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diff --git a/srclib/pcre/doc/html/pcrepattern.html b/srclib/pcre/doc/html/pcrepattern.html deleted file mode 100644 index 1220eb7e02..0000000000 --- a/srclib/pcre/doc/html/pcrepattern.html +++ /dev/null @@ -1,1470 +0,0 @@ -<html> -<head> -<title>pcrepattern specification</title> -</head> -<body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB"> -<h1>pcrepattern man page</h1> -<p> -Return to the <a href="index.html">PCRE index page</a>. -</p> -<p> -This page is part of the PCRE HTML documentation. It was generated automatically -from the original man page. If there is any nonsense in it, please consult the -man page, in case the conversion went wrong. -<br> -<ul> -<li><a name="TOC1" href="#SEC1">PCRE REGULAR EXPRESSION DETAILS</a> -<li><a name="TOC2" href="#SEC2">BACKSLASH</a> -<li><a name="TOC3" href="#SEC3">CIRCUMFLEX AND DOLLAR</a> -<li><a name="TOC4" href="#SEC4">FULL STOP (PERIOD, DOT)</a> -<li><a name="TOC5" href="#SEC5">MATCHING A SINGLE BYTE</a> -<li><a name="TOC6" href="#SEC6">SQUARE BRACKETS AND CHARACTER CLASSES</a> -<li><a name="TOC7" href="#SEC7">POSIX CHARACTER CLASSES</a> -<li><a name="TOC8" href="#SEC8">VERTICAL BAR</a> -<li><a name="TOC9" href="#SEC9">INTERNAL OPTION SETTING</a> -<li><a name="TOC10" href="#SEC10">SUBPATTERNS</a> -<li><a name="TOC11" href="#SEC11">NAMED SUBPATTERNS</a> -<li><a name="TOC12" href="#SEC12">REPETITION</a> -<li><a name="TOC13" href="#SEC13">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a> -<li><a name="TOC14" href="#SEC14">BACK REFERENCES</a> -<li><a name="TOC15" href="#SEC15">ASSERTIONS</a> -<li><a name="TOC16" href="#SEC16">CONDITIONAL SUBPATTERNS</a> -<li><a name="TOC17" href="#SEC17">COMMENTS</a> -<li><a name="TOC18" href="#SEC18">RECURSIVE PATTERNS</a> -<li><a name="TOC19" href="#SEC19">SUBPATTERNS AS SUBROUTINES</a> -<li><a name="TOC20" href="#SEC20">CALLOUTS</a> -</ul> -<br><a name="SEC1" href="#TOC1">PCRE REGULAR EXPRESSION DETAILS</a><br> -<P> -The syntax and semantics of the regular expressions supported by PCRE are -described below. Regular expressions are also described in the Perl -documentation and in a number of books, some of which have copious examples. -Jeffrey Friedl's "Mastering Regular Expressions", published by O'Reilly, covers -regular expressions in great detail. This description of PCRE's regular -expressions is intended as reference material. -</P> -<P> -The original operation of PCRE was on strings of one-byte characters. However, -there is now also support for UTF-8 character strings. To use this, you must -build PCRE to include UTF-8 support, and then call <b>pcre_compile()</b> with -the PCRE_UTF8 option. How this affects pattern matching is mentioned in several -places below. There is also a summary of UTF-8 features in the -<a href="pcre.html#utf8support">section on UTF-8 support</a> -in the main -<a href="pcre.html"><b>pcre</b></a> -page. -</P> -<P> -A regular expression is a pattern that is matched against a subject string from -left to right. Most characters stand for themselves in a pattern, and match the -corresponding characters in the subject. As a trivial example, the pattern -<pre> - The quick brown fox -</pre> -matches a portion of a subject string that is identical to itself. The power of -regular expressions comes from the ability to include alternatives and -repetitions in the pattern. These are encoded in the pattern by the use of -<i>metacharacters</i>, which do not stand for themselves but instead are -interpreted in some special way. -</P> -<P> -There are two different sets of metacharacters: those that are recognized -anywhere in the pattern except within square brackets, and those that are -recognized in square brackets. Outside square brackets, the metacharacters are -as follows: -<pre> - \ general escape character with several uses - ^ assert start of string (or line, in multiline mode) - $ assert end of string (or line, in multiline mode) - . match any character except newline (by default) - [ start character class definition - | start of alternative branch - ( start subpattern - ) end subpattern - ? extends the meaning of ( - also 0 or 1 quantifier - also quantifier minimizer - * 0 or more quantifier - + 1 or more quantifier - also "possessive quantifier" - { start min/max quantifier -</pre> -Part of a pattern that is in square brackets is called a "character class". In -a character class the only metacharacters are: -<pre> - \ general escape character - ^ negate the class, but only if the first character - - indicates character range - [ POSIX character class (only if followed by POSIX syntax) - ] terminates the character class -</pre> -The following sections describe the use of each of the metacharacters. -</P> -<br><a name="SEC2" href="#TOC1">BACKSLASH</a><br> -<P> -The backslash character has several uses. Firstly, if it is followed by a -non-alphanumeric character, it takes away any special meaning that character may -have. This use of backslash as an escape character applies both inside and -outside character classes. -</P> -<P> -For example, if you want to match a * character, you write \* in the pattern. -This escaping action applies whether or not the following character would -otherwise be interpreted as a metacharacter, so it is always safe to precede a -non-alphanumeric with backslash to specify that it stands for itself. In -particular, if you want to match a backslash, you write \\. -</P> -<P> -If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the -pattern (other than in a character class) and characters between a # outside -a character class and the next newline character are ignored. An escaping -backslash can be used to include a whitespace or # character as part of the -pattern. -</P> -<P> -If you want to remove the special meaning from a sequence of characters, you -can do so by putting them between \Q and \E. This is different from Perl in -that $ and @ are handled as literals in \Q...\E sequences in PCRE, whereas in -Perl, $ and @ cause variable interpolation. Note the following examples: -<pre> - Pattern PCRE matches Perl matches - - \Qabc$xyz\E abc$xyz abc followed by the contents of $xyz - \Qabc\$xyz\E abc\$xyz abc\$xyz - \Qabc\E\$\Qxyz\E abc$xyz abc$xyz -</pre> -The \Q...\E sequence is recognized both inside and outside character classes. -<a name="digitsafterbackslash"></a></P> -<br><b> -Non-printing characters -</b><br> -<P> -A second use of backslash provides a way of encoding non-printing characters -in patterns in a visible manner. There is no restriction on the appearance of -non-printing characters, apart from the binary zero that terminates a pattern, -but when a pattern is being prepared by text editing, it is usually easier to -use one of the following escape sequences than the binary character it -represents: -<pre> - \a alarm, that is, the BEL character (hex 07) - \cx "control-x", where x is any character - \e escape (hex 1B) - \f formfeed (hex 0C) - \n newline (hex 0A) - \r carriage return (hex 0D) - \t tab (hex 09) - \ddd character with octal code ddd, or backreference - \xhh character with hex code hh - \x{hhh..