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diff --git a/doc/doc-txt/pcrepattern.txt b/doc/doc-txt/pcrepattern.txt new file mode 100644 index 000000000..1dc800af4 --- /dev/null +++ b/doc/doc-txt/pcrepattern.txt @@ -0,0 +1,1413 @@ +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<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. 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: + + (?<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 (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 (?<! + 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 sev- + eral 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 cur- + rent 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 impossi- + ble to calculate the length of the lookbehind. The \X escape, which can + match different numbers of bytes, is also not permitted. + + 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, 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})(?<!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" pre- + ceded by six characters, the first of which are digits and the last + three of which are not "999". For example, it doesn't match "123abc- + foo". 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 that matches "foo" preceded by three digits and any + three characters that are not "999". + + +CONDITIONAL SUBPATTERNS + + It is possible to cause the matching process to obey a subpattern con- + ditionally or to choose between two alternative subpatterns, depending + on the result of an assertion, or whether a previous capturing subpat- + tern 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 alterna- + 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: + + ( \( )? [^()]+ (?(1) \) ) + + The first part matches an optional opening parenthesis, and if that + character is present, sets it as the first captured substring. The sec- + ond 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-pat- + 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. + + 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 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 that continues up to the + next closing parenthesis. Nested parentheses are not permitted. The + characters that make up a comment play no part in the pattern matching + at all. + + 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. + + +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 provides a facility that allows regular expressions to + recurse (amongst other things). It does this by interpolating Perl code + in the expression at run time, and the code can refer to the expression + itself. A Perl pattern 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 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<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 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. |