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| <div class="refentry"> |
| <a name="glib-regex-syntax"></a><div class="titlepage"></div> |
| <div class="refnamediv"><table width="100%"><tr> |
| <td valign="top"> |
| <h2><span class="refentrytitle">Regular expression syntax</span></h2> |
| <p>Regular expression syntax — |
| syntax and semantics of regular expressions supported by GRegex |
| </p> |
| </td> |
| <td class="gallery_image" valign="top" align="right"></td> |
| </tr></table></div> |
| <div class="refsect1"> |
| <a name="id-1.5.25.3"></a><h2>GRegex regular expression details</h2> |
| <p> |
| A regular expression is a pattern that is matched against a |
| string from left to right. Most characters stand for themselves in a |
| pattern, and match the corresponding characters in the string. As a |
| trivial example, the pattern |
| </p> |
| <pre class="programlisting"> |
| The quick brown fox |
| </pre> |
| <p> |
| matches a portion of a string that is identical to itself. When |
| caseless matching is specified (the <code class="varname">G_REGEX_CASELESS</code> flag), letters are |
| matched independently of case. |
| </p> |
| <p> |
| 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 metacharacters, 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: |
| </p> |
| <div class="table"> |
| <a name="id-1.5.25.3.7"></a><p class="title"><b>Table 1. Metacharacters outside square brackets</b></p> |
| <div class="table-contents"><table class="table" summary="Metacharacters outside square brackets" border="1"> |
| <colgroup> |
| <col align="center"> |
| <col> |
| </colgroup> |
| <thead><tr> |
| <th align="center">Character</th> |
| <th>Meaning</th> |
| </tr></thead> |
| <tbody> |
| <tr> |
| <td align="center">\</td> |
| <td>general escape character with several uses</td> |
| </tr> |
| <tr> |
| <td align="center">^</td> |
| <td>assert start of string (or line, in multiline mode)</td> |
| </tr> |
| <tr> |
| <td align="center">$</td> |
| <td>assert end of string (or line, in multiline mode)</td> |
| </tr> |
| <tr> |
| <td align="center">.</td> |
| <td>match any character except newline (by default)</td> |
| </tr> |
| <tr> |
| <td align="center">[</td> |
| <td>start character class definition</td> |
| </tr> |
| <tr> |
| <td align="center">|</td> |
| <td>start of alternative branch</td> |
| </tr> |
| <tr> |
| <td align="center">(</td> |
| <td>start subpattern</td> |
| </tr> |
| <tr> |
| <td align="center">)</td> |
| <td>end subpattern</td> |
| </tr> |
| <tr> |
| <td align="center">?</td> |
| <td>extends the meaning of (, or 0/1 quantifier, or quantifier minimizer</td> |
| </tr> |
| <tr> |
| <td align="center">*</td> |
| <td>0 or more quantifier</td> |
| </tr> |
| <tr> |
| <td align="center">+</td> |
| <td>1 or more quantifier, also "possessive quantifier"</td> |
| </tr> |
| <tr> |
| <td align="center">{</td> |
| <td>start min/max quantifier</td> |
| </tr> |
| </tbody> |
| </table></div> |
| </div> |
| <br class="table-break"><p> |
| Part of a pattern that is in square brackets is called a "character |
| class". In a character class the only metacharacters are: |
| </p> |
| <div class="table"> |
| <a name="id-1.5.25.3.9"></a><p class="title"><b>Table 2. Metacharacters inside square brackets</b></p> |
| <div class="table-contents"><table class="table" summary="Metacharacters inside square brackets" border="1"> |
| <colgroup> |
| <col align="center"> |
| <col> |
| </colgroup> |
| <thead><tr> |
| <th align="center">Character</th> |
| <th>Meaning</th> |
| </tr></thead> |
| <tbody> |
| <tr> |
| <td align="center">\</td> |
| <td>general escape character</td> |
| </tr> |
| <tr> |
| <td align="center">^</td> |
| <td>negate the class, but only if the first character</td> |
| </tr> |
| <tr> |
| <td align="center">-</td> |
| <td>indicates character range</td> |
| </tr> |
| <tr> |
| <td align="center">[</td> |
| <td>POSIX character class (only if followed by POSIX syntax)</td> |
| </tr> |
| <tr> |
| <td align="center">]</td> |
| <td>terminates the character class</td> |
| </tr> |
| </tbody> |
| </table></div> |
| </div> |
| <br class="table-break"> |
| </div> |
| <div class="refsect1"> |
| <a name="id-1.5.25.4"></a><h2>Backslash</h2> |
| <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 <code class="varname">G_REGEX_EXTENDED</code> |
| option, whitespace in the pattern (other than in a character class) and |
| characters between a # outside a character class and the next newline |
| are ignored. |
| An escaping backslash can be used to include a whitespace or # character |
| as part of the pattern. |
| </p> |
| <p> |
| Note that the C compiler interprets backslash in strings itself, therefore |
| you need to duplicate all \ characters when you put a regular expression |
| in a C string, like "\\d{3}". |
| </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. |
| The \Q...\E sequence is recognized both inside and outside character |
| classes. |
| </p> |
| <div class="refsect2"> |
| <a name="id-1.5.25.4.7"></a><h3>Non-printing characters</h3> |
| <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: |
| </p> |
| <div class="table"> |
| <a name="id-1.5.25.4.7.3"></a><p class="title"><b>Table 3. Non-printing characters</b></p> |
| <div class="table-contents"><table class="table" summary="Non-printing characters" border="1"> |
| <colgroup> |
| <col align="center"> |
| <col> |
| </colgroup> |
| <thead><tr> |
| <th align="center">Escape</th> |
| <th>Meaning</th> |
| </tr></thead> |
| <tbody> |
| <tr> |
| <td align="center">\a</td> |
| <td>alarm, that is, the BEL character (hex 07)</td> |
| </tr> |
| <tr> |
| <td align="center">\cx</td> |
| <td>"control-x", where x is any character</td> |
| </tr> |
| <tr> |
| <td align="center">\e</td> |
| <td>escape (hex 1B)</td> |
| </tr> |
| <tr> |
| <td align="center">\f</td> |
| <td>formfeed (hex 0C)</td> |
| </tr> |
| <tr> |
| <td align="center">\n</td> |
| <td>newline (hex 0A)</td> |
| </tr> |
| <tr> |
| <td align="center">\r</td> |
| <td>carriage return (hex 0D)</td> |
| </tr> |
| <tr> |
| <td align="center">\t</td> |
| <td>tab (hex 09)</td> |
| </tr> |
| <tr> |
| <td align="center">\ddd</td> |
| <td>character with octal code ddd, or backreference</td> |
| </tr> |
| <tr> |
| <td align="center">\xhh</td> |
| <td>character with hex code hh</td> |
| </tr> |
| <tr> |
| <td align="center">\x{hhh..}</td> |
| <td>character with hex code hhh..</td> |
| </tr> |
| </tbody> |
| </table></div> |
| </div> |
| <br class="table-break"><p> |
| 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). 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. 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. 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, GRegex 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 back reference. A |
| description of how this works is given later, following the discussion |
| of parenthesized subpatterns. |
| </p> |
| <p> |
| Inside a character class, or if the decimal number is greater than 9 |
| and there have not been that many capturing subpatterns, GRegex re-reads |
| up to three octal digits following the backslash, and uses them to generate |
| a data character. Any subsequent digits stand for themselves. For example: |
| </p> |
| <div class="table"> |
| <a name="id-1.5.25.4.7.10"></a><p class="title"><b>Table 4. Non-printing characters</b></p> |
| <div class="table-contents"><table class="table" summary="Non-printing characters" border="1"> |
| <colgroup> |