} character with hex code hhh... (UTF-8 mode only) -</pre> -The precise effect of \cx is as follows: if x is a lower case letter, it -is converted to upper case. Then bit 6 of the character (hex 40) is inverted. -Thus \cz becomes hex 1A, but \c{ becomes hex 3B, while \c; becomes hex -7B. -</P> -<P> -After \x, from zero to two hexadecimal digits are read (letters can be in -upper or lower case). In UTF-8 mode, any number of hexadecimal digits may -appear between \x{ and }, but the value of the character code must be less -than 2**31 (that is, the maximum hexadecimal value is 7FFFFFFF). If characters -other than hexadecimal digits appear between \x{ and }, or if there is no -terminating }, this form of escape is not recognized. Instead, the initial -\x will be interpreted as a basic hexadecimal escape, with no following -digits, giving a character whose value is zero. -</P> -<P> -Characters whose value is less than 256 can be defined by either of the two -syntaxes for \x when PCRE is in UTF-8 mode. There is no difference in the -way they are handled. For example, \xdc is exactly the same as \x{dc}. -</P> -<P> -After \0 up to two further octal digits are read. In both cases, if there -are fewer than two digits, just those that are present are used. Thus the -sequence \0\x\07 specifies two binary zeros followed by a BEL character -(code value 7). Make sure you supply two digits after the initial zero if the -pattern character that follows is itself an octal digit. -</P> -<P> -The handling of a backslash followed by a digit other than 0 is complicated. -Outside a character class, PCRE reads it and any following digits as a decimal -number. If the number is less than 10, or if there have been at least that many -previous capturing left parentheses in the expression, the entire sequence is -taken as a <i>back reference</i>. A description of how this works is given -<a href="#backreferences">later,</a> -following the discussion of -<a href="#subpattern">parenthesized subpatterns.</a> -</P> -<P> -Inside a character class, or if the decimal number is greater than 9 and there -have not been that many capturing subpatterns, PCRE re-reads up to three octal -digits following the backslash, and generates a single byte from the least -significant 8 bits of the value. Any subsequent digits stand for themselves. -For example: -<pre> - \040 is another way of writing a space - \40 is the same, provided there are fewer than 40 previous capturing subpatterns - \7 is always a back reference - \11 might be a back reference, or another way of writing a tab - \011 is always a tab - \0113 is a tab followed by the character "3" - \113 might be a back reference, otherwise the character with octal code 113 - \377 might be a back reference, otherwise the byte consisting entirely of 1 bits - \81 is either a back reference, or a binary zero followed by the two characters "8" and "1" -</pre> -Note that octal values of 100 or greater must not be introduced by a leading -zero, because no more than three octal digits are ever read. -</P> -<P> -All the sequences that define a single byte value or a single UTF-8 character -(in UTF-8 mode) can be used both inside and outside character classes. In -addition, inside a character class, the sequence \b is interpreted as the -backspace character (hex 08), and the sequence \X is interpreted as the -character "X". Outside a character class, these sequences have different -meanings -<a href="#uniextseq">(see below).</a> -</P> -<br><b> -Generic character types -</b><br> -<P> -The third use of backslash is for specifying generic character types. The -following are always recognized: -<pre> - \d any decimal digit - \D any character that is not a decimal digit - \s any whitespace character - \S any character that is not a whitespace character - \w any "word" character - \W any "non-word" character -</pre> -Each pair of escape sequences partitions the complete set of characters into -two disjoint sets. Any given character matches one, and only one, of each pair. -</P> -<P> -These character type sequences can appear both inside and outside character -classes. They each match one character of the appropriate type. If the current -matching point is at the end of the subject string, all of them fail, since -there is no character to match. -</P> -<P> -For compatibility with Perl, \s does not match the VT character (code 11). -This makes it different from the the POSIX "space" class. The \s characters -are HT (9), LF (10), FF (12), CR (13), and space (32). -</P> -<P> -A "word" character is an underscore or any character less than 256 that is a -letter or digit. The definition of letters and digits is controlled by PCRE's -low-valued character tables, and may vary if locale-specific matching is taking -place (see -<a href="pcreapi.html#localesupport">"Locale support"</a> -in the -<a href="pcreapi.html"><b>pcreapi</b></a> -page). For example, in the "fr_FR" (French) locale, some character codes -greater than 128 are used for accented letters, and these are matched by \w. -</P> -<P> -In UTF-8 mode, characters with values greater than 128 never match \d, \s, or -\w, and always match \D, \S, and \W. This is true even when Unicode -character property support is available. -<a name="uniextseq"></a></P> -<br><b> -Unicode character properties -</b><br> -<P> -When PCRE is built with Unicode character property support, three additional -escape sequences to match generic character types are available when UTF-8 mode -is selected. They are: -<pre> - \p{<i>xx</i>} a character with the <i>xx</i> property - \P{<i>xx</i>} a character without the <i>xx</i> property - \X an extended Unicode sequence -</pre> -The property names represented by <i>xx</i> above are limited to the -Unicode general category properties. Each character has exactly one such -property, specified by a two-letter abbreviation. For compatibility with Perl, -negation can be specified by including a circumflex between the opening brace -and the property name. For example, \p{^Lu} is the same as \P{Lu}. -</P> -<P> -If only one letter is specified with \p or \P, it includes all the properties -that start with that letter. In this case, in the absence of negation, the -curly brackets in the escape sequence are optional; these two examples have -the same effect: -<pre> - \p{L} - \pL -</pre> -The following property codes are supported: -<pre> - C Other - Cc Control - Cf Format - Cn Unassigned - Co Private use - Cs Surrogate - - L Letter - Ll Lower case letter - Lm Modifier letter - Lo Other letter - Lt Title case letter - Lu Upper case letter - - M Mark - Mc Spacing mark - Me Enclosing mark - Mn Non-spacing mark - - N Number - Nd Decimal number - Nl Letter number - No Other number - - P Punctuation - Pc Connector punctuation - Pd Dash punctuation - Pe Close punctuation - Pf Final punctuation - Pi Initial punctuation - Po Other punctuation - Ps Open punctuation - - S Symbol - Sc Currency symbol - Sk Modifier symbol - Sm Mathematical symbol - So Other symbol - - Z Separator - Zl Line separator - Zp Paragraph separator - Zs Space separator -</pre> -Extended properties such as "Greek" or "InMusicalSymbols" are not supported by -PCRE. -</P> -<P> -Specifying caseless matching does not affect these escape sequences. For -example, \p{Lu} always matches only upper case letters. -</P> -<P> -The \X escape matches any number of Unicode characters that form an extended -Unicode sequence. \X is equivalent to -<pre> - (?>\PM\pM*) -</pre> -That is, it matches a character without the "mark" property, followed by zero -or more characters with the "mark" property, and treats the sequence as an -atomic group -<a href="#atomicgroup">(see below).</a> -Characters with the "mark" property are typically accents that affect the -preceding character. -</P> -<P> -Matching characters by Unicode property is not fast, because PCRE has to search -a structure that contains data for over fifteen thousand characters. That is -why the traditional escape sequences such as \d and \w do not use Unicode -properties in PCRE. -<a name="smallassertions"></a></P> -<br><b> -Simple assertions -</b><br> -<P> -The fourth use of backslash is for certain simple assertions. An assertion -specifies a condition that has to be met at a particular point in a match, -without consuming any characters from the subject string. The use of -subpatterns for more complicated assertions is described -<a href="#bigassertions">below.