicode to include more complicated
  956 kinds of composite character by giving each character a grapheme breaking
  957 property, and creating rules that use these properties to define the boundaries
  958 of extended grapheme clusters. In releases of PCRE later than 8.31, \X matches
  959 one of these clusters.
  960 </P>
  961 <P>
  962 \X always matches at least one character. Then it decides whether to add
  963 additional characters according to the following rules for ending a cluster:
  964 </P>
  965 <P>
  966 1. End at the end of the subject string.
  967 </P>
  968 <P>
  969 2. Do not end between CR and LF; otherwise end after any control character.
  970 </P>
  971 <P>
  972 3. Do not break Hangul (a Korean script) syllable sequences. Hangul characters
  973 are of five types: L, V, T, LV, and LVT. An L character may be followed by an
  974 L, V, LV, or LVT character; an LV or V character may be followed by a V or T
  975 character; an LVT or T character may be follwed only by a T character.
  976 </P>
  977 <P>
  978 4. Do not end before extending characters or spacing marks. Characters with
  979 the "mark" property always have the "extend" grapheme breaking property.
  980 </P>
  981 <P>
  982 5. Do not end after prepend characters.
  983 </P>
  984 <P>
  985 6. Otherwise, end the cluster.
  986 <a name="extraprops"></a></P>
  987 <br><b>
  988 PCRE's additional properties
  989 </b><br>
  990 <P>
  991 As well as the standard Unicode properties described above, PCRE supports four
  992 more that make it possible to convert traditional escape sequences such as \w
  993 and \s to use Unicode properties. PCRE uses these non-standard, non-Perl
  994 properties internally when PCRE_UCP is set. However, they may also be used
  995 explicitly. These properties are:
  996 <pre>
  997   Xan   Any alphanumeric character
  998   Xps   Any POSIX space character
  999   Xsp   Any Perl space character
 1000   Xwd   Any Perl "word" character
 1001 </pre>
 1002 Xan matches characters that have either the L (letter) or the N (number)
 1003 property. Xps matches the characters tab, linefeed, vertical tab, form feed, or
 1004 carriage return, and any other character that has the Z (separator) property.
 1005 Xsp is the same as Xps; it used to exclude vertical tab, for Perl
 1006 compatibility, but Perl changed, and so PCRE followed at release 8.34. Xwd
 1007 matches the same characters as Xan, plus underscore.
 1008 </P>
 1009 <P>
 1010 There is another non-standard property, Xuc, which matches any character that
 1011 can be represented by a Universal Character Name in C++ and other programming
 1012 languages. These are the characters $, @, ` (grave accent), and all characters
 1013 with Unicode code points greater than or equal to U+00A0, except for the
 1014 surrogates U+D800 to U+DFFF. Note that most base (ASCII) characters are
 1015 excluded. (Universal Character Names are of the form \uHHHH or \UHHHHHHHH
 1016 where H is a hexadecimal digit. Note that the Xuc property does not match these
 1017 sequences but the characters that they represent.)
 1018 <a name="resetmatchstart"></a></P>
 1019 <br><b>
 1020 Resetting the match start
 1021 </b><br>
 1022 <P>
 1023 The escape sequence \K causes any previously matched characters not to be
 1024 included in the final matched sequence. For example, the pattern:
 1025 <pre>
 1026   foo\Kbar
 1027 </pre>
 1028 matches "foobar", but reports that it has matched "bar". This feature is
 1029 similar to a lookbehind assertion
 1030 <a href="#lookbehind">(described below).</a>
 1031 However, in this case, the part of the subject before the real match does not
 1032 have to be of fixed length, as lookbehind assertions do. The use of \K does
 1033 not interfere with the setting of
 1034 <a href="#subpattern">captured substrings.</a>
 1035 For example, when the pattern
 1036 <pre>
 1037   (foo)\Kbar
 1038 </pre>
 1039 matches "foobar", the first substring is still set to "foo".
 1040 </P>
 1041 <P>
 1042 Perl documents that the use of \K within assertions is "not well defined". In
 1043 PCRE, \K is acted upon when it occurs inside positive assertions, but is
 1044 ignored in negative assertions. Note that when a pattern such as (?=ab\K)
 1045 matches, the reported start of the match can be greater than the end of the
 1046 match.
 1047 <a name="smallassertions"></a></P>
 1048 <br><b>
 1049 Simple assertions
 1050 </b><br>
 1051 <P>
 1052 The final use of backslash is for certain simple assertions. An assertion
 1053 specifies a condition that has to be met at a particular point in a match,
 1054 without consuming any characters from the subject string. The use of
 1055 subpatterns for more complicated assertions is described
 1056 <a href="#bigassertions">below.</a>
 1057 The backslashed assertions are:
 1058 <pre>
 1059   \b     matches at a word boundary
 1060   \B     matches when not at a word boundary
 1061   \A     matches at the start of the subject
 1062   \Z     matches at the end of the subject
 1063           also matches before a newline at the end of the subject
 1064   \z     matches only at the end of the subject
 1065   \G     matches at the first matching position in the subject
 1066 </pre>
 1067 Inside a character class, \b has a different meaning; it matches the backspace
 1068 character. If any other of these assertions appears in a character class, by
 1069 default it matches the corresponding literal character (for example, \B
 1070 matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
 1071 escape sequence" error is generated instead.
 1072 </P>
 1073 <P>
 1074 A word boundary is a position in the subject string where the current character
 1075 and the previous character do not both match \w or \W (i.e. one matches
 1076 \w and the other matches \W), or the start or end of the string if the
 1077 first or last character matches \w, respectively. In a UTF mode, the meanings
 1078 of \w and \W can be changed by setting the PCRE_UCP option. When this is
 1079 done, it also affects \b and \B. Neither PCRE nor Perl has a separate "start
 1080 of word" or "end of word" metasequence. However, whatever follows \b normally
 1081 determines which it is. For example, the fragment \ba matches "a" at the start
 1082 of a word.
 1083 </P>
 1084 <P>
 1085 The \A, \Z, and \z assertions differ from the traditional circumflex and
 1086 dollar (described in the next section) in that they only ever match at the very
 1087 start and end of the subject string, whatever options are set. Thus, they are
 1088 independent of multiline mode. These three assertions are not affected by the
 1089 PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
 1090 circumflex and dollar metacharacters. However, if the <i>startoffset</i>
 1091 argument of <b>pcre_exec()</b> is non-zero, indicating that matching is to start
 1092 at a point other than the beginning of the subject, \A can never match. The
 1093 difference between \Z and \z is that \Z matches before a newline at the end
 1094 of the string as well as at the very end, whereas \z matches only at the end.
 1095 </P>
 1096 <P>
 1097 The \G assertion is true only when the current matching position is at the
 1098 start point of the match, as specified by the <i>startoffset</i> argument of
 1099 <b>pcre_exec()</b>. It differs from \A when the value of <i>startoffset</i> is
 1100 non-zero. By calling <b>pcre_exec()</b> multiple times with appropriate
 1101 arguments, you can mimic Perl's /g option, and it is in this kind of
 1102 implementation where \G can be useful.
 1103 </P>
 1104 <P>
 1105 Note, however, that PCRE's interpretation of \G, as the start of the current
 1106 match, is subtly different from Perl's, which defines it as the end of the
 1107 previous match. In Perl, these can be different when the previously matched
 1108 string was empty. Because PCRE does just one match at a time, it cannot
 1109 reproduce this behaviour.
 1110 </P>
 1111 <P>
 1112 If all the alternatives of a pattern begin with \G, the expression is anchored
 1113 to the starting match position, and the "anchored" flag is set in the compiled
 1114 regular expression.
 1115 </P>
 1116 <br><a name="SEC6" href="#TOC1">CIRCUMFLEX AND DOLLAR</a><br>
 1117 <P>
 1118 The circumflex and dollar metacharacters are zero-width assertions. That is,
 1119 they test for a particular condition being true without consuming any
 1120 characters from the subject string.
 1121 </P>
 1122 <P>
 1123 Outside a character class, in the default matching mode, the circumflex
 1124 character is an assertion that is true only if the current matching point is at
 1125 the start of the subject string. If the <i>startoffset</i> argument of
 1126 <b>pcre_exec()</b> is non-zero, circumflex can never match if the PCRE_MULTILINE
 1127 option is unset. Inside a character class, circumflex has an entirely different
 1128 meaning
 1129 <a href="#characterclass">(see below).</a>
 1130 </P>
 1131 <P>
 1132 Circumflex need not be the first character of the pattern if a number of
 1133 alternatives are involved, but it should be the first thing in each alternative
 1134 in which it appears if the pattern is ever to match that branch. If all
 1135 possible alternatives start with a circumflex, that is, if the pattern is
 1136 constrained to match only at the start of the subject, it is said to be an
 1137 "anchored" pattern. (There are also other constructs that can cause a pattern
 1138 to be anchored.)
 1139 </P>
 1140 <P>
 1141 The dollar character is an assertion that is true only if the current matching
 1142 point is at the end of the subject string, or immediately before a newline at
 1143 the end of the string (by default). Note, however, that it does not actually
 1144 match the newline. Dollar need not be the last character of the pattern if a
 1145 number of alternatives are involved, but it should be the last item in any
 1146 branch in which it appears. Dollar has no special meaning in a character class.
 1147 </P>
 1148 <P>
 1149 The meaning of dollar can be changed so that it matches only at the very end of
 1150 the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This
 1151 does not affect the \Z assertion.
 1152 </P>
 1153 <P>
 1154 The meanings of the circumflex and dollar characters are changed if the
 1155 PCRE_MULTILINE option is set. When this is the case, a circumflex matches
 1156 immediately after internal newlines as well as at the start of the subject
 1157 string. It does not match after a newline that ends the string. A dollar
 1158 matches before any newlines in the string, as well as at the very end, when
 1159 PCRE_MULTILINE is set. When newline is specified as the two-character
 1160 sequence CRLF, isolated CR and LF characters do not indicate newlines.
 1161 </P>
 1162 <P>
 1163 For example, the pattern /^abc$/ matches the subject string "def\nabc" (where
 1164 \n represents a newline) in multiline mode, but not otherwise. Consequently,
 1165 patterns that are anchored in single line mode because all branches start with
 1166 ^ are not anchored in multiline mode, and a match for circumflex is possible
 1167 when the <i>startoffset</i> argument of <b>pcre_exec()</b> is non-zero. The
 1168 PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
 1169 </P>
 1170 <P>
 1171 Note that the sequences \A, \Z, and \z can be used to match the start and
 1172 end of the subject in both modes, and if all branches of a pattern start with
 1173 \A it is always anchored, whether or not PCRE_MULTILINE is set.
 1174 <a name="fullstopdot"></a></P>
 1175 <br><a name="SEC7" href="#TOC1">FULL STOP (PERIOD, DOT) AND \N</a><br>
 1176 <P>
 1177 Outside a character class, a dot in the pattern matches any one character in
 1178 the subject string except (by default) a character that signifies the end of a
 1179 line.
 1180 </P>
 1181 <P>
 1182 When a line ending is defined as a single character, dot never matches that
 1183 character; when the two-character sequence CRLF is used, dot does not match CR
 1184 if it is immediately followed by LF, but otherwise it matches all characters
 1185 (including isolated CRs and LFs). When any Unicode line endings are being
 1186 recognized, dot does not match CR or LF or any of the other line ending
 1187 characters.
 1188 </P>
 1189 <P>
 1190 The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
 1191 option is set, a dot matches any one character, without exception. If the
 1192 two-character sequence CRLF is present in the subject string, it takes two dots
 1193 to match it.
 1194 </P>
 1195 <P>
 1196 The handling of dot is entirely independent of the handling of circumflex and
 1197 dollar, the only relationship being that they both involve newlines. Dot has no
 1198 special meaning in a character class.
 1199 </P>
 1200 <P>
 1201 The escape sequence \N behaves like a dot, except that it is not affected by
 1202 the PCRE_DOTALL option. In other words, it matches any character except one
 1203 that signifies the end of a line. Perl also uses \N to match characters by
 1204 name; PCRE does not support this.
 1205 </P>
 1206 <br><a name="SEC8" href="#TOC1">MATCHING A SINGLE DATA UNIT</a><br>
 1207 <P>
 1208 Outside a character class, the escape sequence \C matches any one data unit,
 1209 whether or not a UTF mode is set. In the 8-bit library, one data unit is one
 1210 byte; in the 16-bit library it is a 16-bit unit; in the 32-bit library it is
 1211 a 32-bit unit. Unlike a dot, \C always
 1212 matches line-ending characters. The feature is provided in Perl in order to
 1213 match individual bytes in UTF-8 mode, but it is unclear how it can usefully be
 1214 used. Because \C breaks up characters into individual data units, matching one
 1215 unit with \C in a UTF mode means that the rest of the string may start with a
 1216 malformed UTF character. This has undefined results, because PCRE assumes that
 1217 it is dealing with valid UTF strings (and by default it checks this at the
 1218 start of processing unless the PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK or
 1219 PCRE_NO_UTF32_CHECK option is used).
 1220 </P>
 1221 <P>
 1222 PCRE does not allow \C to appear in lookbehind assertions
 1223 <a href="#lookbehind">(described below)</a>
 1224 in a UTF mode, because this would make it impossible to calculate the length of
 1225 the lookbehind.
 1226 </P>
 1227 <P>
 1228 In general, the \C escape sequence is best avoided. However, one
 1229 way of using it that avoids the problem of malformed UTF characters is to use a
 1230 lookahead to check the length of the next character, as in this pattern, which
 1231 could be used with a UTF-8 string (ignore white space and line breaks):
 1232 <pre>
 1233   (?| (?=[\x00-\x7f])(\C) |
 1234       (?=[\x80-\x{7ff}])(\C)(\C) |
 1235       (?=[\x{800}-\x{ffff}])(\C)(\C)(\C) |
 1236       (?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C))
 1237 </pre>
 1238 A group that starts with (?| resets the capturing parentheses numbers in each
 1239 alternative (see
 1240 <a href="#dupsubpatternnumber">"Duplicate Subpattern Numbers"</a>
 1241 below). The assertions at the start of each branch check the next UTF-8
 1242 character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
 1243 character's individual bytes are then captured by the appropriate number of
 1244 groups.