| <col align="center"> |
| <col> |
| </colgroup> |
| <thead><tr> |
| <th align="center">Escape</th> |
| <th>Meaning</th> |
| </tr></thead> |
| <tbody> |
| <tr> |
| <td align="center">\040</td> |
| <td>is another way of writing a space</td> |
| </tr> |
| <tr> |
| <td align="center">\40</td> |
| <td>is the same, provided there are fewer than 40 previous capturing subpatterns</td> |
| </tr> |
| <tr> |
| <td align="center">\7</td> |
| <td>is always a back reference</td> |
| </tr> |
| <tr> |
| <td align="center">\11</td> |
| <td>might be a back reference, or another way of writing a tab</td> |
| </tr> |
| <tr> |
| <td align="center">\011</td> |
| <td>is always a tab</td> |
| </tr> |
| <tr> |
| <td align="center">\0113</td> |
| <td>is a tab followed by the character "3"</td> |
| </tr> |
| <tr> |
| <td align="center">\113</td> |
| <td>might be a back reference, otherwise the character with octal code 113</td> |
| </tr> |
| <tr> |
| <td align="center">\377</td> |
| <td>might be a back reference, otherwise the byte consisting entirely of 1 bits</td> |
| </tr> |
| <tr> |
| <td align="center">\81</td> |
| <td>is either a back reference, or a binary zero followed by the two characters "8" and "1"</td> |
| </tr> |
| </tbody> |
| </table></div> |
| </div> |
| <br class="table-break"><p> |
| 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 character 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 |
| sequences \R and \X are interpreted as the characters "R" and "X", respectively. |
| Outside a character class, these sequences have different meanings (see below). |
| </p> |
| </div> |
| <hr> |
| <div class="refsect2"> |
| <a name="id-1.5.25.4.8"></a><h3>Absolute and relative back references</h3> |
| <p> |
| The sequence \g followed by a positive or negative number, optionally enclosed |
| in braces, is an absolute or relative back reference. Back references are |
| discussed later, following the discussion of parenthesized subpatterns. |
| </p> |
| </div> |
| <hr> |
| <div class="refsect2"> |
| <a name="id-1.5.25.4.9"></a><h3>Generic character types</h3> |
| <p> |
| Another use of backslash is for specifying generic character types. |
| The following are always recognized: |
| </p> |
| <div class="table"> |
| <a name="id-1.5.25.4.9.3"></a><p class="title"><b>Table 5. Generic characters</b></p> |
| <div class="table-contents"><table class="table" summary="Generic characters" border="1"> |
| <colgroup> |
| <col align="center"> |
| <col> |
| </colgroup> |
| <thead><tr> |
| <th align="center">Escape</th> |
| <th>Meaning</th> |
| </tr></thead> |
| <tbody> |
| <tr> |
| <td align="center">\d</td> |
| <td>any decimal digit</td> |
| </tr> |
| <tr> |
| <td align="center">\D</td> |
| <td>any character that is not a decimal digit</td> |
| </tr> |
| <tr> |
| <td align="center">\s</td> |
| <td>any whitespace character</td> |
| </tr> |
| <tr> |
| <td align="center">\S</td> |
| <td>any character that is not a whitespace character</td> |
| </tr> |
| <tr> |
| <td align="center">\w</td> |
| <td>any "word" character</td> |
| </tr> |
| <tr> |
| <td align="center">\W</td> |
| <td>any "non-word" character</td> |
| </tr> |
| </tbody> |
| </table></div> |
| </div> |
| <br class="table-break"><p> |
| 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 passed 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 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.</p> |
| <p> |
| Characters with values greater than 128 never match \d, |
| \s, or \w, and always match \D, \S, and \W. |
| </p> |
| </div> |
| <hr> |
| <div class="refsect2"> |
| <a name="id-1.5.25.4.10"></a><h3>Newline sequences</h3> |
| <p>Outside a character class, the escape sequence \R matches any Unicode |
| newline sequence. |
| This particular group matches either the two-character sequence CR followed by |
| LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab, |
| U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), NEL (next |
| line, U+0085), LS (line separator, U+2028), or PS (paragraph separator, U+2029). |
| The two-character sequence is treated as a single unit that |
| cannot be split. Inside a character class, \R matches the letter "R".</p> |
| </div> |
| <hr> |
| <div class="refsect2"> |
| <a name="id-1.5.25.4.11"></a><h3>Unicode character properties</h3> |
| <p> |
| To support generic character types there are three additional escape |
| sequences, they are: |
| </p> |
| <div class="table"> |
| <a name="id-1.5.25.4.11.3"></a><p class="title"><b>Table 6. Generic character types</b></p> |
| <div class="table-contents"><table class="table" summary="Generic character types" border="1"> |
| <colgroup> |
| <col align="center"> |
| <col> |
| </colgroup> |
| <thead><tr> |
| <th align="center">Escape</th> |
| <th>Meaning</th> |
| </tr></thead> |
| <tbody> |
| <tr> |
| <td align="center">\p{xx}</td> |
| <td>a character with the xx property</td> |
| </tr> |
| <tr> |
| <td align="center">\P{xx}</td> |
| <td>a character without the xx property</td> |
| </tr> |
| <tr> |
| <td align="center">\X</td> |
| <td>an extended Unicode sequence</td> |
| </tr> |
| </tbody> |
| </table></div> |
| </div> |
| <br class="table-break"><p> |
| The property names represented by xx above are limited to the Unicode |
| script names, the general category properties, and "Any", which matches |
| any character (including newline). Other properties such as "InMusicalSymbols" |
| are not currently supported. Note that \P{Any} does not match any characters, |
| so always causes a match failure. |
| </p> |
| <p> |
| Sets of Unicode characters are defined as belonging to certain scripts. A |
| character from one of these sets can be matched using a script name. For |
| example, \p{Greek} or \P{Han}. |
| </p> |
| <p> |
| Those that are not part of an identified script are lumped together as |
| "Common". The current list of scripts can be found in the documentation for |
| the #GUnicodeScript enumeration. Script names for use with \p{} can be |
| found by replacing all spaces with underscores, e.g. for Linear B use |
| \p{Linear_B}. |
| </p> |
| <p> |
| Each character has exactly one general category property, specified by a |
| two-letter abbreviation. For compatibility with Perl, negation can be specified |
| by including a circumflex between the opening brace and the property name. For |
| example, \p{^Lu} is the same as \P{Lu}. |
| </p> |
| <p> |
| If only one letter is specified with \p or \P, it includes all the general |
| category properties that start with that letter. In this case, in the absence |
| of negation, the curly brackets in the escape sequence are optional; these two |
| examples have the same effect: |
| </p> |
| <pre class="programlisting"> |
| \p{L} |
| \pL |
| </pre> |
| <p> |
| In addition to the two-letter category codes listed in the |
| documentation for the #GUnicodeType enumeration, the following |
| general category property codes are supported: |
| </p> |
| <div class="table"> |
| <a name="id-1.5.25.4.11.11"></a><p class="title"><b>Table 7. Property codes</b></p> |
| <div class="table-contents"><table class="table" summary="Property codes" border="1"> |
| <colgroup> |
| <col align="center"> |
| <col> |
| </colgroup> |
| <thead><tr> |
| <th align="center">Code</th> |
| <th>Meaning</th> |
| </tr></thead> |
| <tbody> |
| <tr> |
| <td align="center">C</td> |
| <td>Other</td> |
| </tr> |
| <tr> |
| <td align="center">L</td> |
| <td>Letter</td> |
| </tr> |
| <tr> |