</a> -The backslashed -assertions are: -<pre> - \b matches at a word boundary - \B matches when not at a word boundary - \A matches at start of subject - \Z matches at end of subject or before newline at end - \z matches at end of subject - \G matches at first matching position in subject -</pre> -These assertions may not appear in character classes (but note that \b has a -different meaning, namely the backspace character, inside a character class). -</P> -<P> -A word boundary is a position in the subject string where the current character -and the previous character do not both match \w or \W (i.e. one matches -\w and the other matches \W), or the start or end of the string if the -first or last character matches \w, respectively. -</P> -<P> -The \A, \Z, and \z assertions differ from the traditional circumflex and -dollar (described in the next section) in that they only ever match at the very -start and end of the subject string, whatever options are set. Thus, they are -independent of multiline mode. These three assertions are not affected by the -PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the -circumflex and dollar metacharacters. However, if the <i>startoffset</i> -argument of <b>pcre_exec()</b> is non-zero, indicating that matching is to start -at a point other than the beginning of the subject, \A can never match. The -difference between \Z and \z is that \Z matches before a newline that is the -last character of the string as well as at the end of the string, whereas \z -matches only at the end. -</P> -<P> -The \G assertion is true only when the current matching position is at the -start point of the match, as specified by the <i>startoffset</i> argument of -<b>pcre_exec()</b>. It differs from \A when the value of <i>startoffset</i> is -non-zero. By calling <b>pcre_exec()</b> multiple times with appropriate -arguments, you can mimic Perl's /g option, and it is in this kind of -implementation where \G can be useful. -</P> -<P> -Note, however, that PCRE's interpretation of \G, as the start of the current -match, is subtly different from Perl's, which defines it as the end of the -previous match. In Perl, these can be different when the previously matched -string was empty. Because PCRE does just one match at a time, it cannot -reproduce this behaviour. -</P> -<P> -If all the alternatives of a pattern begin with \G, the expression is anchored -to the starting match position, and the "anchored" flag is set in the compiled -regular expression. -</P> -<br><a name="SEC3" href="#TOC1">CIRCUMFLEX AND DOLLAR</a><br> -<P> -Outside a character class, in the default matching mode, the circumflex -character is an assertion that is true only if the current matching point is -at the start of the subject string. If the <i>startoffset</i> argument of -<b>pcre_exec()</b> is non-zero, circumflex can never match if the PCRE_MULTILINE -option is unset. Inside a character class, circumflex has an entirely different -meaning -<a href="#characterclass">(see below).</a> -</P> -<P> -Circumflex need not be the first character of the pattern if a number of -alternatives are involved, but it should be the first thing in each alternative -in which it appears if the pattern is ever to match that branch. If all -possible alternatives start with a circumflex, that is, if the pattern is -constrained to match only at the start of the subject, it is said to be an -"anchored" pattern. (There are also other constructs that can cause a pattern -to be anchored.) -</P> -<P> -A dollar character is an assertion that is true only if the current matching -point is at the end of the subject string, or immediately before a newline -character that is the last character in the string (by default). Dollar need -not be the last character of the pattern if a number of alternatives are -involved, but it should be the last item in any branch in which it appears. -Dollar has no special meaning in a character class. -</P> -<P> -The meaning of dollar can be changed so that it matches only at the very end of -the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This -does not affect the \Z assertion. -</P> -<P> -The meanings of the circumflex and dollar characters are changed if the -PCRE_MULTILINE option is set. When this is the case, they match immediately -after and immediately before an internal newline character, respectively, in -addition to matching at the start and end of the subject string. For example, -the pattern /^abc$/ matches the subject string "def\nabc" (where \n -represents a newline character) in multiline mode, but not otherwise. -Consequently, patterns that are anchored in single line mode because all -branches start with ^ are not anchored in multiline mode, and a match for -circumflex is possible when the <i>startoffset</i> argument of <b>pcre_exec()</b> -is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is -set. -</P> -<P> -Note that the sequences \A, \Z, and \z can be used to match the start and -end of the subject in both modes, and if all branches of a pattern start with -\A it is always anchored, whether PCRE_MULTILINE is set or not. -</P> -<br><a name="SEC4" href="#TOC1">FULL STOP (PERIOD, DOT)</a><br> -<P> -Outside a character class, a dot in the pattern matches any one character in -the subject, including a non-printing character, but not (by default) newline. -In UTF-8 mode, a dot matches any UTF-8 character, which might be more than one -byte long, except (by default) newline. If the PCRE_DOTALL option is set, -dots match newlines as well. The handling of dot is entirely independent of the -handling of circumflex and dollar, the only relationship being that they both -involve newline characters. Dot has no special meaning in a character class. -</P> -<br><a name="SEC5" href="#TOC1">MATCHING A SINGLE BYTE</a><br> -<P> -Outside a character class, the escape sequence \C matches any one byte, both -in and out of UTF-8 mode. Unlike a dot, it can match a newline. The feature is -provided in Perl in order to match individual bytes in UTF-8 mode. Because it -breaks up UTF-8 characters into individual bytes, what remains in the string -may be a malformed UTF-8 string. For this reason, the \C escape sequence is -best avoided. -</P> -<P> -PCRE does not allow \C to appear in lookbehind assertions -<a href="#lookbehind">(described below),</a> -because in UTF-8 mode this would make it impossible to calculate the length of -the lookbehind. -<a name="characterclass"></a></P> -<br><a name="SEC6" href="#TOC1">SQUARE BRACKETS AND CHARACTER CLASSES</a><br> -<P> -An opening square bracket introduces a character class, terminated by a closing -square bracket. A closing square bracket on its own is not special. If a -closing square bracket is required as a member of the class, it should be the -first data character in the class (after an initial circumflex, if present) or -escaped with a backslash. -</P> -<P> -A character class matches a single character in the subject. In UTF-8 mode, the -character may occupy more than one byte. A matched character must be in the set -of characters defined by the class, unless the first character in the class -definition is a circumflex, in which case the subject character must not be in -the set defined by the class. If a circumflex is actually required as a member -of the class, ensure it is not the first character, or escape it with a -backslash. -</P> -<P> -For example, the character class [aeiou] matches any lower case vowel, while -[^aeiou] matches any character that is not a lower case vowel. Note that a -circumflex is just a convenient notation for specifying the characters that -are in the class by enumerating those that are not. A class that starts with a -circumflex is not an assertion: it still consumes a character from the subject -string, and therefore it fails if the current pointer is at the end of the -string. -</P> -<P> -In UTF-8 mode, characters with values greater than 255 can be included in a -class as a literal string of bytes, or by using the \x{ escaping mechanism. -</P> -<P> -When caseless matching is set, any letters in a class represent both their -upper case and lower case versions, so for example, a caseless [aeiou] matches -"A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a -caseful version would. When running in UTF-8 mode, PCRE supports the concept of -case for characters with values greater than 128 only when it is compiled with -Unicode property support. -</P> -<P> -The newline character is never treated in any special way in character classes, -whatever the setting of the PCRE_DOTALL or PCRE_MULTILINE options is. A class -such as [^a] will always match a newline. -</P> -<P> -The minus (hyphen) character can be used to specify