 1245 <a name="characterclass"></a></P>
 1246 <br><a name="SEC9" href="#TOC1">SQUARE BRACKETS AND CHARACTER CLASSES</a><br>
 1247 <P>
 1248 An opening square bracket introduces a character class, terminated by a closing
 1249 square bracket. A closing square bracket on its own is not special by default.
 1250 However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square
 1251 bracket causes a compile-time error. If a closing square bracket is required as
 1252 a member of the class, it should be the first data character in the class
 1253 (after an initial circumflex, if present) or escaped with a backslash.
 1254 </P>
 1255 <P>
 1256 A character class matches a single character in the subject. In a UTF mode, the
 1257 character may be more than one data unit long. A matched character must be in
 1258 the set of characters defined by the class, unless the first character in the
 1259 class definition is a circumflex, in which case the subject character must not
 1260 be in the set defined by the class. If a circumflex is actually required as a
 1261 member of the class, ensure it is not the first character, or escape it with a
 1262 backslash.
 1263 </P>
 1264 <P>
 1265 For example, the character class [aeiou] matches any lower case vowel, while
 1266 [^aeiou] matches any character that is not a lower case vowel. Note that a
 1267 circumflex is just a convenient notation for specifying the characters that
 1268 are in the class by enumerating those that are not. A class that starts with a
 1269 circumflex is not an assertion; it still consumes a character from the subject
 1270 string, and therefore it fails if the current pointer is at the end of the
 1271 string.
 1272 </P>
 1273 <P>
 1274 In UTF-8 (UTF-16, UTF-32) mode, characters with values greater than 255 (0xffff)
 1275 can be included in a class as a literal string of data units, or by using the
 1276 \x{ escaping mechanism.
 1277 </P>
 1278 <P>
 1279 When caseless matching is set, any letters in a class represent both their
 1280 upper case and lower case versions, so for example, a caseless [aeiou] matches
 1281 "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
 1282 caseful version would. In a UTF mode, PCRE always understands the concept of
 1283 case for characters whose values are less than 128, so caseless matching is
 1284 always possible. For characters with higher values, the concept of case is
 1285 supported if PCRE is compiled with Unicode property support, but not otherwise.
 1286 If you want to use caseless matching in a UTF mode for characters 128 and
 1287 above, you must ensure that PCRE is compiled with Unicode property support as
 1288 well as with UTF support.
 1289 </P>
 1290 <P>
 1291 Characters that might indicate line breaks are never treated in any special way
 1292 when matching character classes, whatever line-ending sequence is in use, and
 1293 whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
 1294 such as [^a] always matches one of these characters.
 1295 </P>
 1296 <P>
 1297 The minus (hyphen) character can be used to specify a range of characters in a
 1298 character class. For example, [d-m] matches any letter between d and m,
 1299 inclusive. If a minus character is required in a class, it must be escaped with
 1300 a backslash or appear in a position where it cannot be interpreted as
 1301 indicating a range, typically as the first or last character in the class, or
 1302 immediately after a range. For example, [b-d-z] matches letters in the range b
 1303 to d, a hyphen character, or z.
 1304 </P>
 1305 <P>
 1306 It is not possible to have the literal character "]" as the end character of a
 1307 range. A pattern such as [W-]46] is interpreted as a class of two characters
 1308 ("W" and "-") followed by a literal string "46]", so it would match "W46]" or
 1309 "-46]". However, if the "]" is escaped with a backslash it is interpreted as
 1310 the end of range, so [W-\]46] is interpreted as a class containing a range
 1311 followed by two other characters. The octal or hexadecimal representation of
 1312 "]" can also be used to end a range.
 1313 </P>
 1314 <P>
 1315 An error is generated if a POSIX character class (see below) or an escape
 1316 sequence other than one that defines a single character appears at a point
 1317 where a range ending character is expected. For example, [z-\xff] is valid,
 1318 but [A-\d] and [A-[:digit:]] are not.
 1319 </P>
 1320 <P>
 1321 Ranges operate in the collating sequence of character values. They can also be
 1322 used for characters specified numerically, for example [\000-\037]. Ranges
 1323 can include any characters that are valid for the current mode.
 1324 </P>
 1325 <P>
 1326 If a range that includes letters is used when caseless matching is set, it
 1327 matches the letters in either case. For example, [W-c] is equivalent to
 1328 [][\\^_`wxyzabc], matched caselessly, and in a non-UTF mode, if character
 1329 tables for a French locale are in use, [\xc8-\xcb] matches accented E
 1330 characters in both cases. In UTF modes, PCRE supports the concept of case for
 1331 characters with values greater than 128 only when it is compiled with Unicode
 1332 property support.
 1333 </P>
 1334 <P>
 1335 The character escape sequences \d, \D, \h, \H, \p, \P, \s, \S, \v,
 1336 \V, \w, and \W may appear in a character class, and add the characters that
 1337 they match to the class. For example, [\dABCDEF] matches any hexadecimal
 1338 digit. In UTF modes, the PCRE_UCP option affects the meanings of \d, \s, \w
 1339 and their upper case partners, just as it does when they appear outside a
 1340 character class, as described in the section entitled
 1341 <a href="#genericchartypes">"Generic character types"</a>
 1342 above. The escape sequence \b has a different meaning inside a character
 1343 class; it matches the backspace character. The sequences \B, \N, \R, and \X
 1344 are not special inside a character class. Like any other unrecognized escape
 1345 sequences, they are treated as the literal characters "B", "N", "R", and "X" by
 1346 default, but cause an error if the PCRE_EXTRA option is set.
 1347 </P>
 1348 <P>
 1349 A circumflex can conveniently be used with the upper case character types to
 1350 specify a more restricted set of characters than the matching lower case type.
 1351 For example, the class [^\W_] matches any letter or digit, but not underscore,
 1352 whereas [\w] includes underscore. A positive character class should be read as
 1353 "something OR something OR ..." and a negative class as "NOT something AND NOT
 1354 something AND NOT ...".
 1355 </P>
 1356 <P>
 1357 The only metacharacters that are recognized in character classes are backslash,
 1358 hyphen (only where it can be interpreted as specifying a range), circumflex
 1359 (only at the start), opening square bracket (only when it can be interpreted as
 1360 introducing a POSIX class name, or for a special compatibility feature - see
 1361 the next two sections), and the terminating closing square bracket. However,
 1362 escaping other non-alphanumeric characters does no harm.
 1363 </P>
 1364 <br><a name="SEC10" href="#TOC1">POSIX CHARACTER CLASSES</a><br>
 1365 <P>
 1366 Perl supports the POSIX notation for character classes. This uses names
 1367 enclosed by [: and :] within the enclosing square brackets. PCRE also supports
 1368 this notation. For example,
 1369 <pre>
 1370   [01[:alpha:]%]
 1371 </pre>
 1372 matches "0", "1", any alphabetic character, or "%". The supported class names
 1373 are:
 1374 <pre>
 1375   alnum    letters and digits
 1376   alpha    letters
 1377   ascii    character codes 0 - 127
 1378   blank    space or tab only
 1379   cntrl    control characters
 1380   digit    decimal digits (same as \d)
 1381   graph    printing characters, excluding space
 1382   lower    lower case letters
 1383   print    printing characters, including space
 1384   punct    printing characters, excluding letters and digits and space
 1385   space    white space (the same as \s from PCRE 8.34)
 1386   upper    upper case letters
 1387   word     "word" characters (same as \w)
 1388   xdigit   hexadecimal digits
 1389 </pre>
 1390 The default "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13),
 1391 and space (32). If locale-specific matching is taking place, the list of space
 1392 characters may be different; there may be fewer or more of them. "Space" used
 1393 to be different to \s, which did not include VT, for Perl compatibility.
 1394 However, Perl changed at release 5.18, and PCRE followed at release 8.34.
 1395 "Space" and \s now match the same set of characters.
 1396 </P>
 1397 <P>
 1398 The name "word" is a Perl extension, and "blank" is a GNU extension from Perl
 1399 5.8. Another Perl extension is negation, which is indicated by a ^ character
 1400 after the colon. For example,
 1401 <pre>
 1402   [12[:^digit:]]
 1403 </pre>
 1404 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX
 1405 syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
 1406 supported, and an error is given if they are encountered.
 1407 </P>
 1408 <P>
 1409 By default, characters with values greater than 128 do not match any of the
 1410 POSIX character classes. However, if the PCRE_UCP option is passed to
 1411 <b>pcre_compile()</b>, some of the classes are changed so that Unicode character
 1412 properties are used. This is achieved by replacing certain POSIX classes by
 1413 other sequences, as follows:
 1414 <pre>
 1415   [:alnum:]  becomes  \p{Xan}
 1416   [:alpha:]  becomes  \p{L}
 1417   [:blank:]  becomes  \h
 1418   [:digit:]  becomes  \p{Nd}
 1419   [:lower:]  becomes  \p{Ll}
 1420   [:space:]  becomes  \p{Xps}
 1421   [:upper:]  becomes  \p{Lu}
 1422   [:word:]   becomes  \p{Xwd}
 1423 </pre>
 1424 Negated versions, such as [:^alpha:] use \P instead of \p. Three other POSIX
 1425 classes are handled specially in UCP mode:
 1426 </P>
 1427 <P>
 1428 [:graph:]
 1429 This matches characters that have glyphs that mark the page when printed. In
 1430 Unicode property terms, it matches all characters with the L, M, N, P, S, or Cf
 1431 properties, except for:
 1432 <pre>
 1433   U+061C           Arabic Letter Mark
 1434   U+180E           Mongolian Vowel Separator
 1435   U+2066 - U+2069  Various "isolate"s
 1437 </PRE>
 1438 </P>
 1439 <P>
 1440 [:print:]
 1441 This matches the same characters as [:graph:] plus space characters that are
 1442 not controls, that is, characters with the Zs property.
 1443 </P>
 1444 <P>
 1445 [:punct:]
 1446 This matches all characters that have the Unicode P (punctuation) property,
 1447 plus those characters whose code points are less than 128 that have the S
 1448 (Symbol) property.
 1449 </P>
 1450 <P>
 1451 The other POSIX classes are unchanged, and match only characters with code
 1452 points less than 128.
 1453 </P>
 1454 <br><a name="SEC11" href="#TOC1">COMPATIBILITY FEATURE FOR WORD BOUNDARIES</a><br>
 1455 <P>
 1456 In the POSIX.2 compliant library that was included in 4.4BSD Unix, the ugly
 1457 syntax [[:&#60;:]] and [[:&#62;:]] is used for matching "start of word" and "end of
 1458 word". PCRE treats these items as follows:
 1459 <pre>
 1460   [[:&#60;:]]  is converted to  \b(?=\w)
 1461   [[:&#62;:]]  is converted to  \b(?&#60;=\w)
 1462 </pre>
 1463 Only these exact character sequences are recognized. A sequence such as
 1464 [a[:&#60;:]b] provokes error for an unrecognized POSIX class name. This support is
 1465 not compatible with Perl. It is provided to help migrations from other
 1466 environments, and is best not used in any new patterns. Note that \b matches
 1467 at the start and the end of a word (see
 1468 <a href="#smallassertions">"Simple assertions"</a>
 1469 above), and in a Perl-style pattern the preceding or following character
 1470 normally shows which is wanted, without the need for the assertions that are
 1471 used above in order to give exactly the POSIX behaviour.
 1472 </P>
 1473 <br><a name="SEC12" href="#TOC1">VERTICAL BAR</a><br>
 1474 <P>
 1475 Vertical bar characters are used to separate alternative patterns. For example,
 1476 the pattern
 1477 <pre>
 1478   gilbert|sullivan
 1479 </pre>
 1480 matches either "gilbert" or "sullivan". Any number of alternatives may appear,
 1481 and an empty alternative is permitted (matching the empty string). The matching
 1482 process tries each alternative in turn, from left to right, and the first one
 1483 that succeeds is used. If the alternatives are within a subpattern
 1484 <a href="#subpattern">(defined below),</a>
 1485 "succeeds" means matching the rest of the main pattern as well as the
 1486 alternative in the subpattern.
 1487 </P>
 1488 <br><a name="SEC13" href="#TOC1">INTERNAL OPTION SETTING</a><br>
 1489 <P>
 1490 The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
 1491 PCRE_EXTENDED options (which are Perl-compatible) can be changed from within
 1492 the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
 1493 The option letters are
 1494 <pre>
 1495   i  for PCRE_CASELESS
 1496   m  for PCRE_MULTILINE
 1497   s  for PCRE_DOTALL
 1498   x  for PCRE_EXTENDED
 1499 </pre>
 1500 For example, (?im) sets caseless, multiline matching. It is also possible to
 1501 unset these options by preceding the letter with a hyphen, and a combined
 1502 setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
 1503 PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
 1504 permitted. If a letter appears both before and after the hyphen, the option is
 1505 unset.
 1506 </P>
 1507 <P>
 1508 The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
 1509 changed in the same way as the Perl-compatible options by using the characters
 1510 J, U and X respectively.
 1511 </P>
 1512 <P>
 1513 When one of these option changes occurs at top level (that is, not inside
 1514 subpattern parentheses), the change applies to the remainder of the pattern
 1515 that follows. An option change within a subpattern (see below for a description
 1516 of subpatterns) affects only that part of the subpattern that follows it, so
 1517 <pre>
 1518   (a(?i)b)c
 1519 </pre>
 1520 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
 1521 By this means, options can be made to have different settings in different
 1522 parts of the pattern. Any changes made in one alternative do carry on
 1523 into subsequent branches within the same subpattern. For example,
 1524 <pre>
 1525   (a(?i)b|c)
 1526 </pre>
 1527 matches "ab", "aB", "c", and "C", even though when matching "C" the first
 1528 branch is abandoned before the option setting. This is because the effects of
 1529 option settings happen at compile time. There would be some very weird
 1530 behaviour otherwise.