| <td align="center">M</td> |
| <td>Mark</td> |
| </tr> |
| <tr> |
| <td align="center">N</td> |
| <td>Number</td> |
| </tr> |
| <tr> |
| <td align="center">P</td> |
| <td>Punctuation</td> |
| </tr> |
| <tr> |
| <td align="center">S</td> |
| <td>Symbol</td> |
| </tr> |
| <tr> |
| <td align="center">Z</td> |
| <td>Separator</td> |
| </tr> |
| </tbody> |
| </table></div> |
| </div> |
| <br class="table-break"><p> |
| The special property L& is also supported: it matches a character that has |
| the Lu, Ll, or Lt property, in other words, a letter that is not classified as |
| a modifier or "other". |
| </p> |
| <p> |
| The long synonyms for these properties that Perl supports (such as \ep{Letter}) |
| are not supported by GRegex, nor is it permitted to prefix any of these |
| properties with "Is". |
| </p> |
| <p> |
| No character that is in the Unicode table has the Cn (unassigned) property. |
| Instead, this property is assumed for any code point that is not in the |
| Unicode table. |
| </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 |
| </p> |
| <pre class="programlisting"> |
| (?>\PM\pM*) |
| </pre> |
| <p> |
| 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 (see below). Characters with the "mark" |
| property are typically accents that affect the preceding character. |
| </p> |
| <p> |
| Matching characters by Unicode property is not fast, because GRegex 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. |
| </p> |
| </div> |
| <hr> |
| <div class="refsect2"> |
| <a name="id-1.5.25.4.12"></a><h3>Simple assertions</h3> |
| <p> |
| The final use of backslash is for certain simple assertions. An |
| assertion specifies a condition that has to be met at a particular point in |
| a match, without consuming any characters from the string. The |
| use of subpatterns for more complicated assertions is described below. |
| The backslashed assertions are: |
| </p> |
| <div class="table"> |
| <a name="id-1.5.25.4.12.3"></a><p class="title"><b>Table 8. Simple assertions</b></p> |
| <div class="table-contents"><table class="table" summary="Simple assertions" border="1"> |
| <colgroup> |
| <col align="center"> |
| <col> |
| </colgroup> |
| <thead><tr> |
| <th align="center">Escape</th> |
| <th>Meaning</th> |
| </tr></thead> |
| <tbody> |
| <tr> |
| <td align="center">\b</td> |
| <td>matches at a word boundary</td> |
| </tr> |
| <tr> |
| <td align="center">\B</td> |
| <td>matches when not at a word boundary</td> |
| </tr> |
| <tr> |
| <td align="center">\A</td> |
| <td>matches at the start of the string</td> |
| </tr> |
| <tr> |
| <td align="center">\Z</td> |
| <td>matches at the end of the string or before a newline at the end of the string</td> |
| </tr> |
| <tr> |
| <td align="center">\z</td> |
| <td>matches only at the end of the string</td> |
| </tr> |
| <tr> |
| <td align="center">\G</td> |
| <td>matches at first matching position in the string</td> |
| </tr> |
| </tbody> |
| </table></div> |
| </div> |
| <br class="table-break"><p> |
| 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 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 string, whatever options are |
| set. Thus, they are independent of multiline mode. These three assertions |
| are not affected by the <code class="varname">G_REGEX_MATCH_NOTBOL</code> or <code class="varname">G_REGEX_MATCH_NOTEOL</code> options, |
| which affect only the behaviour of the circumflex and dollar metacharacters. |
| However, if the start_position argument of a matching function is non-zero, |
| indicating that matching is to start at a point other than the beginning of |
| the string, \A can never match. The difference between \Z and \z is |
| that \Z matches before a newline at the end of the string as well at the |
| very end, 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 start_position argument |
| to the matching functions. It differs from \A when the value of startoffset is |
| non-zero. |
| </p> |
| <p> |
| Note, however, that the 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. |
| </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> |
| </div> |
| </div> |
| <div class="refsect1"> |
| <a name="id-1.5.25.5"></a><h2>Circumflex and dollar</h2> |
| <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 string. If the start_position argument to |
| the matching functions is non-zero, circumflex can never match if the |
| <code class="varname">G_REGEX_MULTILINE</code> option is unset. Inside a character class, circumflex |
| has an entirely different meaning (see below). |
| </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 string, |
| 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 string, or immediately |
| before a newline at the end of 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 <code class="varname">G_REGEX_DOLLAR_ENDONLY</code> 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 |
| <code class="varname">G_REGEX_MULTILINE</code> option is set. When this is the case, |
| a circumflex matches immediately after internal newlines as well as at the |
| start of the string. It does not match after a newline that ends the string. |
| A dollar matches before any newlines in the string, as well as at the very |
| end, when <code class="varname">G_REGEX_MULTILINE</code> is set. When newline is |
| specified as the two-character sequence CRLF, isolated CR and LF characters |
| do not indicate newlines. |
| </p> |
| <p> |
| For example, the pattern /^abc$/ matches the string "def\nabc" (where |
| \n represents a newline) in multiline mode, but not otherwise. Consequently, |
| patterns that are anchored in single line mode because all branches start with |
| ^ are not anchored in multiline mode, and a match for circumflex is possible |
| when the <code class="varname">start_position</code> argument of a matching function |
| is non-zero. The <code class="varname">G_REGEX_DOLLAR_ENDONLY</code> option is ignored |
| if <code class="varname">G_REGEX_MULTILINE</code> is set. |
| </p> |
| <p> |
| Note that the sequences \A, \Z, and \z can be used to match the start and |
| end of the string in both modes, and if all branches of a pattern start with |
| \A it is always anchored, whether or not <code class="varname">G_REGEX_MULTILINE</code> |
| is set. |
| </p> |
| </div> |
| <div class="refsect1"> |
| <a name="id-1.5.25.6"></a><h2>Full stop (period, dot)</h2> |
| <p> |
| Outside a character class, a dot in the pattern matches any one character |
| in the string, including a non-printing character, but not (by |
| default) newline. In UTF-8 a character might be more than one byte long. |
| </p> |
| <p> |
| When a line ending is defined as a single character, dot never matches that |
| character; when the two-character sequence CRLF is used, dot does not match CR |
| if it is immediately followed by LF, but otherwise it matches all characters |
| (including isolated CRs and LFs). When any Unicode line endings are being |
| recognized, dot does not match CR or LF or any of the other line ending |
| characters. |
| </p> |
| <p> |
| If the <code class="varname">G_REGEX_DOTALL</code> flag 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> |
| <p> |
| The behaviour of dot with regard to newlines can be changed. If the |
| <code class="varname">G_REGEX_DOTALL</code> option is set, a dot matches any one |
| character, without exception. If newline is defined as the two-character |
| sequence CRLF, it takes two dots to match it. |
| </p> |