a range of characters in a -character class. For example, [d-m] matches any letter between d and m, -inclusive. If a minus character is required in a class, it must be escaped with -a backslash or appear in a position where it cannot be interpreted as -indicating a range, typically as the first or last character in the class. -</P> -<P> -It is not possible to have the literal character "]" as the end character of a -range. A pattern such as [W-]46] is interpreted as a class of two characters -("W" and "-") followed by a literal string "46]", so it would match "W46]" or -"-46]". However, if the "]" is escaped with a backslash it is interpreted as -the end of range, so [W-\]46] is interpreted as a class containing a range -followed by two other characters. The octal or hexadecimal representation of -"]" can also be used to end a range. -</P> -<P> -Ranges operate in the collating sequence of character values. They can also be -used for characters specified numerically, for example [\000-\037]. In UTF-8 -mode, ranges can include characters whose values are greater than 255, for -example [\x{100}-\x{2ff}]. -</P> -<P> -If a range that includes letters is used when caseless matching is set, it -matches the letters in either case. For example, [W-c] is equivalent to -[][\\^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character -tables for the "fr_FR" locale are in use, [\xc8-\xcb] matches accented E -characters in both cases. In UTF-8 mode, PCRE supports the concept of case for -characters with values greater than 128 only when it is compiled with Unicode -property support. -</P> -<P> -The character types \d, \D, \p, \P, \s, \S, \w, and \W may also appear -in a character class, and add the characters that they match to the class. For -example, [\dABCDEF] matches any hexadecimal digit. A circumflex can -conveniently be used with the upper case character types to specify a more -restricted set of characters than the matching lower case type. For example, -the class [^\W_] matches any letter or digit, but not underscore. -</P> -<P> -The only metacharacters that are recognized in character classes are backslash, -hyphen (only where it can be interpreted as specifying a range), circumflex -(only at the start), opening square bracket (only when it can be interpreted as -introducing a POSIX class name - see the next section), and the terminating -closing square bracket. However, escaping other non-alphanumeric characters -does no harm. -</P> -<br><a name="SEC7" href="#TOC1">POSIX CHARACTER CLASSES</a><br> -<P> -Perl supports the POSIX notation for character classes. This uses names -enclosed by [: and :] within the enclosing square brackets. PCRE also supports -this notation. For example, -<pre> - [01[:alpha:]%] -</pre> -matches "0", "1", any alphabetic character, or "%". The supported class names -are -<pre> - alnum letters and digits - alpha letters - ascii character codes 0 - 127 - blank space or tab only - cntrl control characters - digit decimal digits (same as \d) - graph printing characters, excluding space - lower lower case letters - print printing characters, including space - punct printing characters, excluding letters and digits - space white space (not quite the same as \s) - upper upper case letters - word "word" characters (same as \w) - xdigit hexadecimal digits -</pre> -The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and -space (32). Notice that this list includes the VT character (code 11). This -makes "space" different to \s, which does not include VT (for Perl -compatibility). -</P> -<P> -The name "word" is a Perl extension, and "blank" is a GNU extension from Perl -5.8. Another Perl extension is negation, which is indicated by a ^ character -after the colon. For example, -<pre> - [12[:^digit:]] -</pre> -matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX -syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not -supported, and an error is given if they are encountered. -</P> -<P> -In UTF-8 mode, characters with values greater than 128 do not match any of -the POSIX character classes. -</P> -<br><a name="SEC8" href="#TOC1">VERTICAL BAR</a><br> -<P> -Vertical bar characters are used to separate alternative patterns. For example, -the pattern -<pre> - gilbert|sullivan -</pre> -matches either "gilbert" or "sullivan". Any number of alternatives may appear, -and an empty alternative is permitted (matching the empty string). -The matching process tries each alternative in turn, from left to right, -and the first one that succeeds is used. If the alternatives are within a -subpattern -<a href="#subpattern">(defined below),</a> -"succeeds" means matching the rest of the main pattern as well as the -alternative in the subpattern. -</P> -<br><a name="SEC9" href="#TOC1">INTERNAL OPTION SETTING</a><br> -<P> -The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and -PCRE_EXTENDED options can be changed from within the pattern by a sequence of -Perl option letters enclosed between "(?" and ")". The option letters are -<pre> - i for PCRE_CASELESS - m for PCRE_MULTILINE - s for PCRE_DOTALL - x for PCRE_EXTENDED -</pre> -For example, (?im) sets caseless, multiline matching. It is also possible to -unset these options by preceding the letter with a hyphen, and a combined -setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and -PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also -permitted. If a letter appears both before and after the hyphen, the option is -unset. -</P> -<P> -When an option change occurs at top level (that is, not inside subpattern -parentheses), the change applies to the remainder of the pattern that follows. -If the change is placed right at the start of a pattern, PCRE extracts it into -the global options (and it will therefore show up in data extracted by the -<b>pcre_fullinfo()</b> function). -</P> -<P> -An option change within a subpattern affects only that part of the current -pattern that follows it, so -<pre> - (a(?i)b)c -</pre> -matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used). -By this means, options can be made to have different settings in different -parts of the pattern. Any changes made in one alternative do carry on -into subsequent branches within the same subpattern. For example, -<pre> - (a(?i)b|c) -</pre> -matches "ab", "aB", "c", and "C", even though when matching "C" the first -branch is abandoned before the option setting. This is because the effects of -option settings happen at compile time. There would be some very weird -behaviour otherwise. -</P> -<P> -The PCRE-specific options PCRE_UNGREEDY and PCRE_EXTRA can be changed in the -same way as the Perl-compatible options by using the characters U and X -respectively. The (?X) flag setting is special in that it must always occur -earlier in the pattern than any of the additional features it turns on, even -when it is at top level. It is best to put it at the start. -<a name="subpattern"></a></P> -<br><a name="SEC10" href="#TOC1">SUBPATTERNS</a><br> -<P> -Subpatterns are delimited by parentheses (round brackets), which can be nested. -Turning part of a pattern into a subpattern does two things: -<br> -<br> -1. It localizes a set of alternatives. For example, the pattern -<pre> - cat(aract|erpillar|) -</pre> -matches one of the words "cat", "cataract", or "caterpillar". Without the -parentheses, it would match "cataract", "erpillar" or the empty string. -<br> -<br> -2. It sets up the subpattern as a capturing subpattern. This means that, when -the whole pattern matches, that portion of the subject string that matched the -subpattern is passed back to the caller via the <i>ovector</i> argument of -<b>pcre_exec()</b>. Opening parentheses are counted from left to right (starting -from 1) to obtain numbers for the capturing subpatterns. -</P> -<P> -For example, if the string "the red king" is matched against the pattern -<pre> - the ((red|white) (king|queen)) -</pre> -the captured substrings are "red king", "red", and "king", and are numbered 1, -2, and 3, respectively. -</P> -<P> -The fact that plain parentheses fulfil two functions is not