 1531 </P>
 1532 <P>
 1533 <b>Note:</b> There are other PCRE-specific options that can be set by the
 1534 application when the compiling or matching functions are called. In some cases
 1535 the pattern can contain special leading sequences such as (*CRLF) to override
 1536 what the application has set or what has been defaulted. Details are given in
 1537 the section entitled
 1538 <a href="#newlineseq">"Newline sequences"</a>
 1539 above. There are also the (*UTF8), (*UTF16),(*UTF32), and (*UCP) leading
 1540 sequences that can be used to set UTF and Unicode property modes; they are
 1541 equivalent to setting the PCRE_UTF8, PCRE_UTF16, PCRE_UTF32 and the PCRE_UCP
 1542 options, respectively. The (*UTF) sequence is a generic version that can be
 1543 used with any of the libraries. However, the application can set the
 1544 PCRE_NEVER_UTF option, which locks out the use of the (*UTF) sequences.
 1545 <a name="subpattern"></a></P>
 1546 <br><a name="SEC14" href="#TOC1">SUBPATTERNS</a><br>
 1547 <P>
 1548 Subpatterns are delimited by parentheses (round brackets), which can be nested.
 1549 Turning part of a pattern into a subpattern does two things:
 1550 <br>
 1551 <br>
 1552 1. It localizes a set of alternatives. For example, the pattern
 1553 <pre>
 1554   cat(aract|erpillar|)
 1555 </pre>
 1556 matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
 1557 match "cataract", "erpillar" or an empty string.
 1558 <br>
 1559 <br>
 1560 2. It sets up the subpattern as a capturing subpattern. This means that, when
 1561 the whole pattern matches, that portion of the subject string that matched the
 1562 subpattern is passed back to the caller via the <i>ovector</i> argument of the
 1563 matching function. (This applies only to the traditional matching functions;
 1564 the DFA matching functions do not support capturing.)
 1565 </P>
 1566 <P>
 1567 Opening parentheses are counted from left to right (starting from 1) to obtain
 1568 numbers for the capturing subpatterns. For example, if the string "the red
 1569 king" is matched against the pattern
 1570 <pre>
 1571   the ((red|white) (king|queen))
 1572 </pre>
 1573 the captured substrings are "red king", "red", and "king", and are numbered 1,
 1574 2, and 3, respectively.
 1575 </P>
 1576 <P>
 1577 The fact that plain parentheses fulfil two functions is not always helpful.
 1578 There are often times when a grouping subpattern is required without a
 1579 capturing requirement. If an opening parenthesis is followed by a question mark
 1580 and a colon, the subpattern does not do any capturing, and is not counted when
 1581 computing the number of any subsequent capturing subpatterns. For example, if
 1582 the string "the white queen" is matched against the pattern
 1583 <pre>
 1584   the ((?:red|white) (king|queen))
 1585 </pre>
 1586 the captured substrings are "white queen" and "queen", and are numbered 1 and
 1587 2. The maximum number of capturing subpatterns is 65535.
 1588 </P>
 1589 <P>
 1590 As a convenient shorthand, if any option settings are required at the start of
 1591 a non-capturing subpattern, the option letters may appear between the "?" and
 1592 the ":". Thus the two patterns
 1593 <pre>
 1594   (?i:saturday|sunday)
 1595   (?:(?i)saturday|sunday)
 1596 </pre>
 1597 match exactly the same set of strings. Because alternative branches are tried
 1598 from left to right, and options are not reset until the end of the subpattern
 1599 is reached, an option setting in one branch does affect subsequent branches, so
 1600 the above patterns match "SUNDAY" as well as "Saturday".
 1601 <a name="dupsubpatternnumber"></a></P>
 1602 <br><a name="SEC15" href="#TOC1">DUPLICATE SUBPATTERN NUMBERS</a><br>
 1603 <P>
 1604 Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
 1605 the same numbers for its capturing parentheses. Such a subpattern starts with
 1606 (?| and is itself a non-capturing subpattern. For example, consider this
 1607 pattern:
 1608 <pre>
 1609   (?|(Sat)ur|(Sun))day
 1610 </pre>
 1611 Because the two alternatives are inside a (?| group, both sets of capturing
 1612 parentheses are numbered one. Thus, when the pattern matches, you can look
 1613 at captured substring number one, whichever alternative matched. This construct
 1614 is useful when you want to capture part, but not all, of one of a number of
 1615 alternatives. Inside a (?| group, parentheses are numbered as usual, but the
 1616 number is reset at the start of each branch. The numbers of any capturing
 1617 parentheses that follow the subpattern start after the highest number used in
 1618 any branch. The following example is taken from the Perl documentation. The
 1619 numbers underneath show in which buffer the captured content will be stored.
 1620 <pre>
 1621   # before  ---------------branch-reset----------- after
 1622   / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
 1623   # 1            2         2  3        2     3     4
 1624 </pre>
 1625 A back reference to a numbered subpattern uses the most recent value that is
 1626 set for that number by any subpattern. The following pattern matches "abcabc"
 1627 or "defdef":
 1628 <pre>
 1629   /(?|(abc)|(def))\1/
 1630 </pre>
 1631 In contrast, a subroutine call to a numbered subpattern always refers to the
 1632 first one in the pattern with the given number. The following pattern matches
 1633 "abcabc" or "defabc":
 1634 <pre>
 1635   /(?|(abc)|(def))(?1)/
 1636 </pre>
 1637 If a
 1638 <a href="#conditions">condition test</a>
 1639 for a subpattern's having matched refers to a non-unique number, the test is
 1640 true if any of the subpatterns of that number have matched.
 1641 </P>
 1642 <P>
 1643 An alternative approach to using this "branch reset" feature is to use
 1644 duplicate named subpatterns, as described in the next section.
 1645 </P>
 1646 <br><a name="SEC16" href="#TOC1">NAMED SUBPATTERNS</a><br>
 1647 <P>
 1648 Identifying capturing parentheses by number is simple, but it can be very hard
 1649 to keep track of the numbers in complicated regular expressions. Furthermore,
 1650 if an expression is modified, the numbers may change. To help with this
 1651 difficulty, PCRE supports the naming of subpatterns. This feature was not
 1652 added to Perl until release 5.10. Python had the feature earlier, and PCRE
 1653 introduced it at release 4.0, using the Python syntax. PCRE now supports both
 1654 the Perl and the Python syntax. Perl allows identically numbered subpatterns to
 1655 have different names, but PCRE does not.
 1656 </P>
 1657 <P>
 1658 In PCRE, a subpattern can be named in one of three ways: (?&#60;name&#62;...) or
 1659 (?'name'...) as in Perl, or (?P&#60;name&#62;...) as in Python. References to capturing
 1660 parentheses from other parts of the pattern, such as
 1661 <a href="#backreferences">back references,</a>
 1662 <a href="#recursion">recursion,</a>
 1663 and
 1664 <a href="#conditions">conditions,</a>
 1665 can be made by name as well as by number.
 1666 </P>
 1667 <P>
 1668 Names consist of up to 32 alphanumeric characters and underscores, but must
 1669 start with a non-digit. Named capturing parentheses are still allocated numbers
 1670 as well as names, exactly as if the names were not present. The PCRE API
 1671 provides function calls for extracting the name-to-number translation table
 1672 from a compiled pattern. There is also a convenience function for extracting a
 1673 captured substring by name.
 1674 </P>
 1675 <P>
 1676 By default, a name must be unique within a pattern, but it is possible to relax
 1677 this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate
 1678 names are also always permitted for subpatterns with the same number, set up as
 1679 described in the previous section.) Duplicate names can be useful for patterns
 1680 where only one instance of the named parentheses can match. Suppose you want to
 1681 match the name of a weekday, either as a 3-letter abbreviation or as the full
 1682 name, and in both cases you want to extract the abbreviation. This pattern
 1683 (ignoring the line breaks) does the job:
 1684 <pre>
 1685   (?&#60;DN&#62;Mon|Fri|Sun)(?:day)?|
 1686   (?&#60;DN&#62;Tue)(?:sday)?|
 1687   (?&#60;DN&#62;Wed)(?:nesday)?|
 1688   (?&#60;DN&#62;Thu)(?:rsday)?|
 1689   (?&#60;DN&#62;Sat)(?:urday)?
 1690 </pre>
 1691 There are five capturing substrings, but only one is ever set after a match.
 1692 (An alternative way of solving this problem is to use a "branch reset"
 1693 subpattern, as described in the previous section.)
 1694 </P>
 1695 <P>
 1696 The convenience function for extracting the data by name returns the substring
 1697 for the first (and in this example, the only) subpattern of that name that
 1698 matched. This saves searching to find which numbered subpattern it was.
 1699 </P>
 1700 <P>
 1701 If you make a back reference to a non-unique named subpattern from elsewhere in
 1702 the pattern, the subpatterns to which the name refers are checked in the order
 1703 in which they appear in the overall pattern. The first one that is set is used
 1704 for the reference. For example, this pattern matches both "foofoo" and
 1705 "barbar" but not "foobar" or "barfoo":
 1706 <pre>
 1707   (?:(?&#60;n&#62;foo)|(?&#60;n&#62;bar))\k&#60;n&#62;
 1709 </PRE>
 1710 </P>
 1711 <P>
 1712 If you make a subroutine call to a non-unique named subpattern, the one that
 1713 corresponds to the first occurrence of the name is used. In the absence of
 1714 duplicate numbers (see the previous section) this is the one with the lowest
 1715 number.
 1716 </P>
 1717 <P>
 1718 If you use a named reference in a condition
 1719 test (see the
 1720 <a href="#conditions">section about conditions</a>
 1721 below), either to check whether a subpattern has matched, or to check for
 1722 recursion, all subpatterns with the same name are tested. If the condition is
 1723 true for any one of them, the overall condition is true. This is the same
 1724 behaviour as testing by number. For further details of the interfaces for
 1725 handling named subpatterns, see the
 1726 <a href="pcreapi.html"><b>pcreapi</b></a>
 1727 documentation.
 1728 </P>
 1729 <P>
 1730 <b>Warning:</b> You cannot use different names to distinguish between two
 1731 subpatterns with the same number because PCRE uses only the numbers when
 1732 matching. For this reason, an error is given at compile time if different names
 1733 are given to subpatterns with the same number. However, you can always give the
 1734 same name to subpatterns with the same number, even when PCRE_DUPNAMES is not
 1735 set.
 1736 </P>
 1737 <br><a name="SEC17" href="#TOC1">REPETITION</a><br>
 1738 <P>
 1739 Repetition is specified by quantifiers, which can follow any of the following
 1740 items:
 1741 <pre>
 1742   a literal data character
 1743   the dot metacharacter
 1744   the \C escape sequence
 1745   the \X escape sequence
 1746   the \R escape sequence
 1747   an escape such as \d or \pL that matches a single character
 1748   a character class
 1749   a back reference (see next section)
 1750   a parenthesized subpattern (including assertions)
 1751   a subroutine call to a subpattern (recursive or otherwise)
 1752 </pre>
 1753 The general repetition quantifier specifies a minimum and maximum number of
 1754 permitted matches, by giving the two numbers in curly brackets (braces),
 1755 separated by a comma. The numbers must be less than 65536, and the first must
 1756 be less than or equal to the second. For example:
 1757 <pre>
 1758   z{2,4}
 1759 </pre>
 1760 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
 1761 character. If the second number is omitted, but the comma is present, there is
 1762 no upper limit; if the second number and the comma are both omitted, the
 1763 quantifier specifies an exact number of required matches. Thus
 1764 <pre>
 1765   [aeiou]{3,}
 1766 </pre>
 1767 matches at least 3 successive vowels, but may match many more, while
 1768 <pre>
 1769   \d{8}
 1770 </pre>
 1771 matches exactly 8 digits. An opening curly bracket that appears in a position
 1772 where a quantifier is not allowed, or one that does not match the syntax of a
 1773 quantifier, is taken as a literal character. For example, {,6} is not a
 1774 quantifier, but a literal string of four characters.
 1775 </P>
 1776 <P>
 1777 In UTF modes, quantifiers apply to characters rather than to individual data
 1778 units. Thus, for example, \x{100}{2} matches two characters, each of
 1779 which is represented by a two-byte sequence in a UTF-8 string. Similarly,
 1780 \X{3} matches three Unicode extended grapheme clusters, each of which may be
 1781 several data units long (and they may be of different lengths).
 1782 </P>
 1783 <P>
 1784 The quantifier {0} is permitted, causing the expression to behave as if the
 1785 previous item and the quantifier were not present. This may be useful for
 1786 subpatterns that are referenced as
 1787 <a href="#subpatternsassubroutines">subroutines</a>
 1788 from elsewhere in the pattern (but see also the section entitled
 1789 <a href="#subdefine">"Defining subpatterns for use by reference only"</a>
 1790 below). Items other than subpatterns that have a {0} quantifier are omitted
 1791 from the compiled pattern.
 1792 </P>
 1793 <P>
 1794 For convenience, the three most common quantifiers have single-character
 1795 abbreviations:
 1796 <pre>
 1797   *    is equivalent to {0,}
 1798   +    is equivalent to {1,}
 1799   ?    is equivalent to {0,1}
 1800 </pre>
 1801 It is possible to construct infinite loops by following a subpattern that can
 1802 match no characters with a quantifier that has no upper limit, for example:
 1803 <pre>
 1804   (a?)*
 1805 </pre>
 1806 Earlier versions of Perl and PCRE used to give an error at compile time for
 1807 such patterns. However, because there are cases where this can be useful, such
 1808 patterns are now accepted, but if any repetition of the subpattern does in fact
 1809 match no characters, the loop is forcibly broken.