| <p> |
| The handling of dot is entirely independent of the handling of circumflex and |
| dollar, the only relationship being that they both involve newlines. Dot has no |
| special meaning in a character class. |
| </p> |
| </div> |
| <div class="refsect1"> |
| <a name="id-1.5.25.7"></a><h2>Matching a single byte</h2> |
| <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 always matches any line |
| ending characters. |
| 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> |
| GRegex does not allow \C to appear in lookbehind assertions (described |
| below), because in UTF-8 mode this would make it impossible to calculate |
| the length of the lookbehind. |
| </p> |
| </div> |
| <div class="refsect1"> |
| <a name="id-1.5.25.8"></a><h2>Square brackets and character classes</h2> |
| <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 string. 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 |
| string 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 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. |
| </p> |
| <p> |
| Characters that might indicate line breaks are never treated |
| in any special way when matching character classes, whatever line-ending |
| sequence is in use, and whatever setting of the <code class="varname">G_REGEX_DOTALL</code> |
| and <code class="varname">G_REGEX_MULTILINE</code> options is used. A class such as [^a] |
| always matches one of these characters. |
| </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> |
| 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> |
| </div> |
| <div class="refsect1"> |
| <a name="id-1.5.25.9"></a><h2>Posix character classes</h2> |
| <p> |
| GRegex supports the POSIX notation for character classes. This uses names |
| enclosed by [: and :] within the enclosing square brackets. For example, |
| </p> |
| <pre class="programlisting"> |
| [01[:alpha:]%] |
| </pre> |
| <p> |
| matches "0", "1", any alphabetic character, or "%". The supported class |
| names are |
| </p> |
| <div class="table"> |
| <a name="id-1.5.25.9.5"></a><p class="title"><b>Table 9. Posix classes</b></p> |
| <div class="table-contents"><table class="table" summary="Posix classes" border="1"> |
| <colgroup> |
| <col align="center"> |
| <col> |
| </colgroup> |
| <thead><tr> |
| <th align="center">Name</th> |
| <th>Meaning</th> |
| </tr></thead> |
| <tbody> |
| <tr> |
| <td align="center">alnum</td> |
| <td>letters and digits</td> |
| </tr> |
| <tr> |
| <td align="center">alpha</td> |
| <td>letters</td> |
| </tr> |
| <tr> |
| <td align="center">ascii</td> |
| <td>character codes 0 - 127</td> |
| </tr> |
| <tr> |
| <td align="center">blank</td> |
| <td>space or tab only</td> |
| </tr> |
| <tr> |
| <td align="center">cntrl</td> |
| <td>control characters</td> |
| </tr> |
| <tr> |
| <td align="center">digit</td> |
| <td>decimal digits (same as \d)</td> |
| </tr> |
| <tr> |
| <td align="center">graph</td> |
| <td>printing characters, excluding space</td> |
| </tr> |
| <tr> |
| <td align="center">lower</td> |
| <td>lower case letters</td> |
| </tr> |
| <tr> |
| <td align="center">print</td> |
| <td>printing characters, including space</td> |
| </tr> |
| <tr> |
| <td align="center">punct</td> |
| <td>printing characters, excluding letters and digits</td> |
| </tr> |
| <tr> |
| <td align="center">space</td> |
| <td>white space (not quite the same as \s)</td> |
| </tr> |
| <tr> |
| <td align="center">upper</td> |
| <td>upper case letters</td> |
| </tr> |
| <tr> |
| <td align="center">word</td> |
| <td>"word" characters (same as \w)</td> |
| </tr> |
| <tr> |
| <td align="center">xdigit</td> |
| <td>hexadecimal digits</td> |
| </tr> |
| </tbody> |
| </table></div> |
| </div> |
| <br class="table-break"><p> |
| 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. |
| Another Perl extension is negation, which is indicated by a ^ character |
| after the colon. For example, |
| </p> |
| <pre class="programlisting"> |
| [12[:^digit:]] |
| </pre> |
| <p> |
| matches "1", "2", or any non-digit. GRegex 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> |
| </div> |
| <div class="refsect1"> |
| <a name="id-1.5.25.10"></a><h2>Vertical bar</h2> |
| <p> |
| Vertical bar characters are used to separate alternative patterns. For |
| example, the pattern |
| </p> |
| <pre class="programlisting"> |
| gilbert|sullivan |
| </pre> |
| <p> |
| 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 (defined below), "succeeds" means matching the rest of the main pattern as well as the alternative in the subpattern. |
| </p> |
| </div> |
| <div class="refsect1"> |
| <a name="id-1.5.25.11"></a><h2>Internal option setting</h2> |
| <p> |
| The settings of the <code class="varname">G_REGEX_CASELESS</code>, <code class="varname">G_REGEX_MULTILINE</code>, <code class="varname">G_REGEX_MULTILINE</code>, |
| and <code class="varname">G_REGEX_EXTENDED</code> options can be changed from within the pattern by a |
| sequence of Perl-style option letters enclosed between "(?" and ")". The |
| option letters are |
| </p> |
| <div class="table"> |
| <a name="id-1.5.25.11.3"></a><p class="title"><b>Table 10. Option settings</b></p> |
| <div class="table-contents"><table class="table" summary="Option settings" border="1"> |
| <colgroup> |
| <col align="center"> |
| <col> |
| </colgroup> |
| <thead><tr> |
| <th align="center">Option</th> |
| <th>Flag</th> |
| </tr></thead> |
| <tbody> |
| <tr> |
| <td align="center">i</td> |
| <td><code class="varname">G_REGEX_CASELESS</code></td> |
| </tr> |
| <tr> |
| <td align="center">m</td> |
| <td><code class="varname">G_REGEX_MULTILINE</code></td> |
| </tr> |
| <tr> |
| <td align="center">s</td> |
| <td><code class="varname">G_REGEX_DOTALL</code></td> |
| </tr> |
| <tr> |
| <td align="center">x</td> |
| <td><code class="varname">G_REGEX_EXTENDED</code></td> |
| </tr> |
| </tbody> |
| </table></div> |
| </div> |
| <br class="table-break"><p> |
| 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 <code class="varname">G_REGEX_CASELESS</code> |
| and <code class="varname">G_REGEX_MULTILINE</code> while unsetting <code class="varname">G_REGEX_DOTALL</code> and <code class="varname">G_REGEX_EXTENDED</code>, |
| 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. |
| </p> |
| <p> |
| An option change within a subpattern (see below for a description of subpatterns) |
| affects only that part of the current pattern that follows it, so |
| </p> |
| <pre class="programlisting"> |
| (a(?i)b)c |
| </pre> |
| <p> |
| matches abc and aBc and no other strings (assuming <code class="varname">G_REGEX_CASELESS</code> 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, |
| </p> |
| <pre class="programlisting"> |
| (a(?i)b|c) |
| </pre> |
| <p> |
| 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 options <code class="varname">G_REGEX_UNGREEDY</code> and |
| <code class="varname">G_REGEX_EXTRA</code> and <code class="varname">G_REGEX_DUPNAMES</code> |
| can be changed in the same way as the Perl-compatible options by using |
| the characters U, X and J respectively. |
| </p> |
| </div> |
| <div class="refsect1"> |
| <a name="id-1.5.25.12"></a><h2>Subpatterns</h2> |
| <p> |
| Subpatterns are delimited by parentheses (round brackets), which can be |
| nested. Turning part of a pattern into a subpattern does two things: |
| </p> |
| <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> |
| <li class="listitem"><p> |
| It localizes a set of alternatives. For example, the pattern |