always helpful. -There are often times when a grouping subpattern is required without a -capturing requirement. If an opening parenthesis is followed by a question mark -and a colon, the subpattern does not do any capturing, and is not counted when -computing the number of any subsequent capturing subpatterns. For example, if -the string "the white queen" is matched against the pattern -<pre> - the ((?:red|white) (king|queen)) -</pre> -the captured substrings are "white queen" and "queen", and are numbered 1 and -2. The maximum number of capturing subpatterns is 65535, and the maximum depth -of nesting of all subpatterns, both capturing and non-capturing, is 200. -</P> -<P> -As a convenient shorthand, if any option settings are required at the start of -a non-capturing subpattern, the option letters may appear between the "?" and -the ":". Thus the two patterns -<pre> - (?i:saturday|sunday) - (?:(?i)saturday|sunday) -</pre> -match exactly the same set of strings. Because alternative branches are tried -from left to right, and options are not reset until the end of the subpattern -is reached, an option setting in one branch does affect subsequent branches, so -the above patterns match "SUNDAY" as well as "Saturday". -</P> -<br><a name="SEC11" href="#TOC1">NAMED SUBPATTERNS</a><br> -<P> -Identifying capturing parentheses by number is simple, but it can be very hard -to keep track of the numbers in complicated regular expressions. Furthermore, -if an expression is modified, the numbers may change. To help with this -difficulty, PCRE supports the naming of subpatterns, something that Perl does -not provide. The Python syntax (?P<name>...) is used. Names consist of -alphanumeric characters and underscores, and must be unique within a pattern. -</P> -<P> -Named capturing parentheses are still allocated numbers as well as names. The -PCRE API provides function calls for extracting the name-to-number translation -table from a compiled pattern. There is also a convenience function for -extracting a captured substring by name. For further details see the -<a href="pcreapi.html"><b>pcreapi</b></a> -documentation. -</P> -<br><a name="SEC12" href="#TOC1">REPETITION</a><br> -<P> -Repetition is specified by quantifiers, which can follow any of the following -items: -<pre> - a literal data character - the . metacharacter - the \C escape sequence - the \X escape sequence (in UTF-8 mode with Unicode properties) - an escape such as \d that matches a single character - a character class - a back reference (see next section) - a parenthesized subpattern (unless it is an assertion) -</pre> -The general repetition quantifier specifies a minimum and maximum number of -permitted matches, by giving the two numbers in curly brackets (braces), -separated by a comma. The numbers must be less than 65536, and the first must -be less than or equal to the second. For example: -<pre> - z{2,4} -</pre> -matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special -character. If the second number is omitted, but the comma is present, there is -no upper limit; if the second number and the comma are both omitted, the -quantifier specifies an exact number of required matches. Thus -<pre> - [aeiou]{3,} -</pre> -matches at least 3 successive vowels, but may match many more, while -<pre> - \d{8} -</pre> -matches exactly 8 digits. An opening curly bracket that appears in a position -where a quantifier is not allowed, or one that does not match the syntax of a -quantifier, is taken as a literal character. For example, {,6} is not a -quantifier, but a literal string of four characters. -</P> -<P> -In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual -bytes. Thus, for example, \x{100}{2} matches two UTF-8 characters, each of -which is represented by a two-byte sequence. Similarly, when Unicode property -support is available, \X{3} matches three Unicode extended sequences, each of -which may be several bytes long (and they may be of different lengths). -</P> -<P> -The quantifier {0} is permitted, causing the expression to behave as if the -previous item and the quantifier were not present. -</P> -<P> -For convenience (and historical compatibility) the three most common -quantifiers have single-character abbreviations: -<pre> - * is equivalent to {0,} - + is equivalent to {1,} - ? is equivalent to {0,1} -</pre> -It is possible to construct infinite loops by following a subpattern that can -match no characters with a quantifier that has no upper limit, for example: -<pre> - (a?)* -</pre> -Earlier versions of Perl and PCRE used to give an error at compile time for -such patterns. However, because there are cases where this can be useful, such -patterns are now accepted, but if any repetition of the subpattern does in fact -match no characters, the loop is forcibly broken. -</P> -<P> -By default, the quantifiers are "greedy", that is, they match as much as -possible (up to the maximum number of permitted times), without causing the -rest of the pattern to fail. The classic example of where this gives problems -is in trying to match comments in C programs. These appear between /* and */ -and within the comment, individual * and / characters may appear. An attempt to -match C comments by applying the pattern -<pre> - /\*.*\*/ -</pre> -to the string -<pre> - /* first comment */ not comment /* second comment */ -</pre> -fails, because it matches the entire string owing to the greediness of the .* -item. -</P> -<P> -However, if a quantifier is followed by a question mark, it ceases to be -greedy, and instead matches the minimum number of times possible, so the -pattern -<pre> - /\*.*?\*/ -</pre> -does the right thing with the C comments. The meaning of the various -quantifiers is not otherwise changed, just the preferred number of matches. -Do not confuse this use of question mark with its use as a quantifier in its -own right. Because it has two uses, it can sometimes appear doubled, as in -<pre> - \d??\d -</pre> -which matches one digit by preference, but can match two if that is the only -way the rest of the pattern matches. -</P> -<P> -If the PCRE_UNGREEDY option is set (an option which is not available in Perl), -the quantifiers are not greedy by default, but individual ones can be made -greedy by following them with a question mark. In other words, it inverts the -default behaviour. -</P> -<P> -When a parenthesized subpattern is quantified with a minimum repeat count that -is greater than 1 or with a limited maximum, more memory is required for the -compiled pattern, in proportion to the size of the minimum or maximum. -</P> -<P> -If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent -to Perl's /s) is set, thus allowing the . to match newlines, the pattern is -implicitly anchored, because whatever follows will be tried against every -character position in the subject string, so there is no point in retrying the -overall match at any position after the first. PCRE normally treats such a -pattern as though it were preceded by \A. -</P> -<P> -In cases where it is known that the subject string contains no newlines, it is -worth setting PCRE_DOTALL in order to obtain this optimization, or -alternatively using ^ to indicate anchoring explicitly. -</P> -<P> -However, there is one situation where the optimization cannot be used. When .* -is inside capturing parentheses that are the subject of a backreference -elsewhere in the pattern, a match at the start may fail, and a later one -succeed. Consider, for example: -<pre> - (.