 1810 </P>
 1811 <P>
 1812 By default, the quantifiers are "greedy", that is, they match as much as
 1813 possible (up to the maximum number of permitted times), without causing the
 1814 rest of the pattern to fail. The classic example of where this gives problems
 1815 is in trying to match comments in C programs. These appear between /* and */
 1816 and within the comment, individual * and / characters may appear. An attempt to
 1817 match C comments by applying the pattern
 1818 <pre>
 1819   /\*.*\*/
 1820 </pre>
 1821 to the string
 1822 <pre>
 1823   /* first comment */  not comment  /* second comment */
 1824 </pre>
 1825 fails, because it matches the entire string owing to the greediness of the .*
 1826 item.
 1827 </P>
 1828 <P>
 1829 However, if a quantifier is followed by a question mark, it ceases to be
 1830 greedy, and instead matches the minimum number of times possible, so the
 1831 pattern
 1832 <pre>
 1833   /\*.*?\*/
 1834 </pre>
 1835 does the right thing with the C comments. The meaning of the various
 1836 quantifiers is not otherwise changed, just the preferred number of matches.
 1837 Do not confuse this use of question mark with its use as a quantifier in its
 1838 own right. Because it has two uses, it can sometimes appear doubled, as in
 1839 <pre>
 1840   \d??\d
 1841 </pre>
 1842 which matches one digit by preference, but can match two if that is the only
 1843 way the rest of the pattern matches.
 1844 </P>
 1845 <P>
 1846 If the PCRE_UNGREEDY option is set (an option that is not available in Perl),
 1847 the quantifiers are not greedy by default, but individual ones can be made
 1848 greedy by following them with a question mark. In other words, it inverts the
 1849 default behaviour.
 1850 </P>
 1851 <P>
 1852 When a parenthesized subpattern is quantified with a minimum repeat count that
 1853 is greater than 1 or with a limited maximum, more memory is required for the
 1854 compiled pattern, in proportion to the size of the minimum or maximum.
 1855 </P>
 1856 <P>
 1857 If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
 1858 to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is
 1859 implicitly anchored, because whatever follows will be tried against every
 1860 character position in the subject string, so there is no point in retrying the
 1861 overall match at any position after the first. PCRE normally treats such a
 1862 pattern as though it were preceded by \A.
 1863 </P>
 1864 <P>
 1865 In cases where it is known that the subject string contains no newlines, it is
 1866 worth setting PCRE_DOTALL in order to obtain this optimization, or
 1867 alternatively using ^ to indicate anchoring explicitly.
 1868 </P>
 1869 <P>
 1870 However, there are some cases where the optimization cannot be used. When .*
 1871 is inside capturing parentheses that are the subject of a back reference
 1872 elsewhere in the pattern, a match at the start may fail where a later one
 1873 succeeds. Consider, for example:
 1874 <pre>
 1875   (.*)abc\1
 1876 </pre>
 1877 If the subject is "xyz123abc123" the match point is the fourth character. For
 1878 this reason, such a pattern is not implicitly anchored.
 1879 </P>
 1880 <P>
 1881 Another case where implicit anchoring is not applied is when the leading .* is
 1882 inside an atomic group. Once again, a match at the start may fail where a later
 1883 one succeeds. Consider this pattern:
 1884 <pre>
 1885   (?&#62;.*?a)b
 1886 </pre>
 1887 It matches "ab" in the subject "aab". The use of the backtracking control verbs
 1888 (*PRUNE) and (*SKIP) also disable this optimization.
 1889 </P>
 1890 <P>
 1891 When a capturing subpattern is repeated, the value captured is the substring
 1892 that matched the final iteration. For example, after
 1893 <pre>
 1894   (tweedle[dume]{3}\s*)+
 1895 </pre>
 1896 has matched "tweedledum tweedledee" the value of the captured substring is
 1897 "tweedledee". However, if there are nested capturing subpatterns, the
 1898 corresponding captured values may have been set in previous iterations. For
 1899 example, after
 1900 <pre>
 1901   /(a|(b))+/
 1902 </pre>
 1903 matches "aba" the value of the second captured substring is "b".
 1904 <a name="atomicgroup"></a></P>
 1905 <br><a name="SEC18" href="#TOC1">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a><br>
 1906 <P>
 1907 With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
 1908 repetition, failure of what follows normally causes the repeated item to be
 1909 re-evaluated to see if a different number of repeats allows the rest of the
 1910 pattern to match. Sometimes it is useful to prevent this, either to change the
 1911 nature of the match, or to cause it fail earlier than it otherwise might, when
 1912 the author of the pattern knows there is no point in carrying on.
 1913 </P>
 1914 <P>
 1915 Consider, for example, the pattern \d+foo when applied to the subject line
 1916 <pre>
 1917   123456bar
 1918 </pre>
 1919 After matching all 6 digits and then failing to match "foo", the normal
 1920 action of the matcher is to try again with only 5 digits matching the \d+
 1921 item, and then with 4, and so on, before ultimately failing. "Atomic grouping"
 1922 (a term taken from Jeffrey Friedl's book) provides the means for specifying
 1923 that once a subpattern has matched, it is not to be re-evaluated in this way.
 1924 </P>
 1925 <P>
 1926 If we use atomic grouping for the previous example, the matcher gives up
 1927 immediately on failing to match "foo" the first time. The notation is a kind of
 1928 special parenthesis, starting with (?&#62; as in this example:
 1929 <pre>
 1930   (?&#62;\d+)foo
 1931 </pre>
 1932 This kind of parenthesis "locks up" the  part of the pattern it contains once
 1933 it has matched, and a failure further into the pattern is prevented from
 1934 backtracking into it. Backtracking past it to previous items, however, works as
 1935 normal.
 1936 </P>
 1937 <P>
 1938 An alternative description is that a subpattern of this type matches the string
 1939 of characters that an identical standalone pattern would match, if anchored at
 1940 the current point in the subject string.
 1941 </P>
 1942 <P>
 1943 Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
 1944 the above example can be thought of as a maximizing repeat that must swallow
 1945 everything it can. So, while both \d+ and \d+? are prepared to adjust the
 1946 number of digits they match in order to make the rest of the pattern match,
 1947 (?&#62;\d+) can only match an entire sequence of digits.
 1948 </P>
 1949 <P>
 1950 Atomic groups in general can of course contain arbitrarily complicated
 1951 subpatterns, and can be nested. However, when the subpattern for an atomic
 1952 group is just a single repeated item, as in the example above, a simpler
 1953 notation, called a "possessive quantifier" can be used. This consists of an
 1954 additional + character following a quantifier. Using this notation, the
 1955 previous example can be rewritten as
 1956 <pre>
 1957   \d++foo
 1958 </pre>
 1959 Note that a possessive quantifier can be used with an entire group, for
 1960 example:
 1961 <pre>
 1962   (abc|xyz){2,3}+
 1963 </pre>
 1964 Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
 1965 option is ignored. They are a convenient notation for the simpler forms of
 1966 atomic group. However, there is no difference in the meaning of a possessive
 1967 quantifier and the equivalent atomic group, though there may be a performance
 1968 difference; possessive quantifiers should be slightly faster.
 1969 </P>
 1970 <P>
 1971 The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
 1972 Jeffrey Friedl originated the idea (and the name) in the first edition of his
 1973 book. Mike McCloskey liked it, so implemented it when he built Sun's Java
 1974 package, and PCRE copied it from there. It ultimately found its way into Perl
 1975 at release 5.10.
 1976 </P>
 1977 <P>
 1978 PCRE has an optimization that automatically "possessifies" certain simple
 1979 pattern constructs. For example, the sequence A+B is treated as A++B because
 1980 there is no point in backtracking into a sequence of A's when B must follow.
 1981 </P>
 1982 <P>
 1983 When a pattern contains an unlimited repeat inside a subpattern that can itself
 1984 be repeated an unlimited number of times, the use of an atomic group is the
 1985 only way to avoid some failing matches taking a very long time indeed. The
 1986 pattern
 1987 <pre>
 1988   (\D+|&#60;\d+&#62;)*[!?]
 1989 </pre>
 1990 matches an unlimited number of substrings that either consist of non-digits, or
 1991 digits enclosed in &#60;&#62;, followed by either ! or ?. When it matches, it runs
 1992 quickly. However, if it is applied to
 1993 <pre>
 1994   aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
 1995 </pre>
 1996 it takes a long time before reporting failure. This is because the string can
 1997 be divided between the internal \D+ repeat and the external * repeat in a
 1998 large number of ways, and all have to be tried. (The example uses [!?] rather
 1999 than a single character at the end, because both PCRE and Perl have an
 2000 optimization that allows for fast failure when a single character is used. They
 2001 remember the last single character that is required for a match, and fail early
 2002 if it is not present in the string.) If the pattern is changed so that it uses
 2003 an atomic group, like this:
 2004 <pre>
 2005   ((?&#62;\D+)|&#60;\d+&#62;)*[!?]
 2006 </pre>
 2007 sequences of non-digits cannot be broken, and failure happens quickly.
 2008 <a name="backreferences"></a></P>
 2009 <br><a name="SEC19" href="#TOC1">BACK REFERENCES</a><br>
 2010 <P>
 2011 Outside a character class, a backslash followed by a digit greater than 0 (and
 2012 possibly further digits) is a back reference to a capturing subpattern earlier
 2013 (that is, to its left) in the pattern, provided there have been that many
 2014 previous capturing left parentheses.
 2015 </P>
 2016 <P>
 2017 However, if the decimal number following the backslash is less than 10, it is
 2018 always taken as a back reference, and causes an error only if there are not
 2019 that many capturing left parentheses in the entire pattern. In other words, the
 2020 parentheses that are referenced need not be to the left of the reference for
 2021 numbers less than 10. A "forward back reference" of this type can make sense
 2022 when a repetition is involved and the subpattern to the right has participated
 2023 in an earlier iteration.
 2024 </P>
 2025 <P>
 2026 It is not possible to have a numerical "forward back reference" to a subpattern
 2027 whose number is 10 or more using this syntax because a sequence such as \50 is
 2028 interpreted as a character defined in octal. See the subsection entitled
 2029 "Non-printing characters"
 2030 <a href="#digitsafterbackslash">above</a>
 2031 for further details of the handling of digits following a backslash. There is
 2032 no such problem when named parentheses are used. A back reference to any
 2033 subpattern is possible using named parentheses (see below).
 2034 </P>
 2035 <P>
 2036 Another way of avoiding the ambiguity inherent in the use of digits following a
 2037 backslash is to use the \g escape sequence. This escape must be followed by an
 2038 unsigned number or a negative number, optionally enclosed in braces. These
 2039 examples are all identical:
 2040 <pre>
 2041   (ring), \1
 2042   (ring), \g1
 2043   (ring), \g{1}
 2044 </pre>
 2045 An unsigned number specifies an absolute reference without the ambiguity that
 2046 is present in the older syntax. It is also useful when literal digits follow
 2047 the reference. A negative number is a relative reference. Consider this
 2048 example:
 2049 <pre>
 2050   (abc(def)ghi)\g{-1}
 2051 </pre>
 2052 The sequence \g{-1} is a reference to the most recently started capturing
 2053 subpattern before \g, that is, is it equivalent to \2 in this example.
 2054 Similarly, \g{-2} would be equivalent to \1. The use of relative references
 2055 can be helpful in long patterns, and also in patterns that are created by
 2056 joining together fragments that contain references within themselves.
 2057 </P>
 2058 <P>
 2059 A back reference matches whatever actually matched the capturing subpattern in
 2060 the current subject string, rather than anything matching the subpattern
 2061 itself (see
 2062 <a href="#subpatternsassubroutines">"Subpatterns as subroutines"</a>
 2063 below for a way of doing that). So the pattern
 2064 <pre>
 2065   (sens|respons)e and \1ibility
 2066 </pre>
 2067 matches "sense and sensibility" and "response and responsibility", but not
 2068 "sense and responsibility". If caseful matching is in force at the time of the
 2069 back reference, the case of letters is relevant. For example,
 2070 <pre>
 2071   ((?i)rah)\s+\1
 2072 </pre>
 2073 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
 2074 capturing subpattern is matched caselessly.
 2075 </P>
 2076 <P>
 2077 There are several different ways of writing back references to named
 2078 subpatterns. The .NET syntax \k{name} and the Perl syntax \k&#60;name&#62; or
 2079 \k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
 2080 back reference syntax, in which \g can be used for both numeric and named
 2081 references, is also supported. We could rewrite the above example in any of
 2082 the following ways:
 2083 <pre>
 2084   (?&#60;p1&#62;(?i)rah)\s+\k&#60;p1&#62;
 2085   (?'p1'(?i)rah)\s+\k{p1}
 2086   (?P&#60;p1&#62;(?i)rah)\s+(?P=p1)
 2087   (?&#60;p1&#62;(?i)rah)\s+\g{p1}
 2088 </pre>
 2089 A subpattern that is referenced by name may appear in the pattern before or
 2090 after the reference.
 2091 </P>
 2092 <P>
 2093 There may be more than one back reference to the same subpattern. If a
 2094 subpattern has not actually been used in a particular match, any back
 2095 references to it always fail by default. For example, the pattern
 2096 <pre>
 2097   (a|(bc))\2
 2098 </pre>
 2099 always fails if it starts to match "a" rather than "bc". However, if the
 2100 PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an
 2101 unset value matches an empty string.
 2102 </P>
 2103 <P>
 2104 Because there may be many capturing parentheses in a pattern, all digits
 2105 following a backslash are taken as part of a potential back reference number.
 2106 If the pattern continues with a digit character, some delimiter must be used to
 2107 terminate the back reference. If the PCRE_EXTENDED option is set, this can be
 2108 white space. Otherwise, the \g{ syntax or an empty comment (see
 2109 <a href="#comments">"Comments"</a>
 2110 below) can be used.
 2111 </P>
 2112 <br><b>
 2113 Recursive back references
 2114 </b><br>
 2115 <P>
 2116 A back reference that occurs inside the parentheses to which it refers fails
 2117 when the subpattern is first used, so, for example, (a\1) never matches.
