This file contains the PCRE man page that describes the regular expressions supported by PCRE version 5.0. Note that not all of the features are relevant in the context of Exim. In particular, the version of PCRE that is compiled with Exim does not include UTF-8 support, there is no mechanism for changing the options with which the PCRE functions are called, and features such as callout are not accessible. ----------------------------------------------------------------------------- PCRE(3) PCRE(3) NAME PCRE - Perl-compatible regular expressions 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 books, some of which have copious examples. Jeffrey Friedl's "Mastering Regular Expressions", published by O'Reilly, covers regular expressions in great detail. This descrip- tion of PCRE's regular expressions is intended as reference material. 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 pcre_compile() 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 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 alterna- tives and repetitions in the pattern. These are encoded in the pattern by the use of metacharacters, which do not stand for themselves but instead are interpreted in some special way. There are two different sets of metacharacters: those that are recog- nized anywhere in the pattern except within square brackets, and those that are recognized in square brackets. Outside square brackets, the metacharacters 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 metacharacters 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 metacharacters. BACKSLASH 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. 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 back- slash, 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 # charac- ter as part of the pattern. If you want to remove the special meaning from a sequence of charac- ters, you can do so by putting them between \Q and \E. This is differ- ent from Perl in that $ and @ are handled as literals in \Q...\E sequences in PCRE, whereas in Perl, $ and @ cause variable interpola- tion. 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. Non-printing characters A second use of backslash provides a way of encoding non-printing char- acters 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 hex- adecimal escape, with no following digits, giving a character 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 pattern character that follows is itself an octal digit. The handling of a backslash followed by a digit other than 0 is compli- cated. Outside a character class, PCRE reads it and any following dig- its 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 sin- gle 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), and the sequence \X is interpreted as the character "X". Outside a character class, these sequences have different meanings (see below). Generic character types The third use of backslash is for specifying generic character types. The following are always recognized: \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. These character type sequences can appear both inside and outside char- acter 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. 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 an underscore or any character less than 256 that is a letter or digit. The definition of letters and digits is con- trolled by PCRE's low-valued character tables, and may vary if locale- specific matching is taking place (see "Locale support" in the pcreapi 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. 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 Uni- code character property support is available. Unicode character properties When PCRE is built with Unicode character property support, three addi- tional escape sequences to match generic character types are available when UTF-8 mode is selected. They are: \p{xx} a character with the xx property \P{xx} a character without the xx property \X an extended Unicode sequence The property names represented by xx above are limited to the Unicode general category properties. Each character has exactly one such prop- erty, 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}. 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: \p{L} \pL The following property codes are supported: 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 Extended properties such as "Greek" or "InMusicalSymbols" are not sup- ported by PCRE. Specifying caseless matching does not affect these escape sequences. For example, \p{Lu} always matches only upper case letters. The \X escape matches any number of Unicode characters that form an extended Unicode sequence. \X is equivalent to (?>\PM\pM*) That is, it matches a character without the "mark" property, followed by zero or more characters with the "mark" property, and treats the sequence as an atomic group (see below). Characters with the "mark" property are typically accents that affect the preceding character. 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. Simple assertions The fourth use of backslash is for certain simple assertions. An asser- tion 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 char- acter 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 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 asser- tions 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 startoffset argument of pcre_exec() is non-zero, indi- cating 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 argu- ments, you can mimic Perl's /g option, and it is in this kind of imple- mentation 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 that is true only if the current matching point is at the start of the subject string. If the startoffset argu- ment 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 sub- ject, 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 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. 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 immedi- ately after and immediately before an internal newline character, respectively, in addition to matching at the start and end of the sub- ject string. For example, the pattern /^abc$/ matches the subject string "def\nabc" (where \n represents a newline character) in multi- line 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_DOL- LAR_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 charac- ter 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 han- dling 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 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. PCRE does not allow \C to appear in lookbehind assertions (described below), because in UTF-8 mode this would make it impossible to calcu- late the length of the lookbehind. SQUARE BRACKETS AND CHARACTER CLASSES An opening square bracket introduces a character class, terminated by a closing square bracket. A closing square bracket on its own is not spe- cial. 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 that are in the class by enumerating those that are not. A class that starts with a circumflex is not an assertion: it still con- sumes a character from the subject string, and therefore it 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. 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. 