*)abc\1 -</pre> -If the subject is "xyz123abc123" the match point is the fourth character. For -this reason, such a pattern is not implicitly anchored. -</P> -<P> -When a capturing subpattern is repeated, the value captured is the substring -that matched the final iteration. For example, after -<pre> - (tweedle[dume]{3}\s*)+ -</pre> -has matched "tweedledum tweedledee" the value of the captured substring is -"tweedledee". However, if there are nested capturing subpatterns, the -corresponding captured values may have been set in previous iterations. For -example, after -<pre> - /(a|(b))+/ -</pre> -matches "aba" the value of the second captured substring is "b". -<a name="atomicgroup"></a></P> -<br><a name="SEC13" href="#TOC1">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a><br> -<P> -With both maximizing and minimizing repetition, failure of what follows -normally causes the repeated item to be re-evaluated to see if a different -number of repeats allows the rest of the pattern to match. Sometimes it is -useful to prevent this, either to change the nature of the match, or to cause -it fail earlier than it otherwise might, when the author of the pattern knows -there is no point in carrying on. -</P> -<P> -Consider, for example, the pattern \d+foo when applied to the subject line -<pre> - 123456bar -</pre> -After matching all 6 digits and then failing to match "foo", the normal -action of the matcher is to try again with only 5 digits matching the \d+ -item, and then with 4, and so on, before ultimately failing. "Atomic grouping" -(a term taken from Jeffrey Friedl's book) provides the means for specifying -that once a subpattern has matched, it is not to be re-evaluated in this way. -</P> -<P> -If we use atomic grouping for the previous example, the matcher would give up -immediately on failing to match "foo" the first time. The notation is a kind of -special parenthesis, starting with (?> as in this example: -<pre> - (?>\d+)foo -</pre> -This kind of parenthesis "locks up" the part of the pattern it contains once -it has matched, and a failure further into the pattern is prevented from -backtracking into it. Backtracking past it to previous items, however, works as -normal. -</P> -<P> -An alternative description is that a subpattern of this type matches the string -of characters that an identical standalone pattern would match, if anchored at -the current point in the subject string. -</P> -<P> -Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as -the above example can be thought of as a maximizing repeat that must swallow -everything it can. So, while both \d+ and \d+? are prepared to adjust the -number of digits they match in order to make the rest of the pattern match, -(?>\d+) can only match an entire sequence of digits. -</P> -<P> -Atomic groups in general can of course contain arbitrarily complicated -subpatterns, and can be nested. However, when the subpattern for an atomic -group is just a single repeated item, as in the example above, a simpler -notation, called a "possessive quantifier" can be used. This consists of an -additional + character following a quantifier. Using this notation, the -previous example can be rewritten as -<pre> - \d++foo -</pre> -Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY -option is ignored. They are a convenient notation for the simpler forms of -atomic group. However, there is no difference in the meaning or processing of a -possessive quantifier and the equivalent atomic group. -</P> -<P> -The possessive quantifier syntax is an extension to the Perl syntax. It -originates in Sun's Java package. -</P> -<P> -When a pattern contains an unlimited repeat inside a subpattern that can itself -be repeated an unlimited number of times, the use of an atomic group is the -only way to avoid some failing matches taking a very long time indeed. The -pattern -<pre> - (\D+|<\d+>)*[!?] -</pre> -matches an unlimited number of substrings that either consist of non-digits, or -digits enclosed in <>, followed by either ! or ?. When it matches, it runs -quickly. However, if it is applied to -<pre> - aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa -</pre> -it takes a long time before reporting failure. This is because the string can -be divided between the internal \D+ repeat and the external * repeat in a -large number of ways, and all have to be tried. (The example uses [!?] rather -than a single character at the end, because both PCRE and Perl have an -optimization that allows for fast failure when a single character is used. They -remember the last single character that is required for a match, and fail early -if it is not present in the string.) If the pattern is changed so that it uses -an atomic group, like this: -<pre> - ((?>\D+)|<\d+>)*[!?] -</pre> -sequences of non-digits cannot be broken, and failure happens quickly. -<a name="backreferences"></a></P> -<br><a name="SEC14" href="#TOC1">BACK REFERENCES</a><br> -<P> -Outside a character class, a backslash followed by a digit greater than 0 (and -possibly further digits) is a back reference to a capturing subpattern earlier -(that is, to its left) in the pattern, provided there have been that many -previous capturing left parentheses. -</P> -<P> -However, if the decimal number following the backslash is less than 10, it is -always taken as a back reference, and causes an error only if there are not -that many capturing left parentheses in the entire pattern. In other words, the -parentheses that are referenced need not be to the left of the reference for -numbers less than 10. See the subsection entitled "Non-printing characters" -<a href="#digitsafterbackslash">above</a> -for further details of the handling of digits following a backslash. -</P> -<P> -A back reference matches whatever actually matched the capturing subpattern in -the current subject string, rather than anything matching the subpattern -itself (see -<a href="#subpatternsassubroutines">"Subpatterns as subroutines"</a> -below for a way of doing that). So the pattern -<pre> - (sens|respons)e and \1ibility -</pre> -matches "sense and sensibility" and "response and responsibility", but not -"sense and responsibility". If caseful matching is in force at the time of the -back reference, the case of letters is relevant. For example, -<pre> - ((?i)rah)\s+\1 -</pre> -matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original -capturing subpattern is matched caselessly. -</P> -<P> -Back references to named subpatterns use the Python syntax (?P=name). We could -rewrite the above example as follows: -<pre> - (?<p1>(?i)rah)\s+(?P=p1) -</pre> -There may be more than one back reference to the same subpattern. If a -subpattern has not actually been used in a particular match, any back -references to it always fail. For example, the pattern -<pre> - (a|(bc))\2 -</pre> -always fails if it starts to match "a" rather than "bc". Because there may be -many capturing parentheses in a pattern, all digits following the backslash are -taken as part of a potential back reference number. If the pattern continues -with a digit character, some delimiter must be used to terminate the back -reference. If the PCRE_EXTENDED option is set, this can be whitespace. -Otherwise an empty comment (see -<a href="#comments">"Comments"</a> -below) can be used. -</P> -<P> -A back reference that occurs inside the parentheses to which it refers fails -when the subpattern is first used, so, for example, (a\1) never matches. -However, such references can be useful inside repeated subpatterns. For -example, the pattern -<pre> - (a|b\1)+ -</pre> -matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of -the subpattern, the back reference matches the character string corresponding -to the previous iteration. In order for this to work, the pattern must be such -that the first iteration does not need to match the back reference. This can be -done using alternation, as in the example above, or by a quantifier with a -minimum of zero. -<a name="bigassertions"></a></P> -<br><a name="SEC15" href="#TOC1">ASSERTIONS</a><br> -<P> -An assertion is a test on the characters following or preceding the current -matching point that does not actually consume any characters. The simple -assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are described -<a href="#smallassertions">above.