 2118 However, such references can be useful inside repeated subpatterns. For
 2119 example, the pattern
 2120 <pre>
 2121   (a|b\1)+
 2122 </pre>
 2123 matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of
 2124 the subpattern, the back reference matches the character string corresponding
 2125 to the previous iteration. In order for this to work, the pattern must be such
 2126 that the first iteration does not need to match the back reference. This can be
 2127 done using alternation, as in the example above, or by a quantifier with a
 2128 minimum of zero.
 2129 </P>
 2130 <P>
 2131 Back references of this type cause the group that they reference to be treated
 2132 as an
 2133 <a href="#atomicgroup">atomic group.</a>
 2134 Once the whole group has been matched, a subsequent matching failure cannot
 2135 cause backtracking into the middle of the group.
 2136 <a name="bigassertions"></a></P>
 2137 <br><a name="SEC20" href="#TOC1">ASSERTIONS</a><br>
 2138 <P>
 2139 An assertion is a test on the characters following or preceding the current
 2140 matching point that does not actually consume any characters. The simple
 2141 assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are described
 2142 <a href="#smallassertions">above.</a>
 2143 </P>
 2144 <P>
 2145 More complicated assertions are coded as subpatterns. There are two kinds:
 2146 those that look ahead of the current position in the subject string, and those
 2147 that look behind it. An assertion subpattern is matched in the normal way,
 2148 except that it does not cause the current matching position to be changed.
 2149 </P>
 2150 <P>
 2151 Assertion subpatterns are not capturing subpatterns. If such an assertion
 2152 contains capturing subpatterns within it, these are counted for the purposes of
 2153 numbering the capturing subpatterns in the whole pattern. However, substring
 2154 capturing is carried out only for positive assertions. (Perl sometimes, but not
 2155 always, does do capturing in negative assertions.)
 2156 </P>
 2157 <P>
 2158 WARNING: If a positive assertion containing one or more capturing subpatterns
 2159 succeeds, but failure to match later in the pattern causes backtracking over
 2160 this assertion, the captures within the assertion are reset only if no higher
 2161 numbered captures are already set. This is, unfortunately, a fundamental
 2162 limitation of the current implementation, and as PCRE1 is now in
 2163 maintenance-only status, it is unlikely ever to change.
 2164 </P>
 2165 <P>
 2166 For compatibility with Perl, assertion subpatterns may be repeated; though
 2167 it makes no sense to assert the same thing several times, the side effect of
 2168 capturing parentheses may occasionally be useful. In practice, there only three
 2169 cases:
 2170 <br>
 2171 <br>
 2172 (1) If the quantifier is {0}, the assertion is never obeyed during matching.
 2173 However, it may contain internal capturing parenthesized groups that are called
 2174 from elsewhere via the
 2175 <a href="#subpatternsassubroutines">subroutine mechanism.</a>
 2176 <br>
 2177 <br>
 2178 (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
 2179 were {0,1}. At run time, the rest of the pattern match is tried with and
 2180 without the assertion, the order depending on the greediness of the quantifier.
 2181 <br>
 2182 <br>
 2183 (3) If the minimum repetition is greater than zero, the quantifier is ignored.
 2184 The assertion is obeyed just once when encountered during matching.
 2185 </P>
 2186 <br><b>
 2187 Lookahead assertions
 2188 </b><br>
 2189 <P>
 2190 Lookahead assertions start with (?= for positive assertions and (?! for
 2191 negative assertions. For example,
 2192 <pre>
 2193   \w+(?=;)
 2194 </pre>
 2195 matches a word followed by a semicolon, but does not include the semicolon in
 2196 the match, and
 2197 <pre>
 2198   foo(?!bar)
 2199 </pre>
 2200 matches any occurrence of "foo" that is not followed by "bar". Note that the
 2201 apparently similar pattern
 2202 <pre>
 2203   (?!foo)bar
 2204 </pre>
 2205 does not find an occurrence of "bar" that is preceded by something other than
 2206 "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
 2207 (?!foo) is always true when the next three characters are "bar". A
 2208 lookbehind assertion is needed to achieve the other effect.
 2209 </P>
 2210 <P>
 2211 If you want to force a matching failure at some point in a pattern, the most
 2212 convenient way to do it is with (?!) because an empty string always matches, so
 2213 an assertion that requires there not to be an empty string must always fail.
 2214 The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
 2215 <a name="lookbehind"></a></P>
 2216 <br><b>
 2217 Lookbehind assertions
 2218 </b><br>
 2219 <P>
 2220 Lookbehind assertions start with (?&#60;= for positive assertions and (?&#60;! for
 2221 negative assertions. For example,
 2222 <pre>
 2223   (?&#60;!foo)bar
 2224 </pre>
 2225 does find an occurrence of "bar" that is not preceded by "foo". The contents of
 2226 a lookbehind assertion are restricted such that all the strings it matches must
 2227 have a fixed length. However, if there are several top-level alternatives, they
 2228 do not all have to have the same fixed length. Thus
 2229 <pre>
 2230   (?&#60;=bullock|donkey)
 2231 </pre>
 2232 is permitted, but
 2233 <pre>
 2234   (?&#60;!dogs?|cats?)
 2235 </pre>
 2236 causes an error at compile time. Branches that match different length strings
 2237 are permitted only at the top level of a lookbehind assertion. This is an
 2238 extension compared with Perl, which requires all branches to match the same
 2239 length of string. An assertion such as
 2240 <pre>
 2241   (?&#60;=ab(c|de))
 2242 </pre>
 2243 is not permitted, because its single top-level branch can match two different
 2244 lengths, but it is acceptable to PCRE if rewritten to use two top-level
 2245 branches:
 2246 <pre>
 2247   (?&#60;=abc|abde)
 2248 </pre>
 2249 In some cases, the escape sequence \K
 2250 <a href="#resetmatchstart">(see above)</a>
 2251 can be used instead of a lookbehind assertion to get round the fixed-length
 2252 restriction.
 2253 </P>
 2254 <P>
 2255 The implementation of lookbehind assertions is, for each alternative, to
 2256 temporarily move the current position back by the fixed length and then try to
 2257 match. If there are insufficient characters before the current position, the
 2258 assertion fails.
 2259 </P>
 2260 <P>
 2261 In a UTF mode, PCRE does not allow the \C escape (which matches a single data
 2262 unit even in a UTF mode) to appear in lookbehind assertions, because it makes
 2263 it impossible to calculate the length of the lookbehind. The \X and \R
 2264 escapes, which can match different numbers of data units, are also not
 2265 permitted.
 2266 </P>
 2267 <P>
 2268 <a href="#subpatternsassubroutines">"Subroutine"</a>
 2269 calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
 2270 as the subpattern matches a fixed-length string.
 2271 <a href="#recursion">Recursion,</a>
 2272 however, is not supported.
 2273 </P>
 2274 <P>
 2275 Possessive quantifiers can be used in conjunction with lookbehind assertions to
 2276 specify efficient matching of fixed-length strings at the end of subject
 2277 strings. Consider a simple pattern such as
 2278 <pre>
 2279   abcd$
 2280 </pre>
 2281 when applied to a long string that does not match. Because matching proceeds
 2282 from left to right, PCRE will look for each "a" in the subject and then see if
 2283 what follows matches the rest of the pattern. If the pattern is specified as
 2284 <pre>
 2285   ^.*abcd$
 2286 </pre>
 2287 the initial .* matches the entire string at first, but when this fails (because
 2288 there is no following "a"), it backtracks to match all but the last character,
 2289 then all but the last two characters, and so on. Once again the search for "a"
 2290 covers the entire string, from right to left, so we are no better off. However,
 2291 if the pattern is written as
 2292 <pre>
 2293   ^.*+(?&#60;=abcd)
 2294 </pre>
 2295 there can be no backtracking for the .*+ item; it can match only the entire
 2296 string. The subsequent lookbehind assertion does a single test on the last four
 2297 characters. If it fails, the match fails immediately. For long strings, this
 2298 approach makes a significant difference to the processing time.
 2299 </P>
 2300 <br><b>
 2301 Using multiple assertions
 2302 </b><br>
 2303 <P>
 2304 Several assertions (of any sort) may occur in succession. For example,
 2305 <pre>
 2306   (?&#60;=\d{3})(?&#60;!999)foo
 2307 </pre>
 2308 matches "foo" preceded by three digits that are not "999". Notice that each of
 2309 the assertions is applied independently at the same point in the subject
 2310 string. First there is a check that the previous three characters are all
 2311 digits, and then there is a check that the same three characters are not "999".
 2312 This pattern does <i>not</i> match "foo" preceded by six characters, the first
 2313 of which are digits and the last three of which are not "999". For example, it
 2314 doesn't match "123abcfoo". A pattern to do that is
 2315 <pre>
 2316   (?&#60;=\d{3}...)(?&#60;!999)foo
 2317 </pre>
 2318 This time the first assertion looks at the preceding six characters, checking
 2319 that the first three are digits, and then the second assertion checks that the
 2320 preceding three characters are not "999".
 2321 </P>
 2322 <P>
 2323 Assertions can be nested in any combination. For example,
 2324 <pre>
 2325   (?&#60;=(?&#60;!foo)bar)baz
 2326 </pre>
 2327 matches an occurrence of "baz" that is preceded by "bar" which in turn is not
 2328 preceded by "foo", while
 2329 <pre>
 2330   (?&#60;=\d{3}(?!999)...)foo
 2331 </pre>
 2332 is another pattern that matches "foo" preceded by three digits and any three
 2333 characters that are not "999".
 2334 <a name="conditions"></a></P>
 2335 <br><a name="SEC21" href="#TOC1">CONDITIONAL SUBPATTERNS</a><br>
 2336 <P>
 2337 It is possible to cause the matching process to obey a subpattern
 2338 conditionally or to choose between two alternative subpatterns, depending on
 2339 the result of an assertion, or whether a specific capturing subpattern has
 2340 already been matched. The two possible forms of conditional subpattern are:
 2341 <pre>
 2342   (?(condition)yes-pattern)
 2343   (?(condition)yes-pattern|no-pattern)
 2344 </pre>
 2345 If the condition is satisfied, the yes-pattern is used; otherwise the
 2346 no-pattern (if present) is used. If there are more than two alternatives in the
 2347 subpattern, a compile-time error occurs. Each of the two alternatives may
 2348 itself contain nested subpatterns of any form, including conditional
 2349 subpatterns; the restriction to two alternatives applies only at the level of
 2350 the condition. This pattern fragment is an example where the alternatives are
 2351 complex:
 2352 <pre>
 2353   (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
 2355 </PRE>
 2356 </P>
 2357 <P>
 2358 There are four kinds of condition: references to subpatterns, references to
 2359 recursion, a pseudo-condition called DEFINE, and assertions.
 2360 </P>
 2361 <br><b>
 2362 Checking for a used subpattern by number
 2363 </b><br>
 2364 <P>
 2365 If the text between the parentheses consists of a sequence of digits, the
 2366 condition is true if a capturing subpattern of that number has previously
 2367 matched. If there is more than one capturing subpattern with the same number
 2368 (see the earlier
 2369 <a href="#recursion">section about duplicate subpattern numbers),</a>
 2370 the condition is true if any of them have matched. An alternative notation is
 2371 to precede the digits with a plus or minus sign. In this case, the subpattern
 2372 number is relative rather than absolute. The most recently opened parentheses
 2373 can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
 2374 loops it can also make sense to refer to subsequent groups. The next
 2375 parentheses to be opened can be referenced as (?(+1), and so on. (The value
 2376 zero in any of these forms is not used; it provokes a compile-time error.)
 2377 </P>
 2378 <P>
 2379 Consider the following pattern, which contains non-significant white space to
 2380 make it more readable (assume the PCRE_EXTENDED option) and to divide it into
 2381 three parts for ease of discussion:
 2382 <pre>
 2383   ( \( )?    [^()]+    (?(1) \) )
 2384 </pre>
 2385 The first part matches an optional opening parenthesis, and if that
 2386 character is present, sets it as the first captured substring. The second part
 2387 matches one or more characters that are not parentheses. The third part is a
 2388 conditional subpattern that tests whether or not the first set of parentheses
 2389 matched. If they did, that is, if subject started with an opening parenthesis,
 2390 the condition is true, and so the yes-pattern is executed and a closing
 2391 parenthesis is required. Otherwise, since no-pattern is not present, the
 2392 subpattern matches nothing. In other words, this pattern matches a sequence of
 2393 non-parentheses, optionally enclosed in parentheses.
 2394 </P>
 2395 <P>
 2396 If you were embedding this pattern in a larger one, you could use a relative
 2397 reference:
 2398 <pre>
 2399   ...other stuff... ( \( )?    [^()]+    (?(-1) \) ) ...
 2400 </pre>
 2401 This makes the fragment independent of the parentheses in the larger pattern.
 2402 </P>
 2403 <br><b>
 2404 Checking for a used subpattern by name
 2405 </b><br>
 2406 <P>
 2407 Perl uses the syntax (?(&#60;name&#62;)...) or (?('name')...) to test for a used
 2408 subpattern by name. For compatibility with earlier versions of PCRE, which had
 2409 this facility before Perl, the syntax (?(name)...) is also recognized.
 2410 </P>
 2411 <P>
 2412 Rewriting the above example to use a named subpattern gives this:
 2413 <pre>
 2414   (?&#60;OPEN&#62; \( )?    [^()]+    (?(&#60;OPEN&#62;) \) )
 2415 </pre>
 2416 If the name used in a condition of this kind is a duplicate, the test is
 2417 applied to all subpatterns of the same name, and is true if any one of them has
 2418 matched.
 2419 </P>
 2420 <br><b>
 2421 Checking for pattern recursion
 2422 </b><br>
 2423 <P>
 2424 If the condition is the string (R), and there is no subpattern with the name R,
 2425 the condition is true if a recursive call to the whole pattern or any
 2426 subpattern has been made. If digits or a name preceded by ampersand follow the
 2427 letter R, for example:
 2428 <pre>
 2429   (?(R3)...) or (?(R&name)...)
