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diff --git a/external-libs/pcre/doc/pcrepattern.3 b/external-libs/pcre/doc/pcrepattern.3 deleted file mode 100644 index 101aa311..00000000 --- a/external-libs/pcre/doc/pcrepattern.3 +++ /dev/null @@ -1,1231 +0,0 @@ -.TH PCRE 3 -.SH NAME -PCRE - Perl-compatible regular expressions -.SH PCRE REGULAR EXPRESSION DETAILS -.rs -.sp -The syntax and semantics of the regular expressions supported by PCRE are -described below. Regular expressions are also described in the Perl -documentation and in a number of other books, some of which have copious -examples. Jeffrey Friedl's "Mastering Regular Expressions", published by -O'Reilly, covers them in great detail. The description here is intended as -reference documentation. - -The basic operation of PCRE is on strings of bytes. However, there is also -support for UTF-8 character strings. To use this support you must build PCRE to -include UTF-8 support, and then call \fBpcre_compile()\fR with the PCRE_UTF8 -option. How this affects the pattern matching is mentioned in several places -below. There is also a summary of UTF-8 features in the -.\" HTML <a href="pcre.html#utf8support"> -.\" </a> -section on UTF-8 support -.\" -in the main -.\" HREF -\fBpcre\fR -.\" -page. - -A regular expression is a pattern that is matched against a subject string from -left to right. Most characters stand for themselves in a pattern, and match the -corresponding characters in the subject. As a trivial example, the pattern - - The quick brown fox - -matches a portion of a subject string that is identical to itself. The power of -regular expressions comes from the ability to include alternatives and -repetitions in the pattern. These are encoded in the pattern by the use of -\fImeta-characters\fR, which do not stand for themselves but instead are -interpreted in some special way. - -There are two different sets of meta-characters: those that are recognized -anywhere in the pattern except within square brackets, and those that are -recognized in square brackets. Outside square brackets, the meta-characters are -as follows: - - \\ general escape character with several uses - ^ assert start of string (or line, in multiline mode) - $ assert end of string (or line, in multiline mode) - . match any character except newline (by default) - [ start character class definition - | start of alternative branch - ( start subpattern - ) end subpattern - ? extends the meaning of ( - also 0 or 1 quantifier - also quantifier minimizer - * 0 or more quantifier - + 1 or more quantifier - also "possessive quantifier" - { start min/max quantifier - -Part of a pattern that is in square brackets is called a "character class". In -a character class the only meta-characters are: - - \\ general escape character - ^ negate the class, but only if the first character - - indicates character range - [ POSIX character class (only if followed by POSIX - syntax) - ] terminates the character class - -The following sections describe the use of each of the meta-characters. - -.SH BACKSLASH -.rs -.sp -The backslash character has several uses. Firstly, if it is followed by a -non-alphameric character, it takes away any special meaning that character may -have. This use of backslash as an escape character applies both inside and -outside character classes. - -For example, if you want to match a * character, you write \\* in the pattern. -This escaping action applies whether or not the following character would -otherwise be interpreted as a meta-character, so it is always safe to precede a -non-alphameric with backslash to specify that it stands for itself. In -particular, if you want to match a backslash, you write \\\\. - -If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the -pattern (other than in a character class) and characters between a # outside -a character class and the next newline character are ignored. An escaping -backslash can be used to include a whitespace or # character as part of the -pattern. - -If you want to remove the special meaning from a sequence of characters, you -can do so by putting them between \\Q and \\E. This is different from Perl in -that $ and @ are handled as literals in \\Q...\\E sequences in PCRE, whereas in -Perl, $ and @ cause variable interpolation. Note the following examples: - - Pattern PCRE matches Perl matches - - \\Qabc$xyz\\E abc$xyz abc followed by the - contents of $xyz - \\Qabc\\$xyz\\E abc\\$xyz abc\\$xyz - \\Qabc\\E\\$\\Qxyz\\E abc$xyz abc$xyz - -The \\Q...\\E sequence is recognized both inside and outside character classes. - -A second use of backslash provides a way of encoding non-printing characters -in patterns in a visible manner. There is no restriction on the appearance of -non-printing characters, apart from the binary zero that terminates a pattern, -but when a pattern is being prepared by text editing, it is usually easier to -use one of the following escape sequences than the binary character it -represents: - - \\a alarm, that is, the BEL character (hex 07) - \\cx "control-x", where x is any character - \\e escape (hex 1B) - \\f formfeed (hex 0C) - \\n newline (hex 0A) - \\r carriage return (hex 0D) - \\t tab (hex 09) - \\ddd character with octal code ddd, or backreference - \\xhh character with hex code hh - \\x{hhh..