</a> -</P> -<P> -More complicated assertions are coded as subpatterns. There are two kinds: -those that look ahead of the current position in the subject string, and those -that look behind it. An assertion subpattern is matched in the normal way, -except that it does not cause the current matching position to be changed. -</P> -<P> -Assertion subpatterns are not capturing subpatterns, and may not be repeated, -because it makes no sense to assert the same thing several times. If any kind -of assertion contains capturing subpatterns within it, these are counted for -the purposes of numbering the capturing subpatterns in the whole pattern. -However, substring capturing is carried out only for positive assertions, -because it does not make sense for negative assertions. -</P> -<br><b> -Lookahead assertions -</b><br> -<P> -Lookahead assertions start -with (?= for positive assertions and (?! for negative assertions. For example, -<pre> - \w+(?=;) -</pre> -matches a word followed by a semicolon, but does not include the semicolon in -the match, and -<pre> - foo(?!bar) -</pre> -matches any occurrence of "foo" that is not followed by "bar". Note that the -apparently similar pattern -<pre> - (?!foo)bar -</pre> -does not find an occurrence of "bar" that is preceded by something other than -"foo"; it finds any occurrence of "bar" whatsoever, because the assertion -(?!foo) is always true when the next three characters are "bar". A -lookbehind assertion is needed to achieve the other effect. -</P> -<P> -If you want to force a matching failure at some point in a pattern, the most -convenient way to do it is with (?!) because an empty string always matches, so -an assertion that requires there not to be an empty string must always fail. -<a name="lookbehind"></a></P> -<br><b> -Lookbehind assertions -</b><br> -<P> -Lookbehind assertions start with (?<= for positive assertions and (?<! for -negative assertions. For example, -<pre> - (?<!foo)bar -</pre> -does find an occurrence of "bar" that is not preceded by "foo". The contents of -a lookbehind assertion are restricted such that all the strings it matches must -have a fixed length. However, if there are several alternatives, they do not -all have to have the same fixed length. Thus -<pre> - (?<=bullock|donkey) -</pre> -is permitted, but -<pre> - (?<!dogs?|cats?) -</pre> -causes an error at compile time. Branches that match different length strings -are permitted only at the top level of a lookbehind assertion. This is an -extension compared with Perl (at least for 5.8), which requires all branches to -match the same length of string. An assertion such as -<pre> - (?<=ab(c|de)) -</pre> -is not permitted, because its single top-level branch can match two different -lengths, but it is acceptable if rewritten to use two top-level branches: -<pre> - (?<=abc|abde) -</pre> -The implementation of lookbehind assertions is, for each alternative, to -temporarily move the current position back by the fixed width and then try to -match. If there are insufficient characters before the current position, the -match is deemed to fail. -</P> -<P> -PCRE does not allow the \C escape (which matches a single byte in UTF-8 mode) -to appear in lookbehind assertions, because it makes it impossible to calculate -the length of the lookbehind. The \X escape, which can match different numbers -of bytes, is also not permitted. -</P> -<P> -Atomic groups can be used in conjunction with lookbehind assertions to specify -efficient matching at the end of the subject string. Consider a simple pattern -such as -<pre> - abcd$ -</pre> -when applied to a long string that does not match. Because matching proceeds -from left to right, PCRE will look for each "a" in the subject and then see if -what follows matches the rest of the pattern. If the pattern is specified as -<pre> - ^.*abcd$ -</pre> -the initial .* matches the entire string at first, but when this fails (because -there is no following "a"), it backtracks to match all but the last character, -then all but the last two characters, and so on. Once again the search for "a" -covers the entire string, from right to left, so we are no better off. However, -if the pattern is written as -<pre> - ^(?>.*)(?<=abcd) -</pre> -or, equivalently, using the possessive quantifier syntax, -<pre> - ^.*+(?<=abcd) -</pre> -there can be no backtracking for the .* item; it can match only the entire -string. The subsequent lookbehind assertion does a single test on the last four -characters. If it fails, the match fails immediately. For long strings, this -approach makes a significant difference to the processing time. -</P> -<br><b> -Using multiple assertions -</b><br> -<P> -Several assertions (of any sort) may occur in succession. For example, -<pre> - (?<=\d{3})(?<!999)foo -</pre> -matches "foo" preceded by three digits that are not "999". Notice that each of -the assertions is applied independently at the same point in the subject -string. First there is a check that the previous three characters are all -digits, and then there is a check that the same three characters are not "999". -This pattern does <i>not</i> match "foo" preceded by six characters, the first -of which are digits and the last three of which are not "999". For example, it -doesn't match "123abcfoo". A pattern to do that is -<pre> - (?<=\d{3}...)(?<!999)foo -</pre> -This time the first assertion looks at the preceding six characters, checking -that the first three are digits, and then the second assertion checks that the -preceding three characters are not "999". -</P> -<P> -Assertions can be nested in any combination. For example, -<pre> - (?<=(?<!foo)bar)baz -</pre> -matches an occurrence of "baz" that is preceded by "bar" which in turn is not -preceded by "foo", while -<pre> - (?<=\d{3}(?!999)...)foo -</pre> -is another pattern that matches "foo" preceded by three digits and any three -characters that are not "999". -</P> -<br><a name="SEC16" href="#TOC1">CONDITIONAL SUBPATTERNS</a><br> -<P> -It is possible to cause the matching process to obey a subpattern -conditionally or to choose between two alternative subpatterns, depending on -the result of an assertion, or whether a previous capturing subpattern matched -or not. The two possible forms of conditional subpattern are -<pre> - (?(condition)yes-pattern) - (?(condition)yes-pattern|no-pattern) -</pre> -If the condition is satisfied, the yes-pattern is used; otherwise the -no-pattern (if present) is used. If there are more than two alternatives in the -subpattern, a compile-time error occurs. -</P> -<P> -There are three kinds of condition. If the text between the parentheses -consists of a sequence of digits, the condition is satisfied if the capturing -subpattern of that number has previously matched. The number must be greater -than zero. Consider the following pattern, which contains non-significant white -space to make it more readable (assume the PCRE_EXTENDED option) and to divide -it into three parts for ease of discussion: -<pre> - ( \( )? [^()]+ (?(1) \) ) -</pre> -The first part matches an optional opening parenthesis, and if that -character is present, sets it as the first captured substring. The second part -matches one or more characters that are not parentheses. The third part is a -conditional subpattern that tests whether the first set of parentheses matched -or not. If they did, that is, if subject started with an opening parenthesis, -the condition is true, and so the yes-pattern is executed and a closing -parenthesis is required. Otherwise, since no-pattern is not present, the -subpattern matches nothing. In other words, this pattern matches a sequence of -non-parentheses, optionally enclosed in parentheses. -</P> -<P> -If the condition is the string (R), it is satisfied if a recursive call to the -pattern or subpattern has been made. At "top level", the condition is false. -This is a PCRE extension. Recursive patterns are described in the next section. -</P> -<P> -If the condition is not a sequence of digits or (R), it must be an assertion. -This may be a positive or negative lookahead or lookbehind assertion. Consider -this pattern, again containing non-significant white space, and with the two -alternatives on the second line: -<pre> - (?(?