| cat(aract|erpillar|) matches one of the words "cat", "cataract", or |
| "caterpillar". Without the parentheses, it would match "cataract", |
| "erpillar" or an empty string. |
| </p></li> |
| <li class="listitem"><p> |
| It sets up the subpattern as a capturing subpattern. This means |
| that, when the whole pattern matches, that portion of the |
| string that matched the subpattern can be obtained using <code class="function">g_match_info_fetch()</code>. |
| Opening parentheses are counted from left to right (starting from 1, as |
| subpattern 0 is the whole matched string) to obtain numbers for the |
| capturing subpatterns. |
| </p></li> |
| </ul></div> |
| <p> |
| For example, if the string "the red king" is matched against the pattern |
| </p> |
| <pre class="programlisting"> |
| the ((red|white) (king|queen)) |
| </pre> |
| <p> |
| 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 |
| </p> |
| <pre class="programlisting"> |
| the ((?:red|white) (king|queen)) |
| </pre> |
| <p> |
| the captured substrings are "white queen" and "queen", and are numbered |
| 1 and 2. The maximum number of capturing subpatterns is 65535. |
| </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 |
| </p> |
| <pre class="programlisting"> |
| (?i:saturday|sunday) |
| (?:(?i)saturday|sunday) |
| </pre> |
| <p> |
| 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> |
| </div> |
| <div class="refsect1"> |
| <a name="id-1.5.25.13"></a><h2>Named subpatterns</h2> |
| <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, GRegex supports the naming of |
| subpatterns. A subpattern can be named in one of three ways: (?<name>...) or |
| (?'name'...) as in Perl, or (?P<name>...) as in Python. |
| References to capturing parentheses from other |
| parts of the pattern, such as backreferences, recursion, and conditions, |
| can be made by name as well as by number. |
| </p> |
| <p> |
| Names consist of up to 32 alphanumeric characters and underscores. Named |
| capturing parentheses are still allocated numbers as well as names, exactly as |
| if the names were not present. |
| By default, a name must be unique within a pattern, but it is possible to relax |
| this constraint by setting the <code class="varname">G_REGEX_DUPNAMES</code> option at |
| compile time. This can be useful for patterns where only one instance of the |
| named parentheses can match. Suppose you want to match the name of a weekday, |
| either as a 3-letter abbreviation or as the full name, and in both cases you |
| want to extract the abbreviation. This pattern (ignoring the line breaks) does |
| the job: |
| </p> |
| <pre class="programlisting"> |
| (?<DN>Mon|Fri|Sun)(?:day)?| |
| (?<DN>Tue)(?:sday)?| |
| (?<DN>Wed)(?:nesday)?| |
| (?<DN>Thu)(?:rsday)?| |
| (?<DN>Sat)(?:urday)? |
| </pre> |
| <p> |
| There are five capturing substrings, but only one is ever set after a match. |
| The function for extracting the data by name returns the substring |
| for the first (and in this example, the only) subpattern of that name that |
| matched. This saves searching to find which numbered subpattern it was. If you |
| make a reference to a non-unique named subpattern from elsewhere in the |
| pattern, the one that corresponds to the lowest number is used. |
| </p> |
| </div> |
| <div class="refsect1"> |
| <a name="id-1.5.25.14"></a><h2>Repetition</h2> |
| <p> |
| Repetition is specified by quantifiers, which can follow any of the |
| following items: |
| </p> |
| <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> |
| <li class="listitem"><p>a literal data character</p></li> |
| <li class="listitem"><p>the dot metacharacter</p></li> |
| <li class="listitem"><p>the \C escape sequence</p></li> |
| <li class="listitem"><p>the \X escape sequence (in UTF-8 mode)</p></li> |
| <li class="listitem"><p>the \R escape sequence</p></li> |
| <li class="listitem"><p>an escape such as \d that matches a single character</p></li> |
| <li class="listitem"><p>a character class</p></li> |
| <li class="listitem"><p>a back reference (see next section)</p></li> |
| <li class="listitem"><p>a parenthesized subpattern (unless it is an assertion)</p></li> |
| </ul></div> |
| <p> |
| 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: |
| </p> |
| <pre class="programlisting"> |
| z{2,4} |
| </pre> |
| <p> |
| 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 |
| </p> |
| <pre class="programlisting"> |
| [aeiou]{3,} |
| </pre> |
| <p> |
| matches at least 3 successive vowels, but may match many more, while |
| </p> |
| <pre class="programlisting"> |
| \d{8} |
| </pre> |
| <p> |
| 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, |
| \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, the three most common quantifiers have single-character |
| abbreviations: |
| </p> |
| <div class="table"> |
| <a name="id-1.5.25.14.14"></a><p class="title"><b>Table 11. Abbreviations for quantifiers</b></p> |
| <div class="table-contents"><table class="table" summary="Abbreviations for quantifiers" border="1"> |
| <colgroup> |
| <col align="center"> |
| <col> |
| </colgroup> |
| <thead><tr> |
| <th align="center">Abbreviation</th> |
| <th>Meaning</th> |
| </tr></thead> |
| <tbody> |
| <tr> |
| <td align="center">*</td> |
| <td>is equivalent to {0,}</td> |
| </tr> |
| <tr> |
| <td align="center">+</td> |
| <td>is equivalent to {1,}</td> |
| </tr> |
| <tr> |
| <td align="center">?</td> |
| <td>is equivalent to {0,1}</td> |
| </tr> |
| </tbody> |
| </table></div> |
| </div> |
| <br class="table-break"><p> |
| 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: |
| </p> |
| <pre class="programlisting"> |
| (a?)* |
| </pre> |
| <p> |
| Because there are cases where this can be useful, such patterns are |
| 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 |
| </p> |
| <pre class="programlisting"> |
| /\*.*\*/ |
| </pre> |
| <p> |
| to the string |
| </p> |
| <pre class="programlisting"> |
| /* first comment */ not comment /* second comment */ |
| </pre> |
| <p> |
| 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 |
| </p> |
| <pre class="programlisting"> |
| /\*.*?\*/ |
| </pre> |
| <p> |
| 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 |
| </p> |
| <pre class="programlisting"> |
| \d??\d |
| </pre> |
| <p> |
| 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 <code class="varname">G_REGEX_UNGREEDY</code> flag is set, 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 <code class="varname">G_REGEX_DOTALL</code> flag |
| is set, thus allowing the dot to match newlines, the |
| pattern is implicitly anchored, because whatever follows will be tried |
| against every character position in the string, so there is no |
| point in retrying the overall match at any position after the first. |
| GRegex normally treats such a pattern as though it were preceded by \A. |
| </p> |
| <p> |
| In cases where it is known that the string contains no newlines, it |
| is worth setting <code class="varname">G_REGEX_DOTALL</code> 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 |
| where a later one succeeds. Consider, for example: |
| </p> |
| <pre class="programlisting"> |
| (.*)abc\1 |
| </pre> |
| <p> |
| If the string 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 |
| </p> |
| <pre class="programlisting"> |
| (tweedle[dume]{3}\s*)+ |
| </pre> |
| <p> |
| 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 |
| </p> |
| <pre class="programlisting"> |
| /(a|(b))+/ |
| </pre> |
| <p> |