 2430 </pre>
 2431 the condition is true if the most recent recursion is into a subpattern whose
 2432 number or name is given. This condition does not check the entire recursion
 2433 stack. If the name used in a condition of this kind is a duplicate, the test is
 2434 applied to all subpatterns of the same name, and is true if any one of them is
 2435 the most recent recursion.
 2436 </P>
 2437 <P>
 2438 At "top level", all these recursion test conditions are false.
 2439 <a href="#recursion">The syntax for recursive patterns</a>
 2440 is described below.
 2441 <a name="subdefine"></a></P>
 2442 <br><b>
 2443 Defining subpatterns for use by reference only
 2444 </b><br>
 2445 <P>
 2446 If the condition is the string (DEFINE), and there is no subpattern with the
 2447 name DEFINE, the condition is always false. In this case, there may be only one
 2448 alternative in the subpattern. It is always skipped if control reaches this
 2449 point in the pattern; the idea of DEFINE is that it can be used to define
 2450 subroutines that can be referenced from elsewhere. (The use of
 2451 <a href="#subpatternsassubroutines">subroutines</a>
 2452 is described below.) For example, a pattern to match an IPv4 address such as
 2453 "" could be written like this (ignore white space and line
 2454 breaks):
 2455 <pre>
 2456   (?(DEFINE) (?&#60;byte&#62; 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
 2457   \b (?&byte) (\.(?&byte)){3} \b
 2458 </pre>
 2459 The first part of the pattern is a DEFINE group inside which a another group
 2460 named "byte" is defined. This matches an individual component of an IPv4
 2461 address (a number less than 256). When matching takes place, this part of the
 2462 pattern is skipped because DEFINE acts like a false condition. The rest of the
 2463 pattern uses references to the named group to match the four dot-separated
 2464 components of an IPv4 address, insisting on a word boundary at each end.
 2465 </P>
 2466 <br><b>
 2467 Assertion conditions
 2468 </b><br>
 2469 <P>
 2470 If the condition is not in any of the above formats, it must be an assertion.
 2471 This may be a positive or negative lookahead or lookbehind assertion. Consider
 2472 this pattern, again containing non-significant white space, and with the two
 2473 alternatives on the second line:
 2474 <pre>
 2475   (?(?=[^a-z]*[a-z])
 2476   \d{2}-[a-z]{3}-\d{2}  |  \d{2}-\d{2}-\d{2} )
 2477 </pre>
 2478 The condition is a positive lookahead assertion that matches an optional
 2479 sequence of non-letters followed by a letter. In other words, it tests for the
 2480 presence of at least one letter in the subject. If a letter is found, the
 2481 subject is matched against the first alternative; otherwise it is matched
 2482 against the second. This pattern matches strings in one of the two forms
 2483 dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
 2484 <a name="comments"></a></P>
 2485 <br><a name="SEC22" href="#TOC1">COMMENTS</a><br>
 2486 <P>
 2487 There are two ways of including comments in patterns that are processed by
 2488 PCRE. In both cases, the start of the comment must not be in a character class,
 2489 nor in the middle of any other sequence of related characters such as (?: or a
 2490 subpattern name or number. The characters that make up a comment play no part
 2491 in the pattern matching.
 2492 </P>
 2493 <P>
 2494 The sequence (?# marks the start of a comment that continues up to the next
 2495 closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
 2496 option is set, an unescaped # character also introduces a comment, which in
 2497 this case continues to immediately after the next newline character or
 2498 character sequence in the pattern. Which characters are interpreted as newlines
 2499 is controlled by the options passed to a compiling function or by a special
 2500 sequence at the start of the pattern, as described in the section entitled
 2501 <a href="#newlines">"Newline conventions"</a>
 2502 above. Note that the end of this type of comment is a literal newline sequence
 2503 in the pattern; escape sequences that happen to represent a newline do not
 2504 count. For example, consider this pattern when PCRE_EXTENDED is set, and the
 2505 default newline convention is in force:
 2506 <pre>
 2507   abc #comment \n still comment
 2508 </pre>
 2509 On encountering the # character, <b>pcre_compile()</b> skips along, looking for
 2510 a newline in the pattern. The sequence \n is still literal at this stage, so
 2511 it does not terminate the comment. Only an actual character with the code value
 2512 0x0a (the default newline) does so.
 2513 <a name="recursion"></a></P>
 2514 <br><a name="SEC23" href="#TOC1">RECURSIVE PATTERNS</a><br>
 2515 <P>
 2516 Consider the problem of matching a string in parentheses, allowing for
 2517 unlimited nested parentheses. Without the use of recursion, the best that can
 2518 be done is to use a pattern that matches up to some fixed depth of nesting. It
 2519 is not possible to handle an arbitrary nesting depth.
 2520 </P>
 2521 <P>
 2522 For some time, Perl has provided a facility that allows regular expressions to
 2523 recurse (amongst other things). It does this by interpolating Perl code in the
 2524 expression at run time, and the code can refer to the expression itself. A Perl
 2525 pattern using code interpolation to solve the parentheses problem can be
 2526 created like this:
 2527 <pre>
 2528   $re = qr{\( (?: (?&#62;[^()]+) | (?p{$re}) )* \)}x;
 2529 </pre>
 2530 The (?p{...}) item interpolates Perl code at run time, and in this case refers
 2531 recursively to the pattern in which it appears.
 2532 </P>
 2533 <P>
 2534 Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
 2535 supports special syntax for recursion of the entire pattern, and also for
 2536 individual subpattern recursion. After its introduction in PCRE and Python,
 2537 this kind of recursion was subsequently introduced into Perl at release 5.10.
 2538 </P>
 2539 <P>
 2540 A special item that consists of (? followed by a number greater than zero and a
 2541 closing parenthesis is a recursive subroutine call of the subpattern of the
 2542 given number, provided that it occurs inside that subpattern. (If not, it is a
 2543 <a href="#subpatternsassubroutines">non-recursive subroutine</a>
 2544 call, which is described in the next section.) The special item (?R) or (?0) is
 2545 a recursive call of the entire regular expression.
 2546 </P>
 2547 <P>
 2548 This PCRE pattern solves the nested parentheses problem (assume the
 2549 PCRE_EXTENDED option is set so that white space is ignored):
 2550 <pre>
 2551   \( ( [^()]++ | (?R) )* \)
 2552 </pre>
 2553 First it matches an opening parenthesis. Then it matches any number of
 2554 substrings which can either be a sequence of non-parentheses, or a recursive
 2555 match of the pattern itself (that is, a correctly parenthesized substring).
 2556 Finally there is a closing parenthesis. Note the use of a possessive quantifier
 2557 to avoid backtracking into sequences of non-parentheses.
 2558 </P>
 2559 <P>
 2560 If this were part of a larger pattern, you would not want to recurse the entire
 2561 pattern, so instead you could use this:
 2562 <pre>
 2563   ( \( ( [^()]++ | (?1) )* \) )
 2564 </pre>
 2565 We have put the pattern into parentheses, and caused the recursion to refer to
 2566 them instead of the whole pattern.
 2567 </P>
 2568 <P>
 2569 In a larger pattern, keeping track of parenthesis numbers can be tricky. This
 2570 is made easier by the use of relative references. Instead of (?1) in the
 2571 pattern above you can write (?-2) to refer to the second most recently opened
 2572 parentheses preceding the recursion. In other words, a negative number counts
 2573 capturing parentheses leftwards from the point at which it is encountered.
 2574 </P>
 2575 <P>
 2576 It is also possible to refer to subsequently opened parentheses, by writing
 2577 references such as (?+2). However, these cannot be recursive because the
 2578 reference is not inside the parentheses that are referenced. They are always
 2579 <a href="#subpatternsassubroutines">non-recursive subroutine</a>
 2580 calls, as described in the next section.
 2581 </P>
 2582 <P>
 2583 An alternative approach is to use named parentheses instead. The Perl syntax
 2584 for this is (?&name); PCRE's earlier syntax (?P&#62;name) is also supported. We
 2585 could rewrite the above example as follows:
 2586 <pre>
 2587   (?&#60;pn&#62; \( ( [^()]++ | (?&pn) )* \) )
 2588 </pre>
 2589 If there is more than one subpattern with the same name, the earliest one is
 2590 used.
 2591 </P>
 2592 <P>
 2593 This particular example pattern that we have been looking at contains nested
 2594 unlimited repeats, and so the use of a possessive quantifier for matching
 2595 strings of non-parentheses is important when applying the pattern to strings
 2596 that do not match. For example, when this pattern is applied to
 2597 <pre>
 2598   (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
 2599 </pre>
 2600 it yields "no match" quickly. However, if a possessive quantifier is not used,
 2601 the match runs for a very long time indeed because there are so many different
 2602 ways the + and * repeats can carve up the subject, and all have to be tested
 2603 before failure can be reported.
 2604 </P>
 2605 <P>
 2606 At the end of a match, the values of capturing parentheses are those from
 2607 the outermost level. If you want to obtain intermediate values, a callout
 2608 function can be used (see below and the
 2609 <a href="pcrecallout.html"><b>pcrecallout</b></a>
 2610 documentation). If the pattern above is matched against
 2611 <pre>
 2612   (ab(cd)ef)
 2613 </pre>
 2614 the value for the inner capturing parentheses (numbered 2) is "ef", which is
 2615 the last value taken on at the top level. If a capturing subpattern is not
 2616 matched at the top level, its final captured value is unset, even if it was
 2617 (temporarily) set at a deeper level during the matching process.
 2618 </P>
 2619 <P>
 2620 If there are more than 15 capturing parentheses in a pattern, PCRE has to
 2621 obtain extra memory to store data during a recursion, which it does by using
 2622 <b>pcre_malloc</b>, freeing it via <b>pcre_free</b> afterwards. If no memory can
 2623 be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
 2624 </P>
 2625 <P>
 2626 Do not confuse the (?R) item with the condition (R), which tests for recursion.
 2627 Consider this pattern, which matches text in angle brackets, allowing for
 2628 arbitrary nesting. Only digits are allowed in nested brackets (that is, when
 2629 recursing), whereas any characters are permitted at the outer level.
 2630 <pre>
 2631   &#60; (?: (?(R) \d++  | [^&#60;&#62;]*+) | (?R)) * &#62;
 2632 </pre>
 2633 In this pattern, (?(R) is the start of a conditional subpattern, with two
 2634 different alternatives for the recursive and non-recursive cases. The (?R) item
 2635 is the actual recursive call.
 2636 <a name="recursiondifference"></a></P>
 2637 <br><b>
 2638 Differences in recursion processing between PCRE and Perl
 2639 </b><br>
 2640 <P>
 2641 Recursion processing in PCRE differs from Perl in two important ways. In PCRE
 2642 (like Python, but unlike Perl), a recursive subpattern call is always treated
 2643 as an atomic group. That is, once it has matched some of the subject string, it
 2644 is never re-entered, even if it contains untried alternatives and there is a
 2645 subsequent matching failure. This can be illustrated by the following pattern,
 2646 which purports to match a palindromic string that contains an odd number of
 2647 characters (for example, "a", "aba", "abcba", "abcdcba"):
 2648 <pre>
 2649   ^(.|(.)(?1)\2)$
 2650 </pre>
 2651 The idea is that it either matches a single character, or two identical
 2652 characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
 2653 it does not if the pattern is longer than three characters. Consider the
 2654 subject string "abcba":
 2655 </P>
 2656 <P>
 2657 At the top level, the first character is matched, but as it is not at the end
 2658 of the string, the first alternative fails; the second alternative is taken
 2659 and the recursion kicks in. The recursive call to subpattern 1 successfully
 2660 matches the next character ("b"). (Note that the beginning and end of line
 2661 tests are not part of the recursion).
 2662 </P>
 2663 <P>
 2664 Back at the top level, the next character ("c") is compared with what
 2665 subpattern 2 matched, which was "a". This fails. Because the recursion is
 2666 treated as an atomic group, there are now no backtracking points, and so the
 2667 entire match fails. (Perl is able, at this point, to re-enter the recursion and
 2668 try the second alternative.) However, if the pattern is written with the
 2669 alternatives in the other order, things are different:
 2670 <pre>
 2671   ^((.)(?1)\2|.)$
 2672 </pre>
 2673 This time, the recursing alternative is tried first, and continues to recurse
 2674 until it runs out of characters, at which point the recursion fails. But this
 2675 time we do have another alternative to try at the higher level. That is the big
 2676 difference: in the previous case the remaining alternative is at a deeper
 2677 recursion level, which PCRE cannot use.
 2678 </P>
 2679 <P>
 2680 To change the pattern so that it matches all palindromic strings, not just
 2681 those with an odd number of characters, it is tempting to change the pattern to
 2682 this:
 2683 <pre>
 2684   ^((.)(?1)\2|.?)$
 2685 </pre>
 2686 Again, this works in Perl, but not in PCRE, and for the same reason. When a
 2687 deeper recursion has matched a single character, it cannot be entered again in
 2688 order to match an empty string. The solution is to separate the two cases, and
 2689 write out the odd and even cases as alternatives at the higher level:
 2690 <pre>
 2691   ^(?:((.)(?1)\2|)|((.)(?3)\4|.))
 2692 </pre>
 2693 If you want to match typical palindromic phrases, the pattern has to ignore all
 2694 non-word characters, which can be done like this:
 2695 <pre>
 2696   ^\W*+(?:((.)\W*+(?1)\W*+\2|)|((.)\W*+(?3)\W*+\4|\W*+.\W*+))\W*+$
 2697 </pre>
 2698 If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
 2699 man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
 2700 the use of the possessive quantifier *+ to avoid backtracking into sequences of
 2701 non-word characters. Without this, PCRE takes a great deal longer (ten times or
 2702 more) to match typical phrases, and Perl takes so long that you think it has
 2703 gone into a loop.
 2704 </P>
 2705 <P>
 2706 <b>WARNING</b>: The palindrome-matching patterns above work only if the subject
 2707 string does not start with a palindrome that is shorter than the entire string.
