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diff --git a/doc/doc-txt/pcrepattern.txt b/doc/doc-txt/pcrepattern.txt index b638a00db..9712c86b4 100644 --- a/doc/doc-txt/pcrepattern.txt +++ b/doc/doc-txt/pcrepattern.txt @@ -1,5 +1,5 @@ This file contains the PCRE man page that describes the regular expressions -supported by PCRE version 6.2. Note that not all of the features are relevant +supported by PCRE version 6.7. 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 @@ -110,9 +110,9 @@ BACKSLASH 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. + # outside a character class and the next newline are ignored. An escap- + ing 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 charac- ters, you can do so by putting them between \Q and \E. This is differ- @@ -148,7 +148,7 @@ BACKSLASH \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) + \x{hhh..} character with hex code hhh.. 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 @@ -156,26 +156,24 @@ BACKSLASH 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 character whose - value is zero. + in upper or lower case). Any number of hexadecimal digits may appear + between \x{ and }, but the value of the character code must be less + than 256 in non-UTF-8 mode, and less than 2**31 in UTF-8 mode (that is, + the maximum hexadecimal value is 7FFFFFFF). If characters other than + hexadecimal digits appear between \x{ and }, or if there is no termi- + nating }, this form of escape is not recognized. Instead, the initial + \x will be interpreted as a basic hexadecimal escape, with no following + digits, giving a character whose value is zero. 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}. + two syntaxes for \x. There is no difference in the way they are han- + dled. 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. + After \0 up to two further octal digits are read. If there are fewer + than two digits, just those that are present are used. Thus the + sequence \0\x\07 specifies two binary zeros followed by a BEL character + (code value 7). Make sure you supply two digits after the initial zero + if the pattern character that follows is itself an octal digit. 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- @@ -187,9 +185,11 @@ BACKSLASH 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: + up to three octal digits following the backslash, ane uses them to gen- + erate a data character. Any subsequent digits stand for themselves. In + non-UTF-8 mode, the value of a character specified in octal must be + less than \400. In UTF-8 mode, values up to \777 are permitted. For + example: \040 is another way of writing a space \40 is the same, provided there are fewer than 40 @@ -209,16 +209,15 @@ BACKSLASH 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). + All the sequences that define a single character value 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 third use of backslash is for specifying generic character types. The following are always recognized: \d any decimal digit @@ -229,56 +228,83 @@ BACKSLASH \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, + 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 + 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). + 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). (If + "use locale;" is included in a Perl script, \s may match the VT charac- + ter. In PCRE, it never does.) 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 + 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, + 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. + code character property support is available. The use of locales with + Unicode is discouraged. Unicode character properties When PCRE is built with Unicode character property support, three addi- - tional escape sequences to match generic character types are available + tional escape sequences to match character properties 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 + \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: + script names, the general category properties, and "Any", which matches + any character (including newline). Other properties such as "InMusical- + Symbols" are not currently supported by PCRE. Note that \P{Any} does + not match any characters, so always causes a match failure. + + Sets of Unicode characters are defined as belonging to certain scripts. + A character from one of these sets can be matched using a script name. + For example: + + \p{Greek} + \P{Han} + + Those that are not part of an identified script are lumped together as + "Common". The current list of scripts is: + + Arabic, Armenian, Bengali, Bopomofo, Braille, Buginese, Buhid, Cana- + dian_Aboriginal, Cherokee, Common, Coptic, Cypriot, Cyrillic, Deseret, + Devanagari, Ethiopic, Georgian, Glagolitic, Gothic, Greek, Gujarati, + Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hiragana, Inherited, Kannada, + Katakana, Kharoshthi, Khmer, Lao, Latin, Limbu, Linear_B, Malayalam, + Mongolian, Myanmar, New_Tai_Lue, Ogham, Old_Italic, Old_Persian, Oriya, + Osmanya, Runic, Shavian, Sinhala, Syloti_Nagri, Syriac, Tagalog, Tag- + banwa, Tai_Le, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh, + Ugaritic, Yi. + + Each character has exactly one general category property, specified by + a two-letter abbreviation. For compatibility with Perl, negation can be + specified by including a circumflex between the opening brace and the + property name. For example, \p{^Lu} is the same as \P{Lu}. + + If only one letter is specified with \p or \P, it includes all the gen- + eral category properties that start with that letter. In this case, in + the absence of negation, the curly brackets in the escape sequence are + optional; these two examples have the same effect: \p{L} \pL - The following property codes are supported: + The following general category property codes are supported: C Other Cc Control @@ -324,8 +350,17 @@ BACKSLASH Zp Paragraph separator Zs Space separator - Extended properties such as "Greek" or "InMusicalSymbols" are not sup- - ported by PCRE. + The special property L& is also supported: it matches a character that + has the Lu, Ll, or Lt property, in other words, a letter that is not + classified as a modifier or "other". + + The long synonyms for these properties that Perl supports (such as + \p{Letter}) are not supported by PCRE, nor is it permitted to prefix + any of these properties with "Is". + + No character that is in the Unicode table has the Cn (unassigned) prop- + erty. Instead, this property is assumed for any code point that is not + in the Unicode table. Specifying caseless matching does not affect these escape sequences. For example, \p{Lu} always matches only upper case letters. @@ -378,24 +413,23 @@ BACKSLASH 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- + that \Z matches before a newline at the end of the string as well as at + the very end, 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 + 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 + 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 + 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. @@ -403,68 +437,81 @@ BACKSLASH 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 + 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 + 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. + 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 at the end of the string (by default). Dollar need not + be the last character of the pattern if a number of alternatives are + involved, but it should be the last item in any branch in which it + appears. Dollar has no special meaning in a character class. 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$/ matches the subject - string "def\nabc" (where \n represents a newline character) in multi- - line mode, but not otherwise. Consequently, patterns that are anchored - in single line mode because all branches start with ^ are not anchored - in multiline mode, and a match for circumflex is possible when the - startoffset argument of pcre_exec() is non-zero. The PCRE_DOL- - LAR_ENDONLY option is ignored if PCRE_MULTILINE is set. - - Note that the sequences \A, \Z, and \z can be used to match the start - and end of the subject in both modes, and if all branches of a pattern - start with \A it is always anchored, whether PCRE_MULTILINE is set or - not. + PCRE_MULTILINE option is set. When this is the case, a circumflex + matches immediately after internal newlines as well as at the start of + the subject string. It does not match after a newline that ends the + string. A dollar matches before any newlines in the string, as well as + at the very end, when PCRE_MULTILINE is set. When newline is specified + as the two-character sequence CRLF, isolated CR and LF characters do + not indicate newlines. + + For example, the pattern /^abc$/ matches the subject string "def\nabc" + (where \n represents a newline) in multiline mode, but not otherwise. + Consequently, patterns that are anchored in single line mode because + all branches start with ^ are not anchored in multiline mode, and a + match for circumflex is possible when the 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 or not PCRE_MULTILINE is + set. 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. + ter in the subject string except (by default) a character that signi- + fies the end of a line. In UTF-8 mode, the matched character may be + more than one byte long. When a line ending is defined as a single + character (CR or LF), dot never matches that character; when the two- + character sequence CRLF is used, dot does not match CR if it is immedi- + ately followed by LF, but otherwise it matches all characters (includ- + ing isolated CRs and LFs). + + The behaviour of dot with regard to newlines can be changed. If the + PCRE_DOTALL option is set, a dot matches any one character, without + exception. If newline is defined as the two-character sequence CRLF, it + takes two dots to match it. + + The handling of dot is entirely independent of the handling of circum- + flex and dollar, the only relationship being that they both involve + newlines. 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 + both in and out of UTF-8 mode. Unlike a dot, it always matches CR and + LF. 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. @@ -513,9 +560,11 @@ SQUARE BRACKETS AND CHARACTER CLASSES PCRE is compiled with Unicode property support as well as with UTF-8 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. + Characters that might indicate line breaks (CR and LF) are never + treated in any special way when matching character classes, whatever + line-ending sequence is in use, and whatever setting of the PCRE_DOTALL + and PCRE_MULTILINE options is used. A class such as [^a] always matches + one of these characters. 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 @@ -616,11 +665,10 @@ VERTICAL BAR 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. + 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 @@ -666,12 +714,9 @@ INTERNAL OPTION SETTING 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. + The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA + can be changed in the same way as the Perl-compatible options by using + the characters J, U and X respectively. SUBPATTERNS @@ -683,18 +728,18 @@ SUBPATTERNS cat(aract|erpillar|) - matches one of the words "cat", "cataract", or "caterpillar". Without - the parentheses, it would match "cataract", "erpillar" or the empty + 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 + 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 + 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- + For example, if the string "the red king" is matched against the pat- tern the ((red|white) (king|queen)) @@ -702,50 +747,75 @@ SUBPATTERNS 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 + 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- + 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 + 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 + 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<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 + 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<name>...) is used. References to capturing parentheses from other + parts of the pattern, such as backreferences, recursion, and condi- + tions, can be made by name as well as by number. + + Names consist of up to 32 alphanumeric characters and underscores. + 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. + number translation table from a compiled pattern. There is also a con- + venience function for extracting a captured substring by name. + + By default, a name must be unique within a pattern, but it is possible + to relax this constraint by setting the PCRE_DUPNAMES option at compile + time. This can be useful for patterns where only one instance of the + named parentheses can match. Suppose you want to match the name of a + weekday, either as a 3-letter abbreviation or as the full name, and in + both cases you want to extract the abbreviation. This pattern (ignoring + the line breaks) does the job: + + (?P<DN>Mon|Fri|Sun)(?:day)?