| matches "aba" the value of the second captured substring is "b". |
| </p> |
| </div> |
| <div class="refsect1"> |
| <a name="id-1.5.25.15"></a><h2>Atomic grouping and possessive quantifiers</h2> |
| <p> |
| With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy") |
| repetition, failure of what follows normally causes the repeated |
| item to be re-evaluated to see if a different number |
| of repeats allows the rest of the pattern to match. Sometimes it |
| is useful to prevent this, either to change the nature of the |
| match, or to cause it fail earlier than it otherwise might, when the |
| author of the pattern knows there is no point in carrying on. |
| </p> |
| <p> |
| Consider, for example, the pattern \d+foo when applied to the string |
| </p> |
| <pre class="programlisting"> |
| 123456bar |
| </pre> |
| <p> |
| 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 |
| 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: |
| </p> |
| <pre class="programlisting"> |
| (?>\d+)foo |
| </pre> |
| <p> |
| 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 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 |
| </p> |
| <pre class="programlisting"> |
| \d++foo |
| </pre> |
| <p> |
| Possessive quantifiers are always greedy; the setting of the |
| <code class="varname">G_REGEX_UNGREEDY</code> option is ignored. They are a convenient notation for the |
| simpler forms of atomic group. However, there is no difference in the |
| meaning of a possessive quantifier and the equivalent |
| atomic group, though there may be a performance difference; |
| possessive quantifiers should be slightly faster. |
| </p> |
| <p> |
| The possessive quantifier syntax is an extension to the Perl syntax. |
| It was invented by Jeffrey Friedl in the first edition of his book and |
| then implemented by Mike McCloskey in Sun's Java package. |
| It ultimately found its way into Perl at release 5.10. |
| </p> |
| <p> |
| GRegex has an optimization that automatically "possessifies" certain simple |
| pattern constructs. For example, the sequence A+B is treated as A++B because |
| there is no point in backtracking into a sequence of A's when B must follow. |
| </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 |
| </p> |
| <pre class="programlisting"> |
| (\D+|<\d+>)*[!?] |
| </pre> |
| <p> |
| 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 |
| </p> |
| <pre class="programlisting"> |
| aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa |
| </pre> |
| <p> |
| 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 |
| GRegex has an optimization that allows for fast failure |
| when a single character is used. It 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: |
| </p> |
| <pre class="programlisting"> |
| ((?>\D+)|<\d+>)*[!?] |
| </pre> |
| <p> |
| sequences of non-digits cannot be broken, and failure happens quickly. |
| </p> |
| </div> |
| <div class="refsect1"> |
| <a name="id-1.5.25.16"></a><h2>Back references</h2> |
| <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. A "forward back |
| reference" of this type can make sense when a repetition is involved and |
| the subpattern to the right has participated in an earlier iteration. |
| </p> |
| <p> |
| It is not possible to have a numerical "forward back reference" to subpattern |
| whose number is 10 or more using this syntax because a sequence such as \e50 is |
| interpreted as a character defined in octal. See the subsection entitled |
| "Non-printing characters" above for further details of the handling of digits |
| following a backslash. There is no such problem when named parentheses are used. |
| A back reference to any subpattern is possible using named parentheses (see below). |
| </p> |
| <p> |
| Another way of avoiding the ambiguity inherent in the use of digits following a |
| backslash is to use the \g escape sequence (introduced in Perl 5.10.) |
| This escape must be followed by a positive or a negative number, |
| optionally enclosed in braces. |
| </p> |
| <p> |
| A positive number specifies an absolute reference without the ambiguity that is |
| present in the older syntax. It is also useful when literal digits follow the |
| reference. A negative number is a relative reference. Consider "(abc(def)ghi)\g{-1}", |
| the sequence \g{-1} is a reference to the most recently started capturing |
| subpattern before \g, that is, is it equivalent to \2. Similarly, \g{-2} |
| would be equivalent to \1. The use of relative references can be helpful in |
| long patterns, and also in patterns that are created by joining together |
| fragments that contain references within themselves. |
| </p> |
| <p> |
| A back reference matches whatever actually matched the capturing subpattern |
| in the current string, rather than anything matching |
| the subpattern itself (see "Subpatterns as subroutines" below for a way |
| of doing that). So the pattern |
| </p> |
| <pre class="programlisting"> |
| (sens|respons)e and \1ibility |
| </pre> |
| <p> |
| 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, |
| </p> |
| <pre class="programlisting"> |
| ((?i)rah)\s+\1 |
| </pre> |
| <p> |
| 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 Perl syntax \k<name> or \k'name' |
| or the Python syntax (?P=name). We could rewrite the above example in either of |
| the following ways: |
| </p> |
| <pre class="programlisting"> |
| (?<p1>(?i)rah)\s+\k<p1> |
| (?P<p1>(?i)rah)\s+(?P=p1) |
| </pre> |
| <p> |
| A subpattern that is referenced by name may appear in the pattern before or |
| after the reference. |
| </p> |
| <p> |
| 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 |
| </p> |
| <pre class="programlisting"> |
| (a|(bc))\2 |
| </pre> |
| <p> |
| 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 <code class="varname">G_REGEX_EXTENDED</code> flag is |
| set, this can be whitespace. Otherwise an empty comment (see "Comments" 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 |
| </p> |
| <pre class="programlisting"> |
| (a|b\1)+ |
| </pre> |
| <p> |
| 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. |
| </p> |
| </div> |
| <div class="refsect1"> |
| <a name="id-1.5.25.17"></a><h2>Assertions</h2> |
| <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 above. |
| </p> |
| <p> |
| More complicated assertions are coded as subpatterns. There are two |
| kinds: those that look ahead of the current position in the |
| 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> |
| <div class="refsect2"> |
| <a name="id-1.5.25.17.5"></a><h3>Lookahead assertions</h3> |
| <p> |
| Lookahead assertions start with (?= for positive assertions and (?! for |
| negative assertions. For example, |
| </p> |
| <pre class="programlisting"> |
| \w+(?=;) |
| </pre> |
| <p> |
| matches a word followed by a semicolon, but does not include the semicolon |
| in the match, and |
| </p> |
| <pre class="programlisting"> |
| foo(?!bar) |
| </pre> |
| <p> |
| matches any occurrence of "foo" that is not followed by "bar". Note |
| that the apparently similar pattern |
| </p> |
| <pre class="programlisting"> |
| (?!foo)bar |
| </pre> |
| <p> |
| 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. |
| </p> |
| </div> |
| <hr> |
| <div class="refsect2"> |
| <a name="id-1.5.25.17.6"></a><h3>Lookbehind assertions</h3> |
| <p> |
| Lookbehind assertions start with (?<= for positive assertions and (?<! |
| for negative assertions. For example, |
| </p> |
| <pre class="programlisting"> |