} character with hex code hhh... (UTF-8 mode only) - -The precise effect of \\cx is as follows: if x is a lower case letter, it -is converted to upper case. Then bit 6 of the character (hex 40) is inverted. -Thus \\cz becomes hex 1A, but \\c{ becomes hex 3B, while \\c; becomes hex -7B. - -After \\x, from zero to two hexadecimal digits are read (letters can be in -upper or lower case). In UTF-8 mode, any number of hexadecimal digits may -appear between \\x{ and }, but the value of the character code must be less -than 2**31 (that is, the maximum hexadecimal value is 7FFFFFFF). If characters -other than hexadecimal digits appear between \\x{ and }, or if there is no -terminating }, this form of escape is not recognized. Instead, the initial -\\x will be interpreted as a basic hexadecimal escape, with no following -digits, giving a byte whose value is zero. - -Characters whose value is less than 256 can be defined by either of the two -syntaxes for \\x when PCRE is in UTF-8 mode. There is no difference in the -way they are handled. For example, \\xdc is exactly the same as \\x{dc}. - -After \\0 up to two further octal digits are read. In both cases, if there -are fewer than two digits, just those that are present are used. Thus the -sequence \\0\\x\\07 specifies two binary zeros followed by a BEL character -(code value 7). Make sure you supply two digits after the initial zero if the -character that follows is itself an octal digit. - -The handling of a backslash followed by a digit other than 0 is complicated. -Outside a character class, PCRE reads it and any following digits as a decimal -number. If the number is less than 10, or if there have been at least that many -previous capturing left parentheses in the expression, the entire sequence is -taken as a \fIback reference\fR. A description of how this works is given -later, following the discussion of parenthesized subpatterns. - -Inside a character class, or if the decimal number is greater than 9 and there -have not been that many capturing subpatterns, PCRE re-reads up to three octal -digits following the backslash, and generates a single byte from the least -significant 8 bits of the value. Any subsequent digits stand for themselves. -For example: - - \\040 is another way of writing a space - \\40 is the same, provided there are fewer than 40 - previous capturing subpatterns - \\7 is always a back reference - \\11 might be a back reference, or another way of - writing a tab - \\011 is always a tab - \\0113 is a tab followed by the character "3" - \\113 might be a back reference, otherwise the - character with octal code 113 - \\377 might be a back reference, otherwise - the byte consisting entirely of 1 bits - \\81 is either a back reference, or a binary zero - followed by the two characters "8" and "1" - -Note that octal values of 100 or greater must not be introduced by a leading -zero, because no more than three octal digits are ever read. - -All the sequences that define a single byte value or a single UTF-8 character -(in UTF-8 mode) can be used both inside and outside character classes. In -addition, inside a character class, the sequence \\b is interpreted as the -backspace character (hex 08). Outside a character class it has a different -meaning (see below). - -The third use of backslash is for specifying generic character types: - - \\d any decimal digit - \\D any character that is not a decimal digit - \\s any whitespace character - \\S any character that is not a whitespace character - \\w any "word" character - \\W any "non-word" character - -Each pair of escape sequences partitions the complete set of characters into -two disjoint sets. Any given character matches one, and only one, of each pair. - -In UTF-8 mode, characters with values greater than 255 never match \\d, \\s, or -\\w, and always match \\D, \\S, and \\W. - -For compatibility with Perl, \\s does not match the VT character (code 11). -This makes it different from the the POSIX "space" class. The \\s characters -are HT (9), LF (10), FF (12), CR (13), and space (32). - -A "word" character is any letter or digit or the underscore character, that is, -any character which can be part of a Perl "word". The definition of letters and -digits is controlled by PCRE's character tables, and may vary if locale- -specific matching is taking place (see -.\" HTML <a href="pcreapi.html#localesupport"> -.\" </a> -"Locale support" -.\" -in the -.\" HREF -\fBpcreapi\fR -.\" -page). For example, in the "fr" (French) locale, some character codes greater -than 128 are used for accented letters, and these are matched by \\w. - -These character type sequences can appear both inside and outside character -classes. They each match one character of the appropriate type. If the current -matching point is at the end of the subject string, all of them fail, since -there is no character to match. - -The fourth use of backslash is for certain simple assertions. An assertion -specifies a condition that has to be met at a particular point in a match, -without consuming any characters from the subject string. The use of -subpatterns for more complicated assertions is described below. The backslashed -assertions are - - \\b matches at a word boundary - \\B matches when not at a word boundary - \\A matches at start of subject - \\Z matches at end of subject or before newline at end - \\z matches at end of subject - \\G matches at first matching position in subject - -These assertions may not appear in character classes (but note that \\b has a -different meaning, namely the backspace character, inside a character class). - -A word boundary is a position in the subject string where the current character -and the previous character do not both match \\w or \\W (i.e. one matches -\\w and the other matches \\W), or the start or end of the string if the -first or last character matches \\w, respectively. - -The \\A, \\Z, and \\z assertions differ from the traditional circumflex and -dollar (described below) in that they only ever match at the very start and end -of the subject string, whatever options are set. Thus, they are independent of -multiline mode. - -They are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options. If the -\fIstartoffset\fR argument of \fBpcre_exec()\fR is non-zero, indicating that -matching is to start at a point other than the beginning of the subject, \\A -can never match. The difference between \\Z and \\z is that \\Z matches before -a newline that is the last character of the string as well as at the end of the -string, whereas \\z matches only at the end. - -The \\G assertion is true only when the current matching position is at the -start point of the match, as specified by the \fIstartoffset\fR argument of -\fBpcre_exec()\fR. It differs from \\A when the value of \fIstartoffset\fR is -non-zero. By calling \fBpcre_exec()\fR multiple times with appropriate -arguments, you can mimic Perl's /g option, and it is in this kind of -implementation where \\G can be useful. - -Note, however, that PCRE's interpretation of \\G, as the start of the current -match, is subtly different from Perl's, which defines it as the end of the -previous