/vendor/cegui-0.4.0-custom/src/pcre/doc/html/pcrepattern.html
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- <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