 2708 For example, although "abcba" is correctly matched, if the subject is "ababa",
 2709 PCRE finds the palindrome "aba" at the start, then fails at top level because
 2710 the end of the string does not follow. Once again, it cannot jump back into the
 2711 recursion to try other alternatives, so the entire match fails.
 2712 </P>
 2713 <P>
 2714 The second way in which PCRE and Perl differ in their recursion processing is
 2715 in the handling of captured values. In Perl, when a subpattern is called
 2716 recursively or as a subpattern (see the next section), it has no access to any
 2717 values that were captured outside the recursion, whereas in PCRE these values
 2718 can be referenced. Consider this pattern:
 2719 <pre>
 2720   ^(.)(\1|a(?2))
 2721 </pre>
 2722 In PCRE, this pattern matches "bab". The first capturing parentheses match "b",
 2723 then in the second group, when the back reference \1 fails to match "b", the
 2724 second alternative matches "a" and then recurses. In the recursion, \1 does
 2725 now match "b" and so the whole match succeeds. In Perl, the pattern fails to
 2726 match because inside the recursive call \1 cannot access the externally set
 2727 value.
 2728 <a name="subpatternsassubroutines"></a></P>
 2729 <br><a name="SEC24" href="#TOC1">SUBPATTERNS AS SUBROUTINES</a><br>
 2730 <P>
 2731 If the syntax for a recursive subpattern call (either by number or by
 2732 name) is used outside the parentheses to which it refers, it operates like a
 2733 subroutine in a programming language. The called subpattern may be defined
 2734 before or after the reference. A numbered reference can be absolute or
 2735 relative, as in these examples:
 2736 <pre>
 2737   (...(absolute)...)...(?2)...
 2738   (...(relative)...)...(?-1)...
 2739   (...(?+1)...(relative)...
 2740 </pre>
 2741 An earlier example pointed out that the pattern
 2742 <pre>
 2743   (sens|respons)e and \1ibility
 2744 </pre>
 2745 matches "sense and sensibility" and "response and responsibility", but not
 2746 "sense and responsibility". If instead the pattern
 2747 <pre>
 2748   (sens|respons)e and (?1)ibility
 2749 </pre>
 2750 is used, it does match "sense and responsibility" as well as the other two
 2751 strings. Another example is given in the discussion of DEFINE above.
 2752 </P>
 2753 <P>
 2754 All subroutine calls, whether recursive or not, are always treated as atomic
 2755 groups. That is, once a subroutine has matched some of the subject string, it
 2756 is never re-entered, even if it contains untried alternatives and there is a
 2757 subsequent matching failure. Any capturing parentheses that are set during the
 2758 subroutine call revert to their previous values afterwards.
 2759 </P>
 2760 <P>
 2761 Processing options such as case-independence are fixed when a subpattern is
 2762 defined, so if it is used as a subroutine, such options cannot be changed for
 2763 different calls. For example, consider this pattern:
 2764 <pre>
 2765   (abc)(?i:(?-1))
 2766 </pre>
 2767 It matches "abcabc". It does not match "abcABC" because the change of
 2768 processing option does not affect the called subpattern.
 2769 <a name="onigurumasubroutines"></a></P>
 2770 <br><a name="SEC25" href="#TOC1">ONIGURUMA SUBROUTINE SYNTAX</a><br>
 2771 <P>
 2772 For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or
 2773 a number enclosed either in angle brackets or single quotes, is an alternative
 2774 syntax for referencing a subpattern as a subroutine, possibly recursively. Here
 2775 are two of the examples used above, rewritten using this syntax:
 2776 <pre>
 2777   (?&#60;pn&#62; \( ( (?&#62;[^()]+) | \g&#60;pn&#62; )* \) )
 2778   (sens|respons)e and \g'1'ibility
 2779 </pre>
 2780 PCRE supports an extension to Oniguruma: if a number is preceded by a
 2781 plus or a minus sign it is taken as a relative reference. For example:
 2782 <pre>
 2783   (abc)(?i:\g&#60;-1&#62;)
 2784 </pre>
 2785 Note that \g{...} (Perl syntax) and \g&#60;...&#62; (Oniguruma syntax) are <i>not</i>
 2786 synonymous. The former is a back reference; the latter is a subroutine call.
 2787 </P>
 2788 <br><a name="SEC26" href="#TOC1">CALLOUTS</a><br>
 2789 <P>
 2790 Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl
 2791 code to be obeyed in the middle of matching a regular expression. This makes it
 2792 possible, amongst other things, to extract different substrings that match the
 2793 same pair of parentheses when there is a repetition.
 2794 </P>
 2795 <P>
 2796 PCRE provides a similar feature, but of course it cannot obey arbitrary Perl
 2797 code. The feature is called "callout". The caller of PCRE provides an external
 2798 function by putting its entry point in the global variable <i>pcre_callout</i>
 2799 (8-bit library) or <i>pcre[16|32]_callout</i> (16-bit or 32-bit library).
 2800 By default, this variable contains NULL, which disables all calling out.
 2801 </P>
 2802 <P>
 2803 Within a regular expression, (?C) indicates the points at which the external
 2804 function is to be called. If you want to identify different callout points, you
 2805 can put a number less than 256 after the letter C. The default value is zero.
 2806 For example, this pattern has two callout points:
 2807 <pre>
 2808   (?C1)abc(?C2)def
 2809 </pre>
 2810 If the PCRE_AUTO_CALLOUT flag is passed to a compiling function, callouts are
 2811 automatically installed before each item in the pattern. They are all numbered
 2812 255. If there is a conditional group in the pattern whose condition is an
 2813 assertion, an additional callout is inserted just before the condition. An
 2814 explicit callout may also be set at this position, as in this example:
 2815 <pre>
 2816   (?(?C9)(?=a)abc|def)
 2817 </pre>
 2818 Note that this applies only to assertion conditions, not to other types of
 2819 condition.
 2820 </P>
 2821 <P>
 2822 During matching, when PCRE reaches a callout point, the external function is
 2823 called. It is provided with the number of the callout, the position in the
 2824 pattern, and, optionally, one item of data originally supplied by the caller of
 2825 the matching function. The callout function may cause matching to proceed, to
 2826 backtrack, or to fail altogether.
 2827 </P>
 2828 <P>
 2829 By default, PCRE implements a number of optimizations at compile time and
 2830 matching time, and one side-effect is that sometimes callouts are skipped. If
 2831 you need all possible callouts to happen, you need to set options that disable
 2832 the relevant optimizations. More details, and a complete description of the
 2833 interface to the callout function, are given in the
 2834 <a href="pcrecallout.html"><b>pcrecallout</b></a>
 2835 documentation.
 2836 <a name="backtrackcontrol"></a></P>
 2837 <br><a name="SEC27" href="#TOC1">BACKTRACKING CONTROL</a><br>
 2838 <P>
 2839 Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
 2840 are still described in the Perl documentation as "experimental and subject to
 2841 change or removal in a future version of Perl". It goes on to say: "Their usage
 2842 in production code should be noted to avoid problems during upgrades." The same
 2843 remarks apply to the PCRE features described in this section.
 2844 </P>
 2845 <P>
 2846 The new verbs make use of what was previously invalid syntax: an opening
 2847 parenthesis followed by an asterisk. They are generally of the form
 2848 (*VERB) or (*VERB:NAME). Some may take either form, possibly behaving
 2849 differently depending on whether or not a name is present. A name is any
 2850 sequence of characters that does not include a closing parenthesis. The maximum
 2851 length of name is 255 in the 8-bit library and 65535 in the 16-bit and 32-bit
 2852 libraries. If the name is empty, that is, if the closing parenthesis
 2853 immediately follows the colon, the effect is as if the colon were not there.
 2854 Any number of these verbs may occur in a pattern.
 2855 </P>
 2856 <P>
 2857 Since these verbs are specifically related to backtracking, most of them can be
 2858 used only when the pattern is to be matched using one of the traditional
 2859 matching functions, because these use a backtracking algorithm. With the
 2860 exception of (*FAIL), which behaves like a failing negative assertion, the
 2861 backtracking control verbs cause an error if encountered by a DFA matching
 2862 function.
 2863 </P>
 2864 <P>
 2865 The behaviour of these verbs in
 2866 <a href="#btrepeat">repeated groups,</a>
 2867 <a href="#btassert">assertions,</a>
 2868 and in
 2869 <a href="#btsub">subpatterns called as subroutines</a>
 2870 (whether or not recursively) is documented below.
 2871 <a name="nooptimize"></a></P>
 2872 <br><b>
 2873 Optimizations that affect backtracking verbs
 2874 </b><br>
 2875 <P>
 2876 PCRE contains some optimizations that are used to speed up matching by running
 2877 some checks at the start of each match attempt. For example, it may know the
 2878 minimum length of matching subject, or that a particular character must be
 2879 present. When one of these optimizations bypasses the running of a match, any
 2880 included backtracking verbs will not, of course, be processed. You can suppress
 2881 the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
 2882 when calling <b>pcre_compile()</b> or <b>pcre_exec()</b>, or by starting the
 2883 pattern with (*NO_START_OPT). There is more discussion of this option in the
 2884 section entitled
 2885 <a href="pcreapi.html#execoptions">"Option bits for <b>pcre_exec()</b>"</a>
 2886 in the
 2887 <a href="pcreapi.html"><b>pcreapi</b></a>
 2888 documentation.
 2889 </P>
 2890 <P>
 2891 Experiments with Perl suggest that it too has similar optimizations, sometimes
 2892 leading to anomalous results.
 2893 </P>
 2894 <br><b>
 2895 Verbs that act immediately
 2896 </b><br>
 2897 <P>
 2898 The following verbs act as soon as they are encountered. They may not be
 2899 followed by a name.
 2900 <pre>
 2901    (*ACCEPT)
 2902 </pre>
 2903 This verb causes the match to end successfully, skipping the remainder of the
 2904 pattern. However, when it is inside a subpattern that is called as a
 2905 subroutine, only that subpattern is ended successfully. Matching then continues
 2906 at the outer level. If (*ACCEPT) in triggered in a positive assertion, the
 2907 assertion succeeds; in a negative assertion, the assertion fails.
 2908 </P>
 2909 <P>
 2910 If (*ACCEPT) is inside capturing parentheses, the data so far is captured. For
 2911 example:
 2912 <pre>
 2913   A((?:A|B(*ACCEPT)|C)D)
 2914 </pre>
 2915 This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
 2916 the outer parentheses.
 2917 <pre>
 2918   (*FAIL) or (*F)
 2919 </pre>
 2920 This verb causes a matching failure, forcing backtracking to occur. It is
 2921 equivalent to (?!) but easier to read. The Perl documentation notes that it is
 2922 probably useful only when combined with (?{}) or (??{}). Those are, of course,
 2923 Perl features that are not present in PCRE. The nearest equivalent is the
 2924 callout feature, as for example in this pattern:
 2925 <pre>
 2926   a+(?C)(*FAIL)
 2927 </pre>
 2928 A match with the string "aaaa" always fails, but the callout is taken before
 2929 each backtrack happens (in this example, 10 times).
 2930 </P>
 2931 <br><b>
 2932 Recording which path was taken
 2933 </b><br>
 2934 <P>
 2935 There is one verb whose main purpose is to track how a match was arrived at,
 2936 though it also has a secondary use in conjunction with advancing the match
 2937 starting point (see (*SKIP) below).
 2938 <pre>
 2939   (*MARK:NAME) or (*:NAME)
 2940 </pre>
 2941 A name is always required with this verb. There may be as many instances of
 2942 (*MARK) as you like in a pattern, and their names do not have to be unique.
 2943 </P>
 2944 <P>
 2945 When a match succeeds, the name of the last-encountered (*MARK:NAME),
 2946 (*PRUNE:NAME), or (*THEN:NAME) on the matching path is passed back to the
 2947 caller as described in the section entitled
 2948 <a href="pcreapi.html#extradata">"Extra data for <b>pcre_exec()</b>"</a>
 2949 in the
 2950 <a href="pcreapi.html"><b>pcreapi</b></a>
 2951 documentation. Here is an example of <b>pcretest</b> output, where the /K
 2952 modifier requests the retrieval and outputting of (*MARK) data:
 2953 <pre>
 2954     re&#62; /X(*MARK:A)Y|X(*MARK:B)Z/K
 2955   data&#62; XY
 2956    0: XY
 2957   MK: A
 2958   XZ
 2959    0: XZ
 2960   MK: B
 2961 </pre>
 2962 The (*MARK) name is tagged with "MK:" in this output, and in this example it
 2963 indicates which of the two alternatives matched. This is a more efficient way
 2964 of obtaining this information than putting each alternative in its own
 2965 capturing parentheses.
 2966 </P>
 2967 <P>
 2968 If a verb with a name is encountered in a positive assertion that is true, the
 2969 name is recorded and passed back if it is the last-encountered. This does not
 2970 happen for negative assertions or failing positive assertions.
 2971 </P>
 2972 <P>
 2973 After a partial match or a failed match, the last encountered name in the
 2974 entire match process is returned. For example:
 2975 <pre>
 2976     re&#62; /X(*MARK:A)Y|X(*MARK:B)Z/K
 2977   data&#62; XP
 2978   No match, mark = B
 2979 </pre>
 2980 Note that in this unanchored example the mark is retained from the match
 2981 attempt that started at the letter "X" in the subject. Subsequent match
 2982 attempts starting at "P" and then with an empty string do not get as far as the
 2983 (*MARK) item, but nevertheless do not reset it.
 2984 </P>
 2985 <P>
 2986 If you are interested in (*MARK) values after failed matches, you should
 2987 probably set the PCRE_NO_START_OPTIMIZE option
 2988 <a href="#nooptimize">(see above)</a>
 2989 to ensure that the match is always attempted.