match. In Perl, these can be different when the previously matched -string was empty. Because PCRE does just one match at a time, it cannot -reproduce this behaviour. - -If all the alternatives of a pattern begin with \\G, the expression is anchored -to the starting match position, and the "anchored" flag is set in the compiled -regular expression. - -.SH CIRCUMFLEX AND DOLLAR -.rs -.sp -Outside a character class, in the default matching mode, the circumflex -character is an assertion which is true only if the current matching point is -at the start of the subject string. If the \fIstartoffset\fR argument of -\fBpcre_exec()\fR is non-zero, circumflex can never match if the PCRE_MULTILINE -option is unset. Inside a character class, circumflex has an entirely different -meaning (see below). - -Circumflex need not be the first character of the pattern if a number of -alternatives are involved, but it should be the first thing in each alternative -in which it appears if the pattern is ever to match that branch. If all -possible alternatives start with a circumflex, that is, if the pattern is -constrained to match only at the start of the subject, it is said to be an -"anchored" pattern. (There are also other constructs that can cause a pattern -to be anchored.) - -A dollar character is an assertion which is true only if the current matching -point is at the end of the subject string, or immediately before a newline -character that is the last character in the string (by default). Dollar need -not be the last character of the pattern if a number of alternatives are -involved, but it should be the last item in any branch in which it appears. -Dollar has no special meaning in a character class. - -The meaning of dollar can be changed so that it matches only at the very end of -the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This -does not affect the \\Z assertion. - -The meanings of the circumflex and dollar characters are changed if the -PCRE_MULTILINE option is set. When this is the case, they match immediately -after and immediately before an internal newline character, respectively, in -addition to matching at the start and end of the subject string. For example, -the pattern /^abc$/ matches the subject string "def\\nabc" in multiline mode, -but not otherwise. Consequently, patterns that are anchored in single line mode -because all branches start with ^ are not anchored in multiline mode, and a -match for circumflex is possible when the \fIstartoffset\fR argument of -\fBpcre_exec()\fR is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if -PCRE_MULTILINE is set. - -Note that the sequences \\A, \\Z, and \\z can be used to match the start and -end of the subject in both modes, and if all branches of a pattern start with -\\A it is always anchored, whether PCRE_MULTILINE is set or not. - -.SH FULL STOP (PERIOD, DOT) -.rs -.sp -Outside a character class, a dot in the pattern matches any one character in -the subject, including a non-printing character, but not (by default) newline. -In UTF-8 mode, a dot matches any UTF-8 character, which might be more than one -byte long, except (by default) for newline. If the PCRE_DOTALL option is set, -dots match newlines as well. The handling of dot is entirely independent of the -handling of circumflex and dollar, the only relationship being that they both -involve newline characters. Dot has no special meaning in a character class. - -.SH MATCHING A SINGLE BYTE -.rs -.sp -Outside a character class, the escape sequence \\C matches any one byte, both -in and out of UTF-8 mode. Unlike a dot, it always matches a newline. The -feature is provided in Perl in order to match individual bytes in UTF-8 mode. -Because it breaks up UTF-8 characters into individual bytes, what remains in -the string may be a malformed UTF-8 string. For this reason it is best avoided. - -PCRE does not allow \\C to appear in lookbehind assertions (see below), because -in UTF-8 mode it makes it impossible to calculate the length of the lookbehind. - -.SH SQUARE BRACKETS -.rs -.sp -An opening square bracket introduces a character class, terminated by a closing -square bracket. A closing square bracket on its own is not special. If a -closing square bracket is required as a member of the class, it should be the -first data character in the class (after an initial circumflex, if present) or -escaped with a backslash. - -A character class matches a single character in the subject. In UTF-8 mode, the -character may occupy more than one byte. A matched character must be in the set -of characters defined by the class, unless the first character in the class -definition is a circumflex, in which case the subject character must not be in -the set defined by the class. If a circumflex is actually required as a member -of the class, ensure it is not the first character, or escape it with a -backslash. - -For example, the character class [aeiou] matches any lower case vowel, while -[^aeiou] matches any character that is not a lower case vowel. Note that a -circumflex is just a convenient notation for specifying the characters which -are in the class by enumerating those that are not. It is not an assertion: it -still consumes a character from the subject string, and fails if the current -pointer is at the end of the string. - -In UTF-8 mode, characters with values greater than 255 can be included in a -class as a literal string of bytes, or by using the \\x{ escaping mechanism. - -When caseless matching is set, any letters in a class represent both their -upper case and lower case versions, so for example, a caseless [aeiou] matches -"A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a -caseful version would. PCRE does not support the concept of case for characters -with values greater than 255. - -The newline character is never treated in any special way in character classes, -whatever the setting of the PCRE_DOTALL or PCRE_MULTILINE options is. A class -such as [^a] will always match a newline. - -The minus (hyphen) character can be used to specify a range of characters in a -character class. For example, [d-m] matches any letter between d and m, -inclusive. If a minus character is required in a class, it must be escaped with -a