| + (?P<DN>Tue)(?:sday)?| + (?P<DN>Wed)(?:nesday)?| + (?P<DN>Thu)(?:rsday)?| + (?P<DN>Sat)(?:urday)? + + There are five capturing substrings, but only one is ever set after a + match. The convenience function for extracting the data by name + returns the substring for the first, and in this example, the only, + subpattern of that name that matched. This saves searching to find + which numbered subpattern it was. If you make a reference to a non- + unique named subpattern from elsewhere in the pattern, the one that + corresponds to the lowest number is used. For further details of the + interfaces for handling named subpatterns, see the pcreapi documenta- + tion. REPETITION @@ -954,8 +1024,10 @@ ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS 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. + The possessive quantifier syntax is an extension to the Perl syntax. + Jeffrey Friedl originated the idea (and the name) in the first edition + of his book. Mike McCloskey liked it, so implemented it when he built + Sun's Java package, and PCRE copied it from there. When a pattern contains an unlimited repeat inside a subpattern that can itself be repeated an unlimited number of times, the use of an @@ -996,31 +1068,41 @@ BACK REFERENCES 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. + to the left of the reference for numbers less than 10. A "forward back + reference" of this type can make sense when a repetition is involved + and the subpattern to the right has participated in an earlier itera- + tion. - A back reference matches whatever actually matched the capturing sub- - pattern in the current subject string, rather than anything matching + It is not possible to have a numerical "forward back reference" to sub- + pattern whose number is 10 or more. However, a back reference to any + subpattern is possible using named parentheses (see below). See also + the subsection 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- + 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 + 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). + 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) + (?P<p1>(?i)rah)\s+(?P=p1) + + A subpattern that is referenced by name may appear in the pattern + before or after the reference. 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 @@ -1109,8 +1191,8 @@ ASSERTIONS 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 sev- - eral alternatives, they do not all have to have the same fixed length. - Thus + eral top-level alternatives, they do not all have to have the same + fixed length. Thus (?<=bullock|donkey) @@ -1223,12 +1305,18 @@ CONDITIONAL SUBPATTERNS tives 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 con- - tains 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: + consists of a sequence of digits, or a sequence of alphanumeric charac- + ters and underscores, the condition is satisfied if the capturing sub- + pattern of that number or name has previously matched. There is a pos- + sible ambiguity here, because subpattern names may consist entirely of + digits. PCRE looks first for a named subpattern; if it cannot find one + and the text consists entirely of digits, it looks for a subpattern of + that number, which must be greater than zero. Using subpattern names + that consist entirely of digits is not recommended. + + 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: ( \( )? [^()]+ (?(1) \) ) @@ -1241,12 +1329,16 @@ CONDITIONAL SUBPATTERNS tern 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. + optionally enclosed in parentheses. Rewriting it to use a named subpat- + tern gives this: + + (?P<OPEN> \( )? [^()]+ (?(OPEN) \) ) - 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 condi- - tion is false. This is a PCRE extension. Recursive patterns are - described in the next section. + If the condition is the string (R), and there is no subpattern with the + name R, the condition 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 @@ -1273,8 +1365,8 @@ COMMENTS 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 new- - line character in the pattern. + character class introduces a comment that continues to immediately + after the next newline in the pattern. RECURSIVE PATTERNS @@ -1304,15 +1396,19 @@ RECURSIVE PATTERNS 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): + A recursive subpattern call is always treated as an atomic group. That + is, once it has matched some of the subject string, it is never re- + entered, even if it contains untried alternatives and there is a subse- + quent matching failure. + + 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- + 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 @@ -1393,8 +1489,14 @@ SUBPATTERNS AS SUBROUTINES (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. + two strings. Such references, if given numerically, must follow the + subpattern to which they refer. However, named references can refer to + later subpatterns. + + Like recursive subpatterns, a "subroutine" call is always treated as an + atomic group. That is, once it has matched some of the subject string, + it is never re-entered, even if it contains untried alternatives and + there is a subsequent matching failure. CALLOUTS @@ -1431,5 +1533,5 @@ CALLOUTS gether. A complete description of the interface to the callout function is given in the pcrecallout documentation. -Last updated: 28 February 2005 -Copyright (c) 1997-2005 University of Cambridge. +Last updated: 06 June 2006 +Copyright (c) 1997-2006 University of Cambridge. |