 2990 </P>
 2991 <br><b>
 2992 Verbs that act after backtracking
 2993 </b><br>
 2994 <P>
 2995 The following verbs do nothing when they are encountered. Matching continues
 2996 with what follows, but if there is no subsequent match, causing a backtrack to
 2997 the verb, a failure is forced. That is, backtracking cannot pass to the left of
 2998 the verb. However, when one of these verbs appears inside an atomic group or an
 2999 assertion that is true, its effect is confined to that group, because once the
 3000 group has been matched, there is never any backtracking into it. In this
 3001 situation, backtracking can "jump back" to the left of the entire atomic group
 3002 or assertion. (Remember also, as stated above, that this localization also
 3003 applies in subroutine calls.)
 3004 </P>
 3005 <P>
 3006 These verbs differ in exactly what kind of failure occurs when backtracking
 3007 reaches them. The behaviour described below is what happens when the verb is
 3008 not in a subroutine or an assertion. Subsequent sections cover these special
 3009 cases.
 3010 <pre>
 3011   (*COMMIT)
 3012 </pre>
 3013 This verb, which may not be followed by a name, causes the whole match to fail
 3014 outright if there is a later matching failure that causes backtracking to reach
 3015 it. Even if the pattern is unanchored, no further attempts to find a match by
 3016 advancing the starting point take place. If (*COMMIT) is the only backtracking
 3017 verb that is encountered, once it has been passed <b>pcre_exec()</b> is
 3018 committed to finding a match at the current starting point, or not at all. For
 3019 example:
 3020 <pre>
 3021   a+(*COMMIT)b
 3022 </pre>
 3023 This matches "xxaab" but not "aacaab". It can be thought of as a kind of
 3024 dynamic anchor, or "I've started, so I must finish." The name of the most
 3025 recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
 3026 match failure.
 3027 </P>
 3028 <P>
 3029 If there is more than one backtracking verb in a pattern, a different one that
 3030 follows (*COMMIT) may be triggered first, so merely passing (*COMMIT) during a
 3031 match does not always guarantee that a match must be at this starting point.
 3032 </P>
 3033 <P>
 3034 Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
 3035 unless PCRE's start-of-match optimizations are turned off, as shown in this
 3036 output from <b>pcretest</b>:
 3037 <pre>
 3038     re&#62; /(*COMMIT)abc/
 3039   data&#62; xyzabc
 3040    0: abc
 3041   data&#62; xyzabc\Y
 3042   No match
 3043 </pre>
 3044 For this pattern, PCRE knows that any match must start with "a", so the
 3045 optimization skips along the subject to "a" before applying the pattern to the
 3046 first set of data. The match attempt then succeeds. In the second set of data,
 3047 the escape sequence \Y is interpreted by the <b>pcretest</b> program. It causes
 3048 the PCRE_NO_START_OPTIMIZE option to be set when <b>pcre_exec()</b> is called.
 3049 This disables the optimization that skips along to the first character. The
 3050 pattern is now applied starting at "x", and so the (*COMMIT) causes the match
 3051 to fail without trying any other starting points.
 3052 <pre>
 3053   (*PRUNE) or (*PRUNE:NAME)
 3054 </pre>
 3055 This verb causes the match to fail at the current starting position in the
 3056 subject if there is a later matching failure that causes backtracking to reach
 3057 it. If the pattern is unanchored, the normal "bumpalong" advance to the next
 3058 starting character then happens. Backtracking can occur as usual to the left of
 3059 (*PRUNE), before it is reached, or when matching to the right of (*PRUNE), but
 3060 if there is no match to the right, backtracking cannot cross (*PRUNE). In
 3061 simple cases, the use of (*PRUNE) is just an alternative to an atomic group or
 3062 possessive quantifier, but there are some uses of (*PRUNE) that cannot be
 3063 expressed in any other way. In an anchored pattern (*PRUNE) has the same effect
 3064 as (*COMMIT).
 3065 </P>
 3066 <P>
 3067 The behaviour of (*PRUNE:NAME) is the not the same as (*MARK:NAME)(*PRUNE).
 3068 It is like (*MARK:NAME) in that the name is remembered for passing back to the
 3069 caller. However, (*SKIP:NAME) searches only for names set with (*MARK).
 3070 <pre>
 3071   (*SKIP)
 3072 </pre>
 3073 This verb, when given without a name, is like (*PRUNE), except that if the
 3074 pattern is unanchored, the "bumpalong" advance is not to the next character,
 3075 but to the position in the subject where (*SKIP) was encountered. (*SKIP)
 3076 signifies that whatever text was matched leading up to it cannot be part of a
 3077 successful match. Consider:
 3078 <pre>
 3079   a+(*SKIP)b
 3080 </pre>
 3081 If the subject is "aaaac...", after the first match attempt fails (starting at
 3082 the first character in the string), the starting point skips on to start the
 3083 next attempt at "c". Note that a possessive quantifer does not have the same
 3084 effect as this example; although it would suppress backtracking during the
 3085 first match attempt, the second attempt would start at the second character
 3086 instead of skipping on to "c".
 3087 <pre>
 3088   (*SKIP:NAME)
 3089 </pre>
 3090 When (*SKIP) has an associated name, its behaviour is modified. When it is
 3091 triggered, the previous path through the pattern is searched for the most
 3092 recent (*MARK) that has the same name. If one is found, the "bumpalong" advance
 3093 is to the subject position that corresponds to that (*MARK) instead of to where
 3094 (*SKIP) was encountered. If no (*MARK) with a matching name is found, the
 3095 (*SKIP) is ignored.
 3096 </P>
 3097 <P>
 3098 Note that (*SKIP:NAME) searches only for names set by (*MARK:NAME). It ignores
 3099 names that are set by (*PRUNE:NAME) or (*THEN:NAME).
 3100 <pre>
 3101   (*THEN) or (*THEN:NAME)
 3102 </pre>
 3103 This verb causes a skip to the next innermost alternative when backtracking
 3104 reaches it. That is, it cancels any further backtracking within the current
 3105 alternative. Its name comes from the observation that it can be used for a
 3106 pattern-based if-then-else block:
 3107 <pre>
 3108   ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
 3109 </pre>
 3110 If the COND1 pattern matches, FOO is tried (and possibly further items after
 3111 the end of the group if FOO succeeds); on failure, the matcher skips to the
 3112 second alternative and tries COND2, without backtracking into COND1. If that
 3113 succeeds and BAR fails, COND3 is tried. If subsequently BAZ fails, there are no
 3114 more alternatives, so there is a backtrack to whatever came before the entire
 3115 group. If (*THEN) is not inside an alternation, it acts like (*PRUNE).
 3116 </P>
 3117 <P>
 3118 The behaviour of (*THEN:NAME) is the not the same as (*MARK:NAME)(*THEN).
 3119 It is like (*MARK:NAME) in that the name is remembered for passing back to the
 3120 caller. However, (*SKIP:NAME) searches only for names set with (*MARK).
 3121 </P>
 3122 <P>
 3123 A subpattern that does not contain a | character is just a part of the
 3124 enclosing alternative; it is not a nested alternation with only one
 3125 alternative. The effect of (*THEN) extends beyond such a subpattern to the
 3126 enclosing alternative. Consider this pattern, where A, B, etc. are complex
 3127 pattern fragments that do not contain any | characters at this level:
 3128 <pre>
 3129   A (B(*THEN)C) | D
 3130 </pre>
 3131 If A and B are matched, but there is a failure in C, matching does not
 3132 backtrack into A; instead it moves to the next alternative, that is, D.
 3133 However, if the subpattern containing (*THEN) is given an alternative, it
 3134 behaves differently:
 3135 <pre>
 3136   A (B(*THEN)C | (*FAIL)) | D
 3137 </pre>
 3138 The effect of (*THEN) is now confined to the inner subpattern. After a failure
 3139 in C, matching moves to (*FAIL), which causes the whole subpattern to fail
 3140 because there are no more alternatives to try. In this case, matching does now
 3141 backtrack into A.
 3142 </P>
 3143 <P>
 3144 Note that a conditional subpattern is not considered as having two
 3145 alternatives, because only one is ever used. In other words, the | character in
 3146 a conditional subpattern has a different meaning. Ignoring white space,
 3147 consider:
 3148 <pre>
 3149   ^.*? (?(?=a) a | b(*THEN)c )
 3150 </pre>
 3151 If the subject is "ba", this pattern does not match. Because .*? is ungreedy,
 3152 it initially matches zero characters. The condition (?=a) then fails, the
 3153 character "b" is matched, but "c" is not. At this point, matching does not
 3154 backtrack to .*? as might perhaps be expected from the presence of the |
 3155 character. The conditional subpattern is part of the single alternative that
 3156 comprises the whole pattern, and so the match fails. (If there was a backtrack
 3157 into .*?, allowing it to match "b", the match would succeed.)
 3158 </P>
 3159 <P>
 3160 The verbs just described provide four different "strengths" of control when
 3161 subsequent matching fails. (*THEN) is the weakest, carrying on the match at the
 3162 next alternative. (*PRUNE) comes next, failing the match at the current
 3163 starting position, but allowing an advance to the next character (for an
 3164 unanchored pattern). (*SKIP) is similar, except that the advance may be more
 3165 than one character. (*COMMIT) is the strongest, causing the entire match to
 3166 fail.
 3167 </P>
 3168 <br><b>
 3169 More than one backtracking verb
 3170 </b><br>
 3171 <P>
 3172 If more than one backtracking verb is present in a pattern, the one that is
 3173 backtracked onto first acts. For example, consider this pattern, where A, B,
 3174 etc. are complex pattern fragments:
 3175 <pre>
 3176   (A(*COMMIT)B(*THEN)C|ABD)
 3177 </pre>
 3178 If A matches but B fails, the backtrack to (*COMMIT) causes the entire match to
 3179 fail. However, if A and B match, but C fails, the backtrack to (*THEN) causes
 3180 the next alternative (ABD) to be tried. This behaviour is consistent, but is
 3181 not always the same as Perl's. It means that if two or more backtracking verbs
 3182 appear in succession, all the the last of them has no effect. Consider this
 3183 example:
 3184 <pre>
 3185   ...(*COMMIT)(*PRUNE)...
 3186 </pre>
 3187 If there is a matching failure to the right, backtracking onto (*PRUNE) causes
 3188 it to be triggered, and its action is taken. There can never be a backtrack
 3189 onto (*COMMIT).
 3190 <a name="btrepeat"></a></P>
 3191 <br><b>
 3192 Backtracking verbs in repeated groups
 3193 </b><br>
 3194 <P>
 3195 PCRE differs from Perl in its handling of backtracking verbs in repeated
 3196 groups. For example, consider:
 3197 <pre>
 3198   /(a(*COMMIT)b)+ac/
 3199 </pre>
 3200 If the subject is "abac", Perl matches, but PCRE fails because the (*COMMIT) in
 3201 the second repeat of the group acts.
 3202 <a name="btassert"></a></P>
 3203 <br><b>
 3204 Backtracking verbs in assertions
 3205 </b><br>
 3206 <P>
 3207 (*FAIL) in an assertion has its normal effect: it forces an immediate backtrack.
 3208 </P>
 3209 <P>
 3210 (*ACCEPT) in a positive assertion causes the assertion to succeed without any
 3211 further processing. In a negative assertion, (*ACCEPT) causes the assertion to
 3212 fail without any further processing.
 3213 </P>
 3214 <P>
 3215 The other backtracking verbs are not treated specially if they appear in a
 3216 positive assertion. In particular, (*THEN) skips to the next alternative in the
 3217 innermost enclosing group that has alternations, whether or not this is within
 3218 the assertion.
 3219 </P>
 3220 <P>
 3221 Negative assertions are, however, different, in order to ensure that changing a
 3222 positive assertion into a negative assertion changes its result. Backtracking
 3223 into (*COMMIT), (*SKIP), or (*PRUNE) causes a negative assertion to be true,
 3224 without considering any further alternative branches in the assertion.
 3225 Backtracking into (*THEN) causes it to skip to the next enclosing alternative
 3226 within the assertion (the normal behaviour), but if the assertion does not have
 3227 such an alternative, (*THEN) behaves like (*PRUNE).
 3228 <a name="btsub"></a></P>
 3229 <br><b>
 3230 Backtracking verbs in subroutines
 3231 </b><br>
 3232 <P>
 3233 These behaviours occur whether or not the subpattern is called recursively.
 3234 Perl's treatment of subroutines is different in some cases.
 3235 </P>
 3236 <P>
 3237 (*FAIL) in a subpattern called as a subroutine has its normal effect: it forces
 3238 an immediate backtrack.
 3239 </P>
 3240 <P>
 3241 (*ACCEPT) in a subpattern called as a subroutine causes the subroutine match to
 3242 succeed without any further processing. Matching then continues after the
 3243 subroutine call.
 3244 </P>
 3245 <P>
 3246 (*COMMIT), (*SKIP), and (*PRUNE) in a subpattern called as a subroutine cause
 3247 the subroutine match to fail.
 3248 </P>
 3249 <P>
 3250 (*THEN) skips to the next alternative in the innermost enclosing group within
 3251 the subpattern that has alternatives. If there is no such group within the
 3252 subpattern, (*THEN) causes the subroutine match to fail.
 3253 </P>
 3254 <br><a name="SEC28" href="#TOC1">SEE ALSO</a><br>
 3255 <P>
 3256 <b>pcreapi</b>(3), <b>pcrecallout</b>(3), <b>pcrematching</b>(3),
 3257 <b>pcresyntax</b>(3), <b>pcre</b>(3), <b>pcre16(3)</b>, <b>pcre32(3)</b>.
 3258 </P>
 3259 <br><a name="SEC29" href="#TOC1">AUTHOR</a><br>
 3260 <P>
 3261 Philip Hazel
 3262 <br>
 3263 University Computing Service
 3264 <br>
 3265 Cambridge CB2 3QH, England.
 3266 <br>
 3267 </P>
 3268 <br><a name="SEC30" href="#TOC1">REVISION</a><br>
 3269 <P>
 3270 Last updated: 23 October 2016
 3271 <br>
 3272 Copyright &copy; 1997-2016 University of Cambridge.
 3273 <br>
 3274 <p>
 3275 Return to the <a href="index.html">PCRE index page</a>.
 3276 </p>