| (?<!foo)bar |
| </pre> |
| <p> |
| 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 top-level alternatives, they do not all have to have the same |
| fixed length. Thus |
| </p> |
| <pre class="programlisting"> |
| (?<=bullock|donkey) |
| </pre> |
| <p> |
| is permitted, but |
| </p> |
| <pre class="programlisting"> |
| (?<!dogs?|cats?) |
| </pre> |
| <p> |
| causes an error at compile time. Branches that match different length |
| strings are permitted only at the top level of a lookbehind assertion. |
| An assertion such as |
| </p> |
| <pre class="programlisting"> |
| (?<=ab(c|de)) |
| </pre> |
| <p> |
| 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: |
| </p> |
| <pre class="programlisting"> |
| (?<=abc|abde) |
| </pre> |
| <p> |
| The implementation of lookbehind assertions is, for each alternative, |
| to temporarily move the current position back by the fixed length and |
| then try to match. If there are insufficient characters before the |
| current position, the assertion fails. |
| </p> |
| <p> |
| GRegex 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 and \R escapes, which can |
| match different numbers of bytes, are also not permitted. |
| </p> |
| <p> |
| Possessive quantifiers 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 |
| </p> |
| <pre class="programlisting"> |
| abcd$ |
| </pre> |
| <p> |
| when applied to a long string that does not match. Because matching |
| proceeds from left to right, GRegex will look for each "a" in the string |
| and then see if what follows matches the rest of the pattern. If the |
| pattern is specified as |
| </p> |
| <pre class="programlisting"> |
| ^.*abcd$ |
| </pre> |
| <p> |
| 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 |
| </p> |
| <pre class="programlisting"> |
| ^.*+(?<=abcd) |
| </pre> |
| <p> |
| 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> |
| </div> |
| <hr> |
| <div class="refsect2"> |
| <a name="id-1.5.25.17.7"></a><h3>Using multiple assertions</h3> |
| <p> |
| Several assertions (of any sort) may occur in succession. For example, |
| </p> |
| <pre class="programlisting"> |
| (?<=\d{3})(?<!999)foo |
| </pre> |
| <p> |
| 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 string. First there is a check that the previous three |
| characters are all digits, and then there is a check that the same |
| three characters are not "999". This pattern does not match "foo" preceded |
| by six characters, the first of which are digits and the last |
| three of which are not "999". For example, it doesn’t match "123abcfoo". |
| A pattern to do that is |
| </p> |
| <pre class="programlisting"> |
| (?<=\d{3}...)(?<!999)foo |
| </pre> |
| <p> |
| 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, |
| </p> |
| <pre class="programlisting"> |
| (?<=(?<!foo)bar)baz |
| </pre> |
| <p> |
| matches an occurrence of "baz" that is preceded by "bar" which in turn |
| is not preceded by "foo", while |
| </p> |
| <pre class="programlisting"> |
| (?<=\d{3}(?!999)...)foo |
| </pre> |
| <p> |
| is another pattern that matches "foo" preceded by three digits and any |
| three characters that are not "999". |
| </p> |
| </div> |
| </div> |
| <div class="refsect1"> |
| <a name="id-1.5.25.18"></a><h2>Conditional subpatterns</h2> |
| <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 |
| </p> |
| <pre class="programlisting"> |
| (?(condition)yes-pattern) |
| (?(condition)yes-pattern|no-pattern) |
| </pre> |
| <p> |
| 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 four kinds of condition: references to subpatterns, references to |
| recursion, a pseudo-condition called DEFINE, and assertions. |
| </p> |
| <div class="refsect2"> |
| <a name="id-1.5.25.18.6"></a><h3>Checking for a used subpattern by number</h3> |
| <p> |
| If the text between the parentheses consists of a sequence of digits, the |
| condition is true if the capturing subpattern of that number has previously |
| matched. |
| </p> |
| <p> |
| Consider the following pattern, which contains non-significant white space |
| to make it more readable (assume the <code class="varname">G_REGEX_EXTENDED</code>) |
| and to divide it into three parts for ease of discussion: |
| </p> |
| <pre class="programlisting"> |
| ( \( )? [^()]+ (?(1) \) ) |
| </pre> |
| <p> |
| 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 string 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> |
| </div> |
| <hr> |
| <div class="refsect2"> |
| <a name="id-1.5.25.18.7"></a><h3>Checking for a used subpattern by name</h3> |
| <p> |
| Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used |
| subpattern by name, the Python syntax (?(name)...) is also recognized. However, |
| there is a possible ambiguity with this syntax, because subpattern names may |
| consist entirely of digits. GRegex looks first for a named subpattern; if it |
| cannot find one and the name consists entirely of digits, GRegex looks for a |
| subpattern of that number, which must be greater than zero. Using subpattern |
| names that consist entirely of digits is not recommended. |
| </p> |
| <p> |
| Rewriting the above example to use a named subpattern gives this: |
| </p> |
| <pre class="programlisting"> |
| (?<OPEN> \( )? [^()]+ (?(<OPEN>) \) ) |
| </pre> |
| </div> |
| <hr> |
| <div class="refsect2"> |
| <a name="id-1.5.25.18.8"></a><h3>Checking for pattern recursion</h3> |
| <p> |
| If the condition is the string (R), and there is no subpattern with the name R, |
| the condition is true if a recursive call to the whole pattern or any |
| subpattern has been made. If digits or a name preceded by ampersand follow the |
| letter R, for example: |
| </p> |
| <pre class="programlisting"> |
| (?(R3)...) |
| (?(R&name)...) |
| </pre> |
| <p> |
| the condition is true if the most recent recursion is into the subpattern whose |
| number or name is given. This condition does not check the entire recursion |
| stack. |
| </p> |
| <p> |
| At "top level", all these recursion test conditions are false. Recursive |
| patterns are described below. |
| </p> |
| </div> |
| <hr> |
| <div class="refsect2"> |
| <a name="id-1.5.25.18.9"></a><h3>Defining subpatterns for use by reference only</h3> |
| <p> |
| If the condition is the string (DEFINE), and there is no subpattern with the |
| name DEFINE, the condition is always false. In this case, there may be only one |
| alternative in the subpattern. It is always skipped if control reaches this |
| point in the pattern; the idea of DEFINE is that it can be used to define |
| "subroutines" that can be referenced from elsewhere. (The use of "subroutines" |
| is described below.) For example, a pattern to match an IPv4 address could be |
| written like this (ignore whitespace and line breaks): |
| </p> |
| <pre class="programlisting"> |
| (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) ) |
| \b (?&byte) (\.(?&byte)){3} \b |
| </pre> |
| <p> |
| The first part of the pattern is a DEFINE group inside which a another group |
| named "byte" is defined. This matches an individual component of an IPv4 |
| address (a number less than 256). When matching takes place, this part of the |
| pattern is skipped because DEFINE acts like a false condition. |
| </p> |
| <p> |
| The rest of the pattern uses references to the named group to match the four |
| dot-separated components of an IPv4 address, insisting on a word boundary at |
| each end. |
| </p> |
| </div> |
| <hr> |
| <div class="refsect2"> |