=[^a-z]*[a-z]) - \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} ) -</pre> -The condition is a positive lookahead assertion that matches an optional -sequence of non-letters followed by a letter. In other words, it tests for the -presence of at least one letter in the subject. If a letter is found, the -subject is matched against the first alternative; otherwise it is matched -against the second. This pattern matches strings in one of the two forms -dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits. -<a name="comments"></a></P> -<br><a name="SEC17" href="#TOC1">COMMENTS</a><br> -<P> -The sequence (?# marks the start of a comment that continues up to the next -closing parenthesis. Nested parentheses are not permitted. The characters -that make up a comment play no part in the pattern matching at all. -</P> -<P> -If the PCRE_EXTENDED option is set, an unescaped # character outside a -character class introduces a comment that continues up to the next newline -character in the pattern. -</P> -<br><a name="SEC18" href="#TOC1">RECURSIVE PATTERNS</a><br> -<P> -Consider the problem of matching a string in parentheses, allowing for -unlimited nested parentheses. Without the use of recursion, the best that can -be done is to use a pattern that matches up to some fixed depth of nesting. It -is not possible to handle an arbitrary nesting depth. Perl provides a facility -that allows regular expressions to recurse (amongst other things). It does this -by interpolating Perl code in the expression at run time, and the code can -refer to the expression itself. A Perl pattern to solve the parentheses problem -can be created like this: -<pre> - $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x; -</pre> -The (?p{...}) item interpolates Perl code at run time, and in this case refers -recursively to the pattern in which it appears. Obviously, PCRE cannot support -the interpolation of Perl code. Instead, it supports some special syntax for -recursion of the entire pattern, and also for individual subpattern recursion. -</P> -<P> -The special item that consists of (? followed by a number greater than zero and -a closing parenthesis is a recursive call of the subpattern of the given -number, provided that it occurs inside that subpattern. (If not, it is a -"subroutine" call, which is described in the next section.) The special item -(?R) is a recursive call of the entire regular expression. -</P> -<P> -For example, this PCRE pattern solves the nested parentheses problem (assume -the PCRE_EXTENDED option is set so that white space is ignored): -<pre> - \( ( (?>[^()]+) | (?R) )* \) -</pre> -First it matches an opening parenthesis. Then it matches any number of -substrings which can either be a sequence of non-parentheses, or a recursive -match of the pattern itself (that is a correctly parenthesized substring). -Finally there is a closing parenthesis. -</P> -<P> -If this were part of a larger pattern, you would not want to recurse the entire -pattern, so instead you could use this: -<pre> - ( \( ( (?>[^()]+) | (?1) )* \) ) -</pre> -We have put the pattern into parentheses, and caused the recursion to refer to -them instead of the whole pattern. In a larger pattern, keeping track of -parenthesis numbers can be tricky. It may be more convenient to use named -parentheses instead. For this, PCRE uses (?P>name), which is an extension to -the Python syntax that PCRE uses for named parentheses (Perl does not provide -named parentheses). We could rewrite the above example as follows: -<pre> - (?P<pn> \( ( (?>[^()]+) | (?P>pn) )* \) ) -</pre> -This particular example pattern contains nested unlimited repeats, and so the -use of atomic grouping for matching strings of non-parentheses is important -when applying the pattern to strings that do not match. For example, when this -pattern is applied to -<pre> - (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa() -</pre> -it yields "no match" quickly. However, if atomic grouping is not used, -the match runs for a very long time indeed because there are so many different -ways the + and * repeats can carve up the subject, and all have to be tested -before failure can be reported. -</P> -<P> -At the end of a match, the values set for any capturing subpatterns are those -from the outermost level of the recursion at which the subpattern value is set. -If you want to obtain intermediate values, a callout function can be used (see -the next section and the -<a href="pcrecallout.html"><b>pcrecallout</b></a> -documentation). If the pattern above is matched against -<pre> - (ab(cd)ef) -</pre> -the value for the capturing parentheses is "ef", which is the last value taken -on at the top level. If additional parentheses are added, giving -<pre> - \( ( ( (?>[^()]+) | (?R) )* ) \) - ^ ^ - ^ ^ -</pre> -the string they capture is "ab(cd)ef", the contents of the top level -parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE -has to obtain extra memory to store data during a recursion, which it does by -using <b>pcre_malloc</b>, freeing it via <b>pcre_free</b> afterwards. If no -memory can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error. -</P> -<P> -Do not confuse the (?R) item with the condition (R), which tests for recursion. -Consider this pattern, which matches text in angle brackets, allowing for -arbitrary nesting. Only digits are allowed in nested brackets (that is, when -recursing), whereas any characters are permitted at the outer level. -<pre> - < (?: (?(R) \d++ | [^<>]*+) | (?R)) * > -</pre> -In this pattern, (?(R) is the start of a conditional subpattern, with two -different alternatives for the recursive and non-recursive cases. The (?R) item -is the actual recursive call. -<a name="subpatternsassubroutines"></a></P> -<br><a name="SEC19" href="#TOC1">SUBPATTERNS AS SUBROUTINES</a><br> -<P> -If the syntax for a recursive subpattern reference (either by number or by -name) is used outside the parentheses to which it refers, it operates like a -subroutine in a programming language. An earlier example pointed out that the -pattern -<pre> - (sens|respons)e and \1ibility -</pre> -matches "sense and sensibility" and "response and responsibility", but not -"sense and responsibility". If instead the pattern -<pre> - (sens|respons)e and (?1)ibility -</pre> -is used, it does match "sense and responsibility" as well as the other two -strings. Such references must, however, follow the subpattern to which they -refer. -</P> -<br><a name="SEC20" href="#TOC1">CALLOUTS</a><br> -<P> -Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl -code to be obeyed in the middle of matching a regular expression. This makes it -possible, amongst other things, to extract different substrings that match the -same pair of parentheses when there is a repetition. -</P> -<P> -PCRE provides a similar feature, but of course it cannot obey arbitrary Perl -code. The feature is called "callout". The caller of PCRE provides an external -function by putting its entry point in the global variable <i>pcre_callout</i>. -By default, this variable contains NULL, which disables all calling out. -</P> -<P> -Within a regular expression, (?C) indicates the points at which the external -function is to be called. If you want to identify different callout points, you -can put a number less than 256 after the letter C. The default value is zero. -For example, this pattern has two callout points: -<pre> - (?C1)\dabc(?C2)def -</pre> -If the PCRE_AUTO_CALLOUT flag is passed to <b>pcre_compile()</b>, callouts are -automatically installed before each item in the pattern. They are all numbered -255. -</P> -<P> -During matching, when PCRE reaches a callout point (and <i>pcre_callout</i> is -set), the external function is called. It is provided with the number of the -callout, the position in the pattern, and, optionally, one item of data -originally supplied by the caller of <b>pcre_exec()</b>. The callout function -may cause matching to proceed, to backtrack, or to fail altogether. A complete -description of the interface to the callout function is given in the -<a href="pcrecallout.html"><b>pcrecallout</b></a> -documentation. -</P> -<P> -Last updated: 09 September 2004 -<br> -Copyright © 1997-2004 University of Cambridge. -<p> -Return to the <a href="index.html">PCRE index page</a>. -</p> |