| SYNOPSIS |
| PCRE - Perl-compatible regular expressions |
| |
| DESCRIPTION |
| This document describes the regular expressions supported by the PCRE |
| package. When the package is compiled into the driver, the macro |
| __PCRE__ is defined. |
| |
| Most of this manpage is lifted directly from the original PCRE manpage |
| (dated January 2003). |
| |
| The PCRE library is a set of functions that implement regular |
| expression pattern matching using the same syntax and semantics as |
| Perl 5, with just a few differences (see below). The current |
| implementation corresponds to Perl 5.005, with some additional features |
| from later versions. This includes some experimental, incomplete |
| support for UTF-8 encoded strings. Details of exactly what is and what |
| is not supported are given below. |
| |
| PCRE REGULAR EXPRESSION DETAILS |
| 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 other books, some of which have |
| copious examples. Jeffrey Friedl's "Mastering Regular Expressions", |
| published by O'Reilly, covers them in great detail. The description |
| here is intended as reference documentation. |
| |
| The basic operation of PCRE is on strings of bytes. However, there is |
| also support for UTF-8 character strings. To use this support you must |
| build PCRE to include UTF-8 support, and then call pcre_compile() with |
| the PCRE_UTF8 option. How this affects the pattern matching is |
| mentioned in several places below. There is also a summary of UTF-8 |
| features in the section on UTF-8 support in the main pcre page. |
| |
| A regular expression is a pattern that is matched against a subject |
| string from left to right. Most characters stand for themselves in a |
| pattern, and match the corresponding characters in the subject. As a |
| trivial example, the pattern |
| |
| The quick brown fox |
| |
| matches a portion of a subject string that is identical to itself. 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 meta-characters, which do not stand for |
| themselves but instead are interpreted in some special way. |
| |
| There are two different sets of meta-characters: those that are |
| recognized anywhere in the pattern except within square brackets, and |
| those that are recognized in square brackets. Outside square brackets, |
| the meta-characters are as follows: |
| |
| \ general escape character with several uses |
| ^ assert start of string (or line, in multiline mode) |
| $ assert end of string (or line, in multiline mode) |
| . match any character except newline (by default) |
| [ start character class definition |
| | start of alternative branch |
| ( start 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 |
| |
| Part of a pattern that is in square brackets is called a "character |
| class". In a character class the only meta-characters are: |
| |
| \ 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 |
| |
| The following sections describe the use of each of the meta-characters. |
| |
| BACKSLASH |
| The backslash character has several uses. Firstly, if it is followed by |
| a non-alphameric 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. |
| |
| 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 meta-character, so it is |
| always safe to precede a non-alphameric with backslash to specify that |
| it stands for itself. In particular, if you want to match a backslash, |
| you write \\. |
| |
| 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. |
| |
| 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: |
| |
| 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 |
| |
| The \Q...\E sequence is recognized both inside and outside character |
| classes. |
| |
| 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: |
| |
| \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) |
| |
| 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. |
| |
| 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 |
| dig-its 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 byte whose value |
| is zero. |
| |
| 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}. |
| |
| 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 character that follows is itself an octal digit. |
| |
| 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 back reference. A |
| description of how this works is given later, following the discussion |
| of parenthesized subpatterns. |
| |
| 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: |
| |
| \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" |
| |
| 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. |
| |
| 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). Outside a character |
| class it has a different meaning (see below). |
| |
| The third use of backslash is for specifying generic character types: |
| |
| \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 |
| |
| 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. |
| |
| In UTF-8 mode, characters with values greater than 255 never match \d, |
| \s, or \w, and always match \D, \S, and \W. |
| |
| 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). |
| |
| A "word" character is any letter or digit or the underscore character, |
| that is, any character which can be part of a Perl "word". The |