| <a name="id-1.5.25.18.10"></a><h3>Assertion conditions</h3> |
| <p> |
| If the condition is not in any of the above formats, 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: |
| </p> |
| <pre class="programlisting"> |
| (?(?=[^a-z]*[a-z]) |
| \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} ) |
| </pre> |
| <p> |
| 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 string. If a |
| letter is found, the string 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. |
| </p> |
| </div> |
| </div> |
| <div class="refsect1"> |
| <a name="id-1.5.25.19"></a><h2>Comments</h2> |
| <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 <code class="varname">G_REGEX_EXTENDED</code> option is set, an unescaped # |
| character outside a character class introduces a comment that continues to |
| immediately after the next newline in the pattern. |
| </p> |
| </div> |
| <div class="refsect1"> |
| <a name="id-1.5.25.20"></a><h2>Recursive patterns</h2> |
| <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. |
| </p> |
| <p> |
| For some time, Perl has provided a facility that allows regular expressions to |
| recurse (amongst other things). It does this by interpolating Perl code in the |
| expression at run time, and the code can refer to the expression itself. A Perl |
| pattern using code interpolation to solve the parentheses problem can be |
| created like this: |
| </p> |
| <pre class="programlisting"> |
| $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x; |
| </pre> |
| <p> |
| The (?p{...}) item interpolates Perl code at run time, and in this case refers |
| recursively to the pattern in which it appears. |
| </p> |
| <p> |
| Obviously, GRegex cannot support the interpolation of Perl code. Instead, it |
| supports special syntax for recursion of the entire pattern, and also for |
| individual subpattern recursion. This kind of recursion was introduced into |
| Perl at release 5.10. |
| </p> |
| <p> |
| A 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) or (?0) is |
| a recursive call of the entire regular expression. |
| </p> |
| <p> |
| In GRegex (like Python, but unlike Perl), a recursive subpattern call is always |
| treated as an atomic group. That is, once it has matched some of the subject |
| string, it is never re-entered, even if it contains untried alternatives and |
| there is a subsequent matching failure. |
| </p> |
| <p> |
| This pattern solves the nested parentheses problem (assume the |
| <code class="varname">G_REGEX_EXTENDED</code> option is set so that white space is |
| ignored): |
| </p> |
| <pre class="programlisting"> |
| \( ( (?>[^()]+) | (?R) )* \) |
| </pre> |
| <p> |
| 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: |
| </p> |
| <pre class="programlisting"> |
| ( \( ( (?>[^()]+) | (?1) )* \) ) |
| </pre> |
| <p> |
| 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. |
| The Perl syntax for this is (?&name); GRegex also supports the(?P>name) |
| syntac. We could rewrite the above example as follows: |
| </p> |
| <pre class="programlisting"> |
| (?<pn> \( ( (?>[^()]+) | (?&pn) )* \) ) |
| </pre> |
| <p> |
| If there is more than one subpattern with the same name, the earliest one is |
| used. 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 |
| </p> |
| <pre class="programlisting"> |
| (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa() |
| </pre> |
| <p> |
| 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 string, 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 the pattern above is matched against |
| </p> |
| <pre class="programlisting"> |
| (ab(cd)ef) |
| </pre> |
| <p> |
| 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 |
| </p> |
| <pre class="programlisting"> |
| \( ( ( (?>[^()]+) | (?R) )* ) \) |
| ^ ^ |
| ^ ^ |
| </pre> |
| <p> |
| the string they capture is "ab(cd)ef", the contents of the top level |
| parentheses. |
| </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. |
| </p> |
| <pre class="programlisting"> |
| < (?: (?(R) \d++ | [^<>]*+) | (?R)) * > |
| </pre> |
| <p> |
| 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. |
| </p> |
| </div> |
| <div class="refsect1"> |
| <a name="id-1.5.25.21"></a><h2>Subpatterns as subroutines</h2> |
| <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. The "called" subpattern may |
| be defined before or after the reference. An earlier example pointed out |
| that the pattern |
| </p> |
| <pre class="programlisting"> |
| (sens|respons)e and \1ibility |
| </pre> |
| <p> |
| matches "sense and sensibility" and "response and responsibility", but |
| not "sense and responsibility". If instead the pattern |
| </p> |
| <pre class="programlisting"> |
| (sens|respons)e and (?1)ibility |
| </pre> |
| <p> |
| is used, it does match "sense and responsibility" as well as the other |
| two strings. Another example is given in the discussion of DEFINE above. |
| </p> |
| <p> |
| Like recursive subpatterns, a "subroutine" call is always treated as an atomic |
| group. That is, once it has matched some of the string, it is never |
| re-entered, even if it contains untried alternatives and there is a subsequent |
| matching failure. |
| </p> |
| <p> |
| When a subpattern is used as a subroutine, processing options such as |
| case-independence are fixed when the subpattern is defined. They cannot be |
| changed for different calls. For example, consider this pattern: |
| </p> |
| <pre class="programlisting"> |
| (abc)(?i:(?1)) |
| </pre> |
| <p> |
| It matches "abcabc". It does not match "abcABC" because the change of |
| processing option does not affect the called subpattern. |
| </p> |
| </div> |
| <div class="refsect1"> |
| <a name="id-1.5.25.22"></a><h2>Copyright</h2> |
| <p> |
| This document was copied and adapted from the PCRE documentation, |
| specifically from the man page for pcrepattern. |
| The original copyright note is: |
| </p> |
| <pre class="programlisting"> |
| Copyright (c) 1997-2006 University of Cambridge. |
| |
| Redistribution and use in source and binary forms, with or without |
| modification, are permitted provided that the following conditions are met: |
| |
| * Redistributions of source code must retain the above copyright notice, |
| this list of conditions and the following disclaimer. |
| |
| * Redistributions in binary form must reproduce the above copyright |
| notice, this list of conditions and the following disclaimer in the |
| documentation and/or other materials provided with the distribution. |
| |
| * Neither the name of the University of Cambridge nor the name of Google |
| Inc. nor the names of their contributors may be used to endorse or |
| promote products derived from this software without specific prior |
| written permission. |
| |
| THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" |
| AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE |
| IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE |
| ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE |
| LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR |
| CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF |
| SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS |
| INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN |
| CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) |
| ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE |
| POSSIBILITY OF SUCH DAMAGE. |
| </pre> |
| </div> |
| </div> |
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