backslash or appear in a position where it cannot be interpreted as -indicating a range, typically as the first or last character in the class. - -It is not possible to have the literal character "]" as the end character of a -range. A pattern such as [W-]46] is interpreted as a class of two characters -("W" and "-") followed by a literal string "46]", so it would match "W46]" or -"-46]". However, if the "]" is escaped with a backslash it is interpreted as -the end of range, so [W-\\]46] is interpreted as a single class containing a -range followed by two separate characters. The octal or hexadecimal -representation of "]" can also be used to end a range. - -Ranges operate in the collating sequence of character values. They can also be -used for characters specified numerically, for example [\\000-\\037]. In UTF-8 -mode, ranges can include characters whose values are greater than 255, for -example [\\x{100}-\\x{2ff}]. - -If a range that includes letters is used when caseless matching is set, it -matches the letters in either case. For example, [W-c] is equivalent to -[][\\^_`wxyzabc], matched caselessly, and if character tables for the "fr" -locale are in use, [\\xc8-\\xcb] matches accented E characters in both cases. - -The character types \\d, \\D, \\s, \\S, \\w, and \\W may also appear in a -character class, and add the characters that they match to the class. For -example, [\\dABCDEF] matches any hexadecimal digit. A circumflex can -conveniently be used with the upper case character types to specify a more -restricted set of characters than the matching lower case type. For example, -the class [^\\W_] matches any letter or digit, but not underscore. - -All non-alphameric characters other than \\, -, ^ (at the start) and the -terminating ] are non-special in character classes, but it does no harm if they -are escaped. - -.SH POSIX CHARACTER CLASSES -.rs -.sp -Perl supports the POSIX notation for character classes, which uses names -enclosed by [: and :] within the enclosing square brackets. PCRE also supports -this notation. For example, - - [01[:alpha:]%] - -matches "0", "1", any alphabetic character, or "%". The supported class names -are - - alnum letters and digits - alpha letters - ascii character codes 0 - 127 - blank space or tab only - cntrl control characters - digit decimal digits (same as \\d) - graph printing characters, excluding space - lower lower case letters - print printing characters, including space - punct printing characters, excluding letters and digits - space white space (not quite the same as \\s) - upper upper case letters - word "word" characters (same as \\w) - xdigit hexadecimal digits - -The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and -space (32). Notice that this list includes the VT character (code 11). This -makes "space" different to \\s, which does not include VT (for Perl -compatibility). - -The name "word" is a Perl extension, and "blank" is a GNU extension from Perl -5.8. Another Perl extension is negation, which is indicated by a ^ character -after the colon. For example, - - [12[:^digit:]] - -matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX -syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not -supported, and an error is given if they are encountered. - -In UTF-8 mode, characters with values greater than 255 do not match any of -the POSIX character classes. - -.SH VERTICAL BAR -.rs -.sp -Vertical bar characters are used to separate alternative patterns. For example, -the pattern - - gilbert|sullivan - -matches either "gilbert" or "sullivan". Any number of alternatives may appear, -and an empty alternative is permitted (matching the empty string). -The matching process tries each alternative in turn, from left to right, -and the first one that succeeds is used. If the alternatives are within a -subpattern (defined below), "succeeds" means matching the rest of the main -pattern as well as the alternative in the subpattern. - -.SH INTERNAL OPTION SETTING -.rs -.sp -The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and -PCRE_EXTENDED options can be changed from within the pattern by a sequence of -Perl option letters enclosed between "(?" and ")". The option letters are - - i for PCRE_CASELESS - m for PCRE_MULTILINE - s for PCRE_DOTALL - x for PCRE_EXTENDED - -For example, (?im) sets caseless, multiline matching. It is also possible to -unset these options by preceding the letter with a hyphen, and a combined -setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and -PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also -permitted. If a letter appears both before and after the hyphen, the option is -unset. - -When an option change occurs at top level (that is, not inside subpattern -parentheses), the change applies to the remainder of the pattern that follows. -If the change is placed right at the start of a pattern, PCRE extracts it into -the global options (and it will therefore show up in data extracted by the -\fBpcre_fullinfo()\fR function). - -An option change within a subpattern affects only that part of the current -pattern that follows it, so - - (a(?i)b)c - -matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used). -By this means, options can be made to have different settings in different -parts of the pattern. Any changes made in one alternative do carry on -into subsequent branches within the same subpattern. For example, - - (a(?i)b|c) - -matches "ab", "aB", "c", and "C", even though when matching "C" the first -branch is abandoned before the option setting. This is because the effects of -option settings happen at compile time. There would be some very weird -behaviour otherwise. - -The PCRE-specific options PCRE_UNGREEDY and PCRE_EXTRA can be changed in the -same way as the Perl-compatible options by using the characters U and X -respectively. The (?X) flag setting is special in that it must always occur -earlier in the pattern than any of the additional features it turns on, even -when it is at top level. It is best put at the start. - -.SH SUBPATTERNS -.rs -.sp -Subpatterns are delimited by parentheses (round brackets), which can be nested. -Marking part of a pattern as a subpattern does two things: - -1. It localizes a set of alternatives. For