| definition of letters and digits is controlled by PCRE's character |
| tables, and may vary if locale-specific matching is taking place (see |
| "Locale support" in the pcreapi page). For example, in the "fr" |
| (French) locale, some character codes greater than 128 are used for |
| accented letters, and these are matched by \w. |
| |
| 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. |
| |
| 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 below. The backslashed assertions are: |
| |
| \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 |
| |
| These assertions may not appear in character classes (but note that \b |
| has a different meaning, namely the backspace character, inside a |
| character class). |
| |
| 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. |
| |
| The \A, \Z, and \z assertions differ from the traditional circumflex |
| and dollar (described below) 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. |
| |
| They are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options. If the |
| startoffset argument of pcre_exec() 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. |
| |
| The \G assertion is true only when the current matching position is at |
| the start point of the match, as specified by the startoffset argument |
| of pcre_exec(). It differs from \A when the value of startoffset is |
| non-zero. By calling pcre_exec() multiple times with appropriate |
| arguments, you can mimic Perl's /g option, and it is in this kind of |
| implementation where \G can be useful. |
| |
| Note, however, that 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. |
| |
| If all the alternatives of a pattern begin with \G, the expression is |
| anchored to the starting match position, and the "anchored" flag is set |
| in the compiled regular expression. |
| |
| CIRCUMFLEX AND DOLLAR |
| Outside a character class, in the default matching mode, the circumflex |
| character is an assertion which is true only if the current matching |
| point is at the start of the subject string. If the startoffset |
| argument of pcre_exec() is non-zero, circumflex can never match if the |
| PCRE_MULTILINE option is unset. Inside a character class, circumflex |
| has an entirely different meaning (see below). |
| |
| Circumflex need not be the first character of the pattern if a number |
| of alternatives are involved, but it should be the first thing in each |
| alternative in which it appears if the pattern is ever to match that |
| branch. If all possible alternatives start with a circumflex, that is, |
| if the pattern is constrained to match only at the start of the |
| subject, it is said to be an "anchored" pattern. (There are also other |
| constructs that can cause a pattern to be anchored.) |
| |
| A dollar character is an assertion which 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. |
| |
| 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. |
| |
| 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" in multiline mode, but not otherwise. Consequently, |
| patterns that are anchored in single line mode because all branches |
| start with ^ are not anchored in multiline mode, and a match for |
| circumflex is possible when the startoffset argument of pcre_exec() is |
| non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE |
| is set. |
| |
| 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. |
| |
| FULL STOP (PERIOD, DOT) |
| 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) for |
| 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. |
| |
| MATCHING A SINGLE BYTE |
| 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 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 it is best avoided. |
| |
| PCRE does not allow \C to appear in lookbehind assertions (see below), |
| because in UTF-8 mode it makes it impossible to calculate the length of |
| the lookbehind. |
| |
| SQUARE BRACKETS |
| 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. |
| |
| 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. |
| |
| 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 which are in the class by enumerating those that are not. It |
| is not an assertion: it still consumes a character from the subject |
| string, and fails if the current pointer is at the end of the string. |
| |
| 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. |
| |
| 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. PCRE does not support the |
| concept of case for characters with values greater than 255. |
| |
| 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. |
| |
| 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. |
| |
| 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 single class containing a range followed |
| by two separate characters. The octal or hexadecimal representation of |
| "]" can also be used to end a range. |
| |
| 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}]. |
| |
| 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 if character tables for the |
| "fr" locale are in use, [\xc8-\xcb] matches accented E characters in |
| both cases. |
| |
| The character types \d, \D, \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. |
| |
| All non-alphameric characters other than \, -, ^ (at the start) and the |
| terminating ] are non-special in character classes, but it does no harm |
| if they are escaped. |
| |
| POSIX CHARACTER CLASSES |
| Perl supports the POSIX notation for character classes, which uses |
| names enclosed by [: and :] within the enclosing square brackets. PCRE |
| also supports this notation. For example, |
| |
| [01[:alpha:]%] |
| |
| matches "0", "1", any alphabetic character, or "%". The supported class |
| names are |
| |
| alnum letters and digits |
| alpha letters |
| ascii character codes 0 - 127 |
| blank space or tab only |
| cntrl control characters |
| digit decimal digits (same as \d) |
| graph printing characters, excluding space |
| lower lower case letters |
| print printing characters, including space |
| punct printing characters, excluding letters and digits |
| space white space (not quite the same as \s) |
| upper upper case letters |
| word "word" characters (same as \w) |
| xdigit hexadecimal digits |
| |
| 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). |
| |
| The name "word" is a Perl extension, and "blank" is a GNU extension |
| from Perl 5.8. Another Perl extension is negation, which is indicated |
| by a ^ character after the colon. For example, |
| |
| [12[:^digit:]] |
| |
| matches "1", "2", or any non-digit. 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. |
| |
| In UTF-8 mode, characters with values greater than 255 do not match any |
| of the POSIX character classes. |
| |
| VERTICAL BAR |
| Vertical bar characters are used to separate alternative patterns. For |
| example, the pattern |
| |
| gilbert|sullivan |
| |
| matches either "gilbert" or "sullivan". Any number of alternatives may |
| appear, and an empty alternative is permitted (matching the empty |
| string). The matching process tries each alternative in turn, from |
| left to right, and the first one that succeeds is used. If the |
| alternatives are within a subpattern (defined below), "succeeds" means |
| matching the rest of the main pattern as well as the alternative in the |
| subpattern. |
| |
| INTERNAL OPTION SETTING |
| 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 |
| |
| i for PCRE_CASELESS |
| m for PCRE_MULTILINE |
| s for PCRE_DOTALL |
| x for PCRE_EXTENDED |
| |
| 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. |
| |
| 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 pcre_fullinfo() function). |
| |
| An option change within a subpattern affects only that part of the |
| current pattern that follows it, so |
| |
| (a(?i)b)c |
| |
| 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, |
| |
| (a(?i)b|c) |
| |
| matches "ab", "aB", "c", and "C", even though when matching "C" the |
| first branch is abandoned before the option setting. This is because |
| the effects of option settings happen at compile time. There would be |
| some very weird behaviour otherwise. |
| |
| 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 put at the start. |
| |
| SUBPATTERNS |
| Subpatterns are delimited by parentheses (round brackets), which can be |
| nested. Marking part of a pattern as a subpattern does two things: |
| |
| 1. 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 the empty |
| string. |
| |
| 2. It sets up the subpattern as a capturing subpattern (as defined |
| above). When the whole pattern matches, that portion of the subject |
| string that matched the subpattern is passed back to the caller via the |
| ovector argument of pcre_exec(). Opening parentheses are counted from |
| left to right (starting from 1) to obtain the numbers of the capturing |
| subpatterns. |
| |
| For example, if the string "the red king" is matched against the |
| pattern |
| |
| the ((red|white) (king|queen)) |
| |
| the captured substrings are "red king", "red", and "king", and are |
| numbered 1, 2, and 3, respectively. |
| |
| The fact that plain parentheses fulfil two functions is not always |
| helpful. There are often times when 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 |
| |
| the ((?:red|white) (king|queen)) |
| |
| 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 |
| noncapturing, is 200. |
| |
| 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 |
| |
| (?i:saturday|sunday) |
| (?:(?i)saturday|sunday) |
| |
| match exactly the same set of strings. Because alternative branches are |
| tried from left to right, and options are not reset until the end of |
| the subpattern is reached, an option setting in one branch does affect |
| subsequent branches, so the above patterns match "SUNDAY" as well as |
| "Saturday". |
| |
| NAMED SUBPATTERNS |
| 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 the 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. |
| |
| 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. For further details |
| see the pcreapi documentation. |
| |
| REPETITION |
| Repetition is specified by quantifiers, which can follow any of the |
| following items: |
| |
| a literal data character |
| the . metacharacter |
| the \C escape sequence |
| escapes such as \d that match single characters |
| a character class |
| a back reference (see next section) |
| a parenthesized subpattern (unless it is an assertion) |
| |
| 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: |
| |
| z{2,4} |
| |
| matches "zz", "zzz", or "zzzz". A closing brace on its own is not a |
| special character. If the second number is omitted, but the comma is |
| present, there is no upper limit; if the second number and the comma |
| are both omitted, the quantifier specifies an exact number of required |
| matches. Thus |
| |
| [aeiou]{3,} |
| |
| matches at least 3 successive vowels, but may match many more, while |
| |
| \d{8} |
| |
| 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. |
| |
| 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. |
| |
| The quantifier {0} is permitted, causing the expression to behave as if |
| the previous item and the quantifier were not present. |
| |
| For convenience (and historical compatibility) the three most common |
| quantifiers have single-character abbreviations: |
| |
| * is equivalent to {0,} |
| + is equivalent to {1,} |
| ? is equivalent to {0,1} |
| |
| It is possible to construct infinite loops by following a subpattern |
| that can match no characters with a quantifier that has no upper limit, |
| for example: |
| |
| (a?)* |
| |
| 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. |
| |
| 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 the sequences /* and */ and within the sequence, |
| individual * and / characters may appear. An attempt to match C |
| comments by applying the pattern |
| |
| /\*.*\*/ |
| |
| to the string |
| |
| /* first command */ not comment /* second comment */ |
| |
| fails, because it matches the entire string owing to the greediness of |
| the .* item. |
| |
| However, if a quantifier is followed by a question mark, it ceases to |
| be greedy, and instead matches the minimum number of times possible, so |
| the pattern |
| |
| /\*.*?\*/ |
| |
| does the right thing with 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 |
| |
| \d??\d |
| |
| which matches one digit by preference, but can match two if that is the |
| only way the rest of the pattern matches. |
| |
| If the 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. |
| |
| When a parenthesized subpattern is quantified with a minimum repeat |
| count that is greater than 1 or with a limited maximum, more store is |
| required for the compiled pattern, in proportion to the size of the |
| minimum or maximum. |
| |
| If a pattern starts with .* or .{0,} and the 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. |
| |
| 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. |
| |
| 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: |
| |
| (.*)abc\1 |
| |
| If the subject is "xyz123abc123" the match point is the fourth |
| character. For this reason, such a pattern is not implicitly anchored. |
| |
| When a capturing subpattern is repeated, the value captured is the |
| substring that matched the final iteration. For example, after |
| |
| (tweedle[dume]{3}\s*)+ |
| |
| has matched "tweedledum tweedledee" the value of the captured substring |
| is "tweedledee". However, if there are nested capturing subpatterns, |
| the corresponding captured values may have been set in previous |
| iterations. For example, after |
| |
| /(a|(b))+/ |
| |
| matches "aba" the value of the second captured substring is "b". |
| |
| ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS |
| With both maximizing 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. |
| |
| Consider, for example, the pattern \d+foo when applied to the subject |
| line |
| |
| 123456bar |
| |
| After matching all 6 digits and then failing to match "foo", the normal |
| action of the matcher is to try again with only 5 digits matching the |
| \d+ item, and then with 4, and so on, before ultimately failing. |
| "Atomic grouping" (a term taken from Jeffrey Friedl's book) provides |
| the means for specifying that once a subpattern has matched, it is not |
| to be re-evaluated in this way. |
| |
| 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: |
| |
| (?>\d+)foo |
| |
| 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. |
| |
| 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. |
| |
| 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. |
| |
| 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 |
| |
| \d++bar |
| |
| 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. |
| |
| The possessive quantifier syntax is an extension to the Perl syntax. It |
| originates in Sun's Java package. |
| |
| 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 |
| |
| (\D+|<\d+>)*[!?] |
| |
| matches an unlimited number of substrings that either consist of non- |
| digits, or digits enclosed in <>, followed by either ! or ?. When it |
| matches, it runs quickly. However, if it is applied to |
| |
| aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa |
| |
| it takes a long time before reporting failure. This is because the |
| string can be divided between the two repeats in a large number of |
| ways, and all have to be tried. (The example used [!?] 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 to |
| |
| ((?>\D+)|<\d+>)*[!?] |
| |
| sequences of non-digits cannot be broken, and failure happens quickly. |
| |
| BACK REFERENCES |
| 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. |
| |
| 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 |
| section entitled "Backslash" above for further details of the handling |
| of digits following a backslash. |
| |
| A back reference matches whatever actually matched the capturing |
| subpattern in the current subject string, rather than anything matching |
| the subpattern itself (see "Subpatterns as subroutines" below for a way |
| of doing that). So the pattern |
| |
| (sens|respons)e and \1ibility |
| |
| matches "sense and sensibility" and "response and responsibility", but |
| not "sense and responsibility". If caseful matching is in force at the |
| time of the back reference, the case of letters is relevant. For |
| example, |
| |
| ((?i)rah)\s+\1 |
| |
| matches "rah rah" and "RAH RAH", but not "RAH rah", even though the |
| original capturing subpattern is matched caselessly. |
| |
| Back references to named subpatterns use the Python syntax (?P=name). |
| We could rewrite the above example as follows: |
| |
| (?<p1>(?i)rah)\s+(?P=p1) |
| |
| 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 |
| |
| (a|(bc))\2 |
| |
| 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 can be used. |
| |
| 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 |
| |
| (a|b\1)+ |
| |
| 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. |
| |
| ASSERTIONS |
| 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. 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. Lookahead |
| assertions start with (?= for positive assertions and (?! for negative |
| assertions. For example, |
| |
| \w+(?=;) |
| |
| matches a word followed by a semicolon, but does not include the |
| semicolon in the match, and |
| |
| foo(?!bar) |
| |
| matches any occurrence of "foo" that is not followed by "bar". Note |
| that the apparently similar pattern |
| |
| (?!foo)bar |
| |
| does not find an occurrence of "bar" that is preceded by something |
| other than "foo"; it finds any occurrence of "bar" whatsoever, because |
| the assertion (?!foo) is always true when the next three characters are |
| "bar". A lookbehind assertion is needed to achieve this effect. |
| |
| If you want to force a matching failure at some point in a pattern, the |
| most convenient way to do it is with (?!) because an empty string |
| always matches, so an assertion that requires there not to be an empty |
| string must always fail. |
| |
| Lookbehind assertions start with (?<= for positive assertions and (?<! |
| for negative assertions. For example, |
| |
| (?<!foo)bar |
| |
| does find an occurrence of "bar" that is not preceded by "foo". The |
| contents of a lookbehind assertion are restricted such that all the |
| strings it matches must have a fixed length. However, if there are |
| several alternatives, they do not all have to have the same fixed |
| length. Thus |
| |
| (?<=bullock|donkey) |
| |
| is permitted, but |
| |
| (?<!dogs?|cats?) |
| |
| causes an error at compile time. Branches that match different length |
| strings are permitted only at the top level of a lookbehind assertion. |
| This is an extension compared with Perl (at least for 5.8), which |
| requires all branches to match the same length of string. An assertion |
| such as |
| |
| (?<=ab(c|de)) |
| |
| 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: |
| |
| (?<=abc|abde) |
| |
| The implementation of lookbehind assertions is, for each alternative, |
| to temporarily move the current position back by the fixed width and |
| then try to match. If there are insufficient characters before the |
| current position, the match is deemed to fail. |
| |
| PCRE does not allow the \C escape (which matches a single byte in UTF-8 |
| mode) to appear in lookbehind assertions, because it makes it |
| impossible to calculate the length of the lookbehind. |
| |
| Atomic groups can be used in conjunction with lookbehind assertions to |
| specify efficient matching at the end of the subject string. Consider a |
| simple pattern such as |
| |
| abcd$ |
| |
| when applied to a long string that does not match. Because matching |
| proceeds from left to right, PCRE will look for each "a" in the subject |
| and then see if what follows matches the rest of the pattern. If the |
| pattern is specified as |
| |
| ^.*abcd$ |
| |
| the initial .* matches the entire string at first, but when this fails |
| (because there is no following "a"), it backtracks to match all but the |
| last character, then all but the last two characters, and so on. Once |
| again the search for "a" covers the entire string, from right to left, |
| so we are no better off. However, if the pattern is written as |
| |
| ^(?>.*)(?<=abcd) |
| |
| or, equivalently, |
| |
| ^.*+(?<=abcd) |
| |
| 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. |
| |
| Several assertions (of any sort) may occur in succession. For example, |
| |
| (?<=\d{3})(?<!999)foo |
| |
| matches "foo" preceded by three digits that are not "999". Notice that |
| each of the assertions is applied independently at the same point in |
| the subject string. First there is a check that the previous three |
| characters are all digits, and then there is a check that the same |
| three characters are not "999". This pattern does not match "foo" |
| preceded by six characters, the first of which are digits and the last |
| three of which are not "999". For example, it doesn't match |
| "123abcfoo". A pattern to do that is |
| |
| (?<=\d{3}...)(?<!999)foo |
| |
| This time the first assertion looks at the preceding six characters, |
| checking that the first three are digits, and then the second assertion |
| checks that the preceding three characters are not "999". |
| |
| Assertions can be nested in any combination. For example, |
| |
| (?<=(?<!foo)bar)baz |
| |
| matches an occurrence of "baz" that is preceded by "bar" which in turn |
| is not preceded by "foo", while |
| |
| (?<=\d{3}(?!999)...)foo |
| |
| is another pattern which matches "foo" preceded by three digits and any |
| three characters that are not "999". |
| |
| 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. |
| |
| CONDITIONAL SUBPATTERNS |
| 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 |
| |
| (?(condition)yes-pattern) |
| (?(condition)yes-pattern|no-pattern) |
| |
| If the condition is satisfied, the yes-pattern is used; otherwise the |
| no-pattern (if present) is used. If there are more than two |
| alternatives in the subpattern, a compile-time error occurs. |
| |
| There are three kinds of condition. If the text between the parentheses |
| consists of a sequence of digits, the condition is satisfied if the |
| capturing subpattern of that number has previously matched. The number |
| must be greater than zero. Consider the following pattern, which |
| contains non-significant white space to make it more readable (assume |
| the PCRE_EXTENDED option) and to divide it into three parts for ease of |
| discussion: |
| |
| ( \( )? [^()]+ (?(1) \) ) |
| |
| The first part matches an optional opening parenthesis, and if that |
| character is present, sets it as the first captured substring. The |
| second part matches one or more characters that are not parentheses. |
| The third part is a conditional subpattern that tests whether the first |
| set of parentheses matched or not. If they did, that is, if subject |
| started with an opening parenthesis, the condition is true, and so the |
| yes-pattern is executed and a closing parenthesis is required. |
| Otherwise, since no-pattern is not present, the subpattern matches |
| nothing. In other words, this pattern matches a sequence of |
| non-parentheses, optionally enclosed in parentheses. |
| |
| If the condition is the string (R), it is satisfied if a recursive call |
| to the pattern or subpattern has been made. At "top level", the |
| condition is false. This is a PCRE extension. Recursive patterns are |
| described in the next section. |
| |
| If the condition is not a sequence of digits or (R), it must be an |
| assertion. This may be a positive or negative lookahead or lookbehind |
| assertion. Consider this pattern, again containing non-significant |
| white space, and with the two alternatives on the second line: |
| |
| (?(?=[^a-z]*[a-z]) |
| \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} ) |
| |
| The condition is a positive lookahead assertion that matches an |
| optional sequence of non-letters followed by a letter. In other words, |
| it tests for the presence of at least one letter in the subject. If a |
| letter is found, the subject is matched against the first alternative; |
| otherwise it is matched against the second. This pattern matches |
| strings in one of the two forms dd-aaa-dd or dd-dd-dd, where aaa are |
| letters and dd are digits. |
| |
| COMMENTS |
| The sequence (?# marks the start of a comment which 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. |
| |
| If the PCRE_EXTENDED option is set, an unescaped # character outside a |
| character class introduces a comment that continues up to the next |
| newline character in the pattern. |
| |
| RECURSIVE PATTERNS |
| Consider the problem of matching a string in parentheses, allowing for |
| unlimited nested parentheses. Without the use of recursion, the best |
| that can be done is to use a pattern that matches up to some fixed |
| depth of nesting. It is not possible to handle an arbitrary nesting |
| depth. Perl has provided an experimental facility that allows regular |
| expressions to recurse (amongst other things). It does this by |
| interpolating Perl code in the expression at run time, and the code can |
| refer to the expression itself. A Perl pattern to solve the parentheses |
| problem can be created like this: |
| |
| $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x; |
| |
| The (?p{...}) item interpolates Perl code at run time, and in this case |
| refers recursively to the pattern in which it appears. Obviously, PCRE |
| cannot support the interpolation of Perl code. Instead, it supports |
| some special syntax for recursion of the entire pattern, and also for |
| individual subpattern recursion. |
| |
| The special item that consists of (? followed by a number greater than |
| zero and a closing parenthesis is a recursive call of the subpattern of |
| the given number, provided that it occurs inside that subpattern. (If |
| not, it is a "subroutine" call, which is described in the next |
| section.) The special item (?R) is a recursive call of the entire |
| regular expression. |
| |
| For example, this PCRE pattern solves the nested parentheses problem |
| (assume the PCRE_EXTENDED option is set so that white space is |
| ignored): |
| |
| \( ( (?>[^()]+) | (?R) )* \) |
| |
| First it matches an opening parenthesis. Then it matches any number of |
| substrings which can either be a sequence of non-parentheses, or a |
| recursive match of the pattern itself (that is a correctly |
| parenthesized substring). Finally there is a closing parenthesis. |
| |
| If this were part of a larger pattern, you would not want to recurse |
| the entire pattern, so instead you could use this: |
| |
| ( \( ( (?>[^()]+) | (?1) )* \) ) |
| |
| We have put the pattern into parentheses, and caused the recursion to |
| refer to them instead of the whole pattern. In a larger pattern, |
| keeping track of parenthesis numbers can be tricky. It may be more |
| convenient to use named parentheses instead. For this, PCRE uses |
| (?P>name), which is an extension to the Python syntax that PCRE uses |
| for named parentheses (Perl does not provide named parentheses). We |
| could rewrite the above example as follows: |
| |
| (?P<pn> \( ( (?>[^()]+) | (?P>pn) )* \) ) |
| |
| 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 |
| |
| (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa() |
| |
| it yields "no match" quickly. However, if atomic grouping is not used, |
| the match runs for a very long time indeed because there are so many |
| different ways the + and * repeats can carve up the subject, and all |
| have to be tested before failure can be reported. |
| |
| At the end of a match, the values set for any capturing subpatterns are |
| those from the outermost level of the recursion at which the subpattern |
| value is set. If you want to obtain intermediate values, a callout |
| function can be used (see below and the pcrecallout documentation). If |
| the pattern above is matched against |
| |
| (ab(cd)ef) |
| |
| 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 |
| |
| \( ( ( (?>[^()]+) | (?R) )* ) \) |
| ^ ^ |
| ^ ^ |
| |
| the string they capture is "ab(cd)ef", the contents of the top level |
| parentheses. If there are more than 15 capturing parentheses in a |
| pattern, PCRE has to obtain extra memory to store data during a |
| recursion, which it does by using pcre_malloc, freeing it via pcre_free |
| afterwards. If no memory can be obtained, the match fails with the |
| PCRE_ERROR_NOMEMORY error. |
| |
| Do not confuse the (?R) item with the condition (R), which tests for |
| recursion. Consider this pattern, which matches text in angle |
| brackets, allowing for arbitrary nesting. Only digits are allowed in |
| nested brackets (that is, when recursing), whereas any characters are |
| permitted at the outer level. |
| |
| < (?: (?(R) \d++ | [^<>]*+) | (?R)) * > |
| |
| In this pattern, (?(R) is the start of a conditional subpattern, with |
| two different alternatives for the recursive and non-recursive cases. |
| The (?R) item is the actual recursive call. |
| |
| SUBPATTERNS AS SUBROUTINES |
| If the syntax for a recursive subpattern reference (either by number or |
| by name) is used outside the parentheses to which it refers, it |
| operates like a subroutine in a programming language. An earlier |
| example pointed out that the pattern |
| |
| (sens|respons)e and \1ibility |
| |
| matches "sense and sensibility" and "response and responsibility", but |
| not "sense and responsibility". If instead the pattern |
| |
| (sens|respons)e and (?1)ibility |
| |
| is used, it does match "sense and responsibility" as well as the other |
| two strings. Such references must, however, follow the subpattern to |
| which they refer. |
| |
| CALLOUTS |
| Perl has a feature whereby using the sequence (?{...}) causes arbitrary |
| Perl code to be obeyed in the middle of matching a regular expression. |
| This makes it possible, amongst other things, to extract different |
| substrings that match the same pair of parentheses when there is a |
| repetition. |
| |
| PCRE provides a similar feature, but of course it cannot obey arbitrary |
| Perl code. The feature is called "callout". The caller of PCRE provides |
| an external function by putting its entry point in the global variable |
| pcre_callout. By default, this variable contains NULL, which disables |
| all calling out. |
| |
| Within a regular expression, (?C) indicates the points at which the |
| external function is to be called. If you want to identify different |
| callout points, you can put a number less than 256 after the letter C. |
| The default value is zero. For example, this pattern has two callout |
| points: |
| |
| (?C1)abc(?C2)def |
| |
| During matching, when PCRE reaches a callout point (and pcre_callout is |
| set), the external function is called. It is provided with the number |
| of the callout, and, optionally, one item of data originally supplied |
| by the caller of pcre_exec(). The callout function may cause matching |
| to backtrack, or to fail altogether. A complete description of the |
| interface to the callout function is given in the pcrecallout |
| documentation. |
| |
| DIFFERENCES FROM PERL |
| This section escribes the differences in the ways that PCRE and Perl |
| handle regular expressions. The differences described here are with |
| respect to Perl 5.8. |
| |
| 1. PCRE does not have full UTF-8 support. Details of what it does have |
| are given in the section on UTF-8 support in the main pcre page. |
| |
| 2. PCRE does not allow repeat quantifiers on lookahead assertions. |
| Perl permits them, but they do not mean what you might think. For |
| example, (?!a){3} does not assert that the next three characters are |
| not "a". It just asserts that the next character is not "a" three |
| times. |
| |
| 3. Capturing subpatterns that occur inside negative lookahead |
| assertions are counted, but their entries in the offsets vector are |
| never set. Perl sets its numerical variables from any such patterns |
| that are matched before the assertion fails to match something |
| (thereby succeeding), but only if the negative lookahead assertion |
| contains just one branch. |
| |
| 4. Though binary zero characters are supported in the subject string, |
| they are not allowed in a pattern string because it is passed as a |
| normal C string, terminated by zero. The escape sequence "\0" can be |
| used in the pattern to represent a binary zero. |
| |
| 5. The following Perl escape sequences are not supported: \l, \u, \L, |
| \U, \P, \p, \N, and \X. In fact these are implemented by Perl's general |
| string-handling and are not part of its pattern matching engine. If any |
| of these are encountered by PCRE, an error is generated. |
| |
| 6. PCRE does support the \Q...\E escape for quoting substrings. |
| Characters in between are treated as literals. This is slightly |
| different from Perl in that $ and @ are also handled as literals inside |
| the quotes. In Perl, they cause variable interpolation (but of course |
| PCRE does not have variables). Note the following examples: |
| |
| 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$xyz |
| |
| The \Q...\E sequence is recognized both inside and outside character |
| classes. |
| |
| 7. Fairly obviously, PCRE does not support the (?{code}) and |
| (?p{code}) constructions. However, there is some experimental support |
| for recursive patterns using the non-Perl items (?R), (?number) and |
| (?P>name). Also, the PCRE "callout" feature allows an external function |
| to be called during pattern matching. |
| |
| 8. There are some differences that are concerned with the settings of |
| captured strings when part of a pattern is repeated. For example, |
| matching "aba" against the pattern /^(a(b)?)+$/ in Perl leaves $2 |
| unset, but in PCRE it is set to "b". |
| |
| 9. PCRE provides some extensions to the Perl regular expression |
| facilities: |
| |
| (a) Although lookbehind assertions must match fixed length strings, |
| each alternative branch of a lookbehind assertion can match a different |
| length of string. Perl requires them all to have the same length. |
| |
| (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $ |
| meta-character matches only at the very end of the string. |
| |
| (c) If PCRE_EXTRA is set, a backslash followed by a letter with no |
| special meaning is faulted. |
| |
| (d) If PCRE_UNGREEDY is set, the greediness of the repetition |
| quantifiers is inverted, that is, by default they are not greedy, but |
| if followed by a question mark they are. |
| |
| (e) PCRE_ANCHORED can be used to force a pattern to be tried only at |
| the first matching position in the subject string. |
| |
| (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and |
| PCRE_NO_AUTO_CAPTURE options for pcre_exec() have no Perl equivalents. |
| |
| (g) The (?R), (?number), and (?P>name) constructs allows for recursive |
| pattern matching (Perl can do this using the (?p{code}) construct, |
| which PCRE cannot support.) |
| |
| (h) PCRE supports named capturing substrings, using the Python syntax. |
| |
| (i) PCRE supports the possessive quantifier "++" syntax, taken from |
| Sun's Java package. |
| |
| (j) The (R) condition, for testing recursion, is a PCRE extension. |
| |
| (k) The callout facility is PCRE-specific. |
| |
| NOTES |
| The \< and \> metacharacters from Henry Spencers package |
| are not available in PCRE, but can be emulated with \b, |
| as required, also in conjunction with \W or \w. |
| |
| In LDMud, backtracks are limited by the EVAL_COST runtime |
| limit, to avoid freezing the driver with a match |
| like regexp(({"=XX==================="}), "X(.+)+X"). |
| |
| LDMud doesn't support PCRE callouts. |
| |
| LIMITATIONS |
| There are some size limitations in PCRE but it is hoped that |
| they will never in practice be relevant. The maximum length |
| of a compiled pattern is 65539 (sic) bytes. All values in |
| repeating quantifiers must be less than 65536. There |
| maximum number of capturing subpatterns is 65535. There is no |
| limit to the number of non-capturing subpatterns, but the |
| maximum depth of nesting of all kinds of parenthesized |
| subpattern, including capturing subpatterns, assertions, |
| and other types of subpattern, is 200. |
| |
| The maximum length of a subject string is the largest |
| positive number that an integer variable can hold. However, |
| PCRE uses recursion to handle subpatterns and indefinite |
| repetition. This means that the available stack space may |
| limit the size of a subject string that can be processed by |
| certain patterns. |
| |
| AUTHOR |
| Philip Hazel <ph10@cam.ac.uk> |
| University Computing Service, |
| New Museums Site, |
| Cambridge CB2 3QG, England. |
| Phone: +44 1223 334714 |
| |
| SEE ALSO |
| regexp(C), hsregexp(C) |