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 charac- ters 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 charac- ter 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 inter- preted 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. 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 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. 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 circum- flex 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. 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. POSIX CHARACTER CLASSES 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, [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 128 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 alterna- tives are within a subpattern (defined below), "succeeds" means match- ing the rest of the main pattern as well as the alternative in the sub- pattern. 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 possi- ble to unset these options by preceding the letter with a hyphen, and a combined setting and unsetting such as (?im-sx), which sets PCRE_CASE- LESS 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 subpat- tern 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 cur- rent 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 to put it at the start. SUBPATTERNS Subpatterns are delimited by parentheses (round brackets), which can be nested. Turning part of a pattern into 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. 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 ovector argument of pcre_exec(). Opening parentheses are counted from left to right (starting from 1) to obtain numbers for the capturing subpatterns. For example, if the string "the red king" is matched against the pat- tern the ((red|white) (king|queen)) the captured substrings are "red king", "red", and "king", and are num- bered 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 captur- ing, 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 non- capturing, 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 expres- sions. Furthermore, if an expression is modified, the numbers may change. To help with this difficulty, PCRE supports the naming of sub- patterns, something that Perl does not provide. The Python syntax (?P...) 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. There is also a con- venience function for extracting a captured substring by name. For fur- ther 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 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) The general repetition quantifier specifies a minimum and maximum num- ber 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 exam- ple, {,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 char- acters, 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). 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 bro- ken. 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 /\*.*\*/ to the string /* first comment */ 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 memory 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 (equiv- alent 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 new- lines, it is worth setting PCRE_DOTALL in order to obtain this opti- mization, 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 charac- ter. For this reason, such a pattern is not implicitly anchored. When a capturing subpattern is repeated, the value captured is the sub- string 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 itera- tions. 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 dif- ferent number of repeats allows the rest of the pattern to match. Some- times 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 nota- tion 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 con- tains 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 pre- pared 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++foo 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 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 charac- ter 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: ((?>\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 sub- pattern 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 pat- tern. In other words, the parentheses that are referenced need not be to the left of the reference for numbers less than 10. See the subsec- tion entitled "Non-printing characters" above for further details of the handling of digits following a backslash. A back reference matches whatever actually matched the capturing sub- pattern 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 exam- ple, ((?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: (?(?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 (see "Com- ments" below) 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 sub- patterns. For example, the pattern (a|b\1)+ matches any number of "a"s and also "aba", "ababbaa" etc. At each iter- ation 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. 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 sub- patterns in the whole pattern. However, substring capturing is carried out only for positive assertions, because it does not make sense for negative assertions. Lookahead assertions 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 semi- colon 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 the other 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 Lookbehind assertions start with (?<= for positive assertions and (?.*)(?<=abcd) or, equivalently, using the possessive quantifier syntax, ^.*+(?<=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. Using multiple assertions Several assertions (of any sort) may occur in succession. For example, (?<=\d{3})(?[^()]+) | (?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 sec- tion.) 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 parenthe- sized 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, keep- ing track of parenthesis numbers can be tricky. It may be more conve- nient 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 \( ( (?>[^()]+) | (?P>pn) )* \) ) 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 the next section and the pcrecallout documen- tation). 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 pat- tern, PCRE has to obtain extra memory to store data during a recursion, which it does by using pcre_malloc, freeing it via pcre_free after- wards. 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 brack- ets, allowing for arbitrary nesting. Only digits are allowed in nested brackets (that is, when recursing), whereas any characters are permit- ted 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 oper- ates 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 sub- strings that match the same pair of parentheses when there is a repeti- tion. 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 If the PCRE_AUTO_CALLOUT flag is passed to pcre_compile(), callouts are automatically installed before each item in the pattern. They are all numbered 255. 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, the position in the pattern, and, optionally, one item of data originally supplied by the caller of pcre_exec(). The callout function may cause matching to proceed, to backtrack, or to fail alto- gether. A complete description of the interface to the callout function is given in the pcrecallout documentation. Last updated: 09 September 2004 Copyright (c) 1997-2004 University of Cambridge.