example, the pattern - - cat(aract|erpillar|) - -matches one of the words "cat", "cataract", or "caterpillar". Without the -parentheses, it would match "cataract", "erpillar" or the empty string. - -2. It sets up the subpattern as a capturing subpattern (as defined above). -When the whole pattern matches, that portion of the subject string that matched -the subpattern is passed back to the caller via the \fIovector\fR argument of -\fBpcre_exec()\fR. Opening parentheses are counted from left to right (starting -from 1) to obtain the numbers of the capturing subpatterns. - -For example, if the string "the red king" is matched against the pattern - - the ((red|white) (king|queen)) - -the captured substrings are "red king", "red", and "king", and are numbered 1, -2, and 3, respectively. - -The fact that plain parentheses fulfil two functions is not always helpful. -There are often times when a grouping subpattern is required without a -capturing requirement. If an opening parenthesis is followed by a question mark -and a colon, the subpattern does not do any capturing, and is not counted when -computing the number of any subsequent capturing subpatterns. For example, if -the string "the white queen" is matched against the pattern - - the ((?:red|white) (king|queen)) - -the captured substrings are "white queen" and "queen", and are numbered 1 and -2. The maximum number of capturing subpatterns is 65535, and the maximum depth -of nesting of all subpatterns, both capturing and 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". - -.SH NAMED SUBPATTERNS -.rs -.sp -Identifying capturing parentheses by number is simple, but it can be very hard -to keep track of the numbers in complicated regular expressions. Furthermore, -if an expression is modified, the numbers may change. To help with the -difficulty, PCRE supports the naming of subpatterns, something that Perl does -not provide. The Python syntax (?P<name>...) is used. Names consist of -alphanumeric characters and underscores, and must be unique within a pattern. - -Named capturing parentheses are still allocated numbers as well as names. The -PCRE API provides function calls for extracting the name-to-number translation -table from a compiled pattern. For further details see the -.\" HREF -\fBpcreapi\fR -.\" -documentation. - -.SH REPETITION -.rs -.sp -Repetition is specified by quantifiers, which can follow any of the following -items: - - a literal data character - the . metacharacter - the \\C escape sequence - escapes such as \\d that match single characters - a character class - a back reference (see next section) - a parenthesized subpattern (unless it is an assertion) - -The general repetition quantifier specifies a minimum and maximum number of -permitted matches, by giving the two numbers in curly brackets (braces), -separated by a comma. The numbers must be less than 65536, and the first must -be less than or equal to the second. For example: - - z{2,4} - -matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special -character. If the second number is omitted, but the comma is present, there is -no upper limit; if the second number and the comma are both omitted, the -quantifier specifies an exact number of required matches. Thus - - [aeiou]{3,} - -matches at least 3 successive vowels, but may match many more, while - - \\d{8} - -matches exactly 8 digits. An opening curly bracket that appears in a position -where a quantifier is not allowed, or one that does not match the syntax of a -quantifier, is taken as a literal character. For example, {,6} is not a -quantifier, but a literal string of four characters. - -In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual -bytes. Thus, for example, \\x{100}{2} matches two UTF-8 characters, each of -which is represented by a two-byte sequence. - -The quantifier {0} is permitted, causing the expression to behave as if the -previous item and the quantifier were not present. - -For convenience (and historical compatibility) the three most common -quantifiers have single-character abbreviations: - - * is equivalent to {0,} - + is equivalent to {1,} - ? is equivalent to {0,1} - -It is possible to construct infinite loops by following a subpattern that can -match no characters with a quantifier that has no upper limit, for example: - - (a?)* - -Earlier versions of Perl and PCRE used to give an error at compile time for -such patterns. However, because there are cases where this can be useful, such -patterns are now accepted, but if any repetition of the subpattern does in fact -match no characters, the loop is forcibly broken. - -By default, the quantifiers are "greedy", that is, they match as much as -possible (up to the maximum number of permitted times), without causing the -rest of the pattern to fail. The classic example of where this gives problems -is in trying to match comments in C programs. These appear between the -sequences /* and */ and within the sequence, individual * and / characters may -appear. An attempt to match C comments by applying the pattern - - /\\*.*\\*/ - -to the string - - /* first command */ not comment /* second comment */ - -fails, because it matches the entire string owing to the greediness of the .* -item. - -However, if a quantifier is followed by a question mark, it ceases to be -greedy, and instead matches the minimum number of times possible, so the -pattern - - /\\*.*?\\*/ - -does the right thing with the C comments. The meaning of the various -quantifiers is not otherwise changed, just the preferred number of matches. -Do not confuse this use of question mark with its use as a quantifier in its -own right. Because it has two uses, it can sometimes appear doubled, as in - - \\d??\\d - -which matches one digit by preference, but can match two if that is the only -way the rest of the pattern matches. - -If the PCRE_UNGREEDY option is set (an option which is not available in Perl), -the quantifiers are not greedy by default, but individual ones can be made -greedy by following them with a question mark. In other words, it inverts the -default behaviour. - -When a parenthesized subpattern is quantified with a minimum repeat count that -is greater than 1 or with a limited maximum, more store is required for the -compiled pattern, in proportion to the size of the minimum or maximum. - -If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent -to Perl's /s) is set, thus allowing the . to match newlines, the pattern is -implicitly anchored, because whatever follows will be tried against every -character position in the subject string, so there is no point in retrying the -overall match at any position after the first. PCRE normally treats such a -pattern as though it were preceded by \\A. - -In cases where it is known that the subject string contains no newlines, it is -worth setting PCRE_DOTALL in order to obtain this optimization, or -alternatively using ^ to indicate anchoring explicitly. - -However, there is one situation where the optimization cannot be used. When .* -is inside capturing parentheses that are the subject of a backreference -elsewhere in the pattern, a match at the start may fail, and a later one -succeed. Consider, for example: - - (.*)abc\\1 - -If the subject is "xyz123abc123" the match point is the fourth character. For -this reason, such a pattern is not implicitly anchored. - -When a capturing subpattern is repeated, the value captured is the substring -that matched the final iteration. For example, after - - (tweedle[dume]{3}\\s*)+ - -has matched "tweedledum tweedledee" the value of the captured substring is -"tweedledee". However, if there are nested capturing subpatterns, the -corresponding captured values may have been set in previous iterations. For -example, after - - /(a|(b))+/ - -matches "aba" the value of the second captured substring is "b". - -.SH ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS -.rs -.sp -With both maximizing and minimizing repetition, failure of what follows -normally causes the repeated item to be re-evaluated to see if a different -number of repeats allows the rest of the pattern to match. Sometimes it is -useful to prevent this, either to change the nature of the match, or to cause -it fail earlier than it otherwise might, when the author of the pattern knows -there is no point in carrying on. - -Consider, for example, the pattern \\d+foo when applied to the subject line - - 123456bar - -After matching all 6 digits and then failing to match "foo", the normal -action of the matcher is to try again with only 5 digits matching the \\d+ -item, and then with 4, and so on, before ultimately failing. "Atomic grouping" -(a term taken from Jeffrey Friedl's book) provides the means for specifying -that once a subpattern has matched, it is not to be re-evaluated in this way. - -If we use atomic grouping for the previous example, the matcher would give up -immediately on failing to match "foo" the first time. The notation is a kind of -special parenthesis, starting with (?> as in this example: - - (?>\\d+)foo - -This kind of parenthesis "locks up" the part of the pattern it contains once -it has matched, and a failure further into the pattern is prevented from -backtracking into it. Backtracking past it to previous items, however, works as -normal. - -An alternative description is that a subpattern of this type matches the string -of characters that an identical standalone pattern would match, if anchored at -the current point in the subject string. - -Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as -the above example can be thought of as a maximizing repeat that must swallow -everything it can. So, while both \\d+ and \\d+? are prepared to adjust the -number of digits they match in order to make the rest of the pattern match, -(?>\\d+) can only match an entire sequence of digits. - -Atomic groups in general can of course contain arbitrarily complicated -subpatterns, and can be nested. However, when the subpattern for an atomic -group is just a single repeated item, as in the example above, a simpler -notation, called a "possessive quantifier" can be used. This consists of an -additional + character following a quantifier. Using this notation, the -previous example can be rewritten as - - \\d++bar - -Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY -option is ignored. They are a convenient notation for the simpler forms of -atomic group. However, there is no difference in the meaning or processing of a -possessive quantifier and the equivalent atomic group. - -The possessive quantifier syntax is an extension to the Perl syntax. It -originates in Sun's Java package. - -When a pattern contains an unlimited repeat inside a subpattern that can itself -be repeated an unlimited number of times, the use of an atomic group is the -only way to avoid some failing matches taking a very long time indeed. The -pattern - - (\\D+|<\\d+>)*[!?] - -matches an unlimited number of substrings that either consist of non-digits, or -digits enclosed in <>, followed by either ! or ?. When it matches, it runs -quickly. However, if it is applied to - - aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa - -it takes a long time before reporting failure. This is because the string can -be divided between the two repeats in a large number of ways, and all have to -be tried. (The example used [!?] rather than a single character at the end, -because both PCRE and Perl have an optimization that allows for fast failure -when a single character is used. They remember the last single character that -is required for a match, and fail early if it is not present in the string.) -If the pattern is changed to - - ((?>\\D+)|<\\d+>)*[!?] - -sequences of non-digits cannot be broken, and failure happens quickly. - -.SH BACK REFERENCES -.rs -.sp -Outside a character class, a backslash followed by a digit greater than 0 (and -possibly further digits) is a back reference to a capturing subpattern earlier -(that is, to its left) in the pattern, provided there have been that many -previous capturing left parentheses. - -However, if the decimal number following the backslash is less than 10, it is -always taken as a back reference, and causes an error only if there are not -that many capturing left parentheses in the entire pattern. In other words, the -parentheses that are referenced need not be to the left of the reference for -numbers less than 10. See the section entitled "Backslash" above for further -details of the handling of digits following a backslash. - -A back reference matches whatever actually matched the capturing subpattern in -the current subject string, rather than anything matching the subpattern -itself (see -.\" HTML <a href="#subpatternsassubroutines"> -.\" </a> -"Subpatterns as subroutines" -.\" -below for a way of doing that). So the pattern - - (sens|respons)e and \\1ibility - -matches "sense and sensibility" and "response and responsibility", but not -"sense and responsibility". If caseful matching is in force at the time of the -back reference, the case of letters is relevant. For example, - - ((?i)rah)\\s+\\1 - -matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original -capturing subpattern is matched caselessly. - -Back references to named subpatterns use the Python syntax (?P=name). We could -rewrite the above example as follows: - - (?<p1>(?i)rah)\\s+(?P=p1) - -There may be more than one back reference to the same subpattern. If a -subpattern has not actually been used in a particular match, any back -references to it always fail. For example, the pattern - - (a|(bc))\\2 - -always fails if it starts to match "a" rather than "bc". Because there may be -many capturing parentheses in a pattern, all digits following the backslash are -taken as part of a potential back reference number. If the pattern continues -with a digit character, some delimiter must be used to terminate the back -reference. If the PCRE_EXTENDED option is set, this can be whitespace. -Otherwise an empty comment can be used. - -A back reference that occurs inside the parentheses to which it refers fails -when the subpattern is first used, so, for example, (a\\1) never matches. -However, such references can be useful inside repeated subpatterns. For -example, the pattern - - (a|b\\1)+ - -matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of -the subpattern, the back reference matches the character string corresponding -to the previous iteration. In order for this to work, the pattern must be such -that the first iteration does not need to match the back reference. This can be -done using alternation, as in the example above, or by a quantifier with a -minimum of zero. - -.SH ASSERTIONS -.rs -.sp -An assertion is a test on the characters following or preceding the current -matching point that does not actually consume any characters. The simple -assertions coded as \\b, \\B, \\A, \\G, \\Z, \\z, ^ and $ are described above. -More complicated assertions are coded as subpatterns. There are two kinds: -those that look ahead of the current position in the subject string, and those -that look behind it. - -An assertion subpattern is matched in the normal way, except that it does not -cause the current matching position to be changed. Lookahead assertions start -with (?= for positive assertions and (?! for negative assertions. For example, - - \\w+(?=;) - -matches a word followed by a semicolon, but does not include the semicolon in -the match, and - - foo(?!bar) - -matches any occurrence of "foo" that is not followed by "bar". Note that the -apparently similar pattern - - (?!foo)bar - -does not find an occurrence of "bar" that is preceded by something other than -"foo"; it finds any occurrence of "bar" whatsoever, because the assertion -(?!foo) is always true when the next three characters are "bar". A -lookbehind assertion is needed to achieve this effect. - -If you want to force a matching failure at some point in a pattern, the most -convenient way to do it is with (?!) because an empty string always matches, so -an assertion that requires there not to be an empty string must always fail. - -Lookbehind assertions start with (?<= for positive assertions and (?<! for -negative assertions. For example, - - (?<!foo)bar - -does find an occurrence of "bar" that is not preceded by "foo". The contents of -a lookbehind assertion are restricted such that all the strings it matches must -have a fixed length. However, if there are several alternatives, they do not -all have to have the same fixed length. Thus - - (?<=bullock|donkey) - -is permitted, but - - (?<!dogs?|cats?) - -causes an error at compile time. Branches that match different length strings -are permitted only at the top level of a lookbehind assertion. This is an -extension compared with Perl (at least for 5.8), which requires all branches to -match the same length of string. An assertion such as - - (?<=ab(c|de)) - -is not permitted, because its single top-level branch can match two different -lengths, but it is acceptable if rewritten to use two top-level branches: - - (?<=abc|abde) - -The implementation of lookbehind assertions is, for each alternative, to -temporarily move the current position back by the fixed width and then try to -match. If there are insufficient characters before the current position, the -match is deemed to fail. - -PCRE does not allow the \\C escape (which matches a single byte in UTF-8 mode) -to appear in lookbehind assertions, because it makes it impossible to calculate -the length of the lookbehind. - -Atomic groups can be used in conjunction with lookbehind assertions to specify -efficient matching at the end of the subject string. Consider a simple pattern -such as - - abcd$ - -when applied to a long string that does not match. Because matching proceeds -from left to right, PCRE will look for each "a" in the subject and then see if -what follows matches the rest of the pattern. If the pattern is specified as - - ^.*abcd$ - -the initial .* matches the entire string at first, but when this fails (because -there is no following "a"), it backtracks to match all but the last character, -then all but the last two characters, and so on. Once again the search for "a" -covers the entire string, from right to left, so we are no better off. However, -if the pattern is written as - - ^(?>.*)(?<=abcd) - -or, equivalently, - - ^.*+(?<=abcd) - -there can be no backtracking for the .* item; it can match only the entire -string. The subsequent lookbehind assertion does a single test on the last four -characters. If it fails, the match fails immediately. For long strings, this -approach makes a significant difference to the processing time. - -Several assertions (of any sort) may occur in succession. For example, - - (?<=\\d{3})(?<!999)foo - -matches "foo" preceded by three digits that are not "999". Notice that each of -the assertions is applied independently at the same point in the subject -string. First there is a check that the previous three characters are all -digits, and then there is a check that the same three characters are not "999". -This pattern does \fInot\fR match "foo" preceded by six characters, the first -of which are digits and the last three of which are not "999". For example, it -doesn't match "123abcfoo". A pattern to do that is - - (?<=\\d{3}...)(?<!999)foo - -This time the first assertion looks at the preceding six characters, checking -that the first three are digits, and then the second assertion checks that the -preceding three characters are not "999". - -Assertions can be nested in any combination. For example, - - (?<=(?<!foo)bar)baz - -matches an occurrence of "baz" that is preceded by "bar" which in turn is not -preceded by "foo", while - - (?<=\\d{3}(?!999)...)foo - -is another pattern which matches "foo" preceded by three digits and any three -characters that are not "999". - -Assertion subpatterns are not capturing subpatterns, and may not be repeated, -because it makes no sense to assert the same thing several times. If any kind -of assertion contains capturing subpatterns within it, these are counted for -the purposes of numbering the capturing subpatterns in the whole pattern. -However, substring capturing is carried out only for positive assertions, -because it does not make sense for negative assertions. - -.SH CONDITIONAL SUBPATTERNS -.rs -.sp -It is possible to cause the matching process to obey a subpattern -conditionally or to choose between two alternative subpatterns, depending on -the result of an assertion, or whether a previous capturing subpattern matched -or not. The two possible forms of conditional subpattern are - - (?(condition)yes-pattern) - (?(condition)yes-pattern|no-pattern) - -If the condition is satisfied, the yes-pattern is used; otherwise the -no-pattern (if present) is used. If there are more than two alternatives in the -subpattern, a compile-time error occurs. - -There are three kinds of condition. If the text between the parentheses -consists of a sequence of digits, the condition is satisfied if the capturing -subpattern of that number has previously matched. The number must be greater -than zero. Consider the following pattern, which contains non-significant white -space to make it more readable (assume the PCRE_EXTENDED option) and to divide -it into three parts for ease of discussion: - - ( \\( )? [^()]+ (?(1) \\) ) - -The first part matches an optional opening parenthesis, and if that -character is present, sets it as the first captured substring. The second part -matches one or more characters that are not parentheses. The third part is a -conditional subpattern that tests whether the first set of parentheses matched -or not. If they did, that is, if subject started with an opening parenthesis, -the condition is true, and so the yes-pattern is executed and a closing -parenthesis is required. Otherwise, since no-pattern is not present, the -subpattern matches nothing. In other words, this pattern matches a sequence of -non-parentheses, optionally enclosed in parentheses. - -If the condition is the string (R), it is satisfied if a recursive call to the -pattern or subpattern has been made. At "top level", the condition is false. -This is a PCRE extension. Recursive patterns are described in the next section. - -If the condition is not a sequence of digits or (R), it must be an assertion. -This may be a positive or negative lookahead or lookbehind assertion. Consider -this pattern, again containing non-significant white space, and with the two -alternatives on the second line: - - (?(?=[^a-z]*[a-z]) - \\d{2}-[a-z]{3}-\\d{2} | \\d{2}-\\d{2}-\\d{2} ) - -The condition is a positive lookahead assertion that matches an optional -sequence of non-letters followed by a letter. In other words, it tests for the -presence of at least one letter in the subject. If a letter is found, the -subject is matched against the first alternative; otherwise it is matched -against the second. This pattern matches strings in one of the two forms -dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits. - -.SH COMMENTS -.rs -.sp -The sequence (?# marks the start of a comment which continues up to the next -closing parenthesis. Nested parentheses are not permitted. The characters -that make up a comment play no part in the pattern matching at all. - -If the PCRE_EXTENDED option is set, an unescaped # character outside a -character class introduces a comment that continues up to the next newline -character in the pattern. - -.SH RECURSIVE PATTERNS -.rs -.sp -Consider the problem of matching a string in parentheses, allowing for -unlimited nested parentheses. Without the use of recursion, the best that can -be done is to use a pattern that matches up to some fixed depth of nesting. It -is not possible to handle an arbitrary nesting depth. Perl has provided an -experimental facility that allows regular expressions to recurse (amongst other -things). It does this by interpolating Perl code in the expression at run time, -and the code can refer to the expression itself. A Perl pattern to solve the -parentheses problem can be created like this: - - $re = qr{\\( (?: (?>[^()]+) | (?p{$re}) )* \\)}x; - -The (?p{...}) item interpolates Perl code at run time, and in this case refers -recursively to the pattern in which it appears. Obviously, PCRE cannot support -the interpolation of Perl code. Instead, it supports some special syntax for -recursion of the entire pattern, and also for individual subpattern recursion. - -The special item that consists of (? followed by a number greater than zero and -a closing parenthesis is a recursive call of the subpattern of the given -number, provided that it occurs inside that subpattern. (If not, it is a -"subroutine" call, which is described in the next section.) The special item -(?R) is a recursive call of the entire regular expression. - -For example, this PCRE pattern solves the nested parentheses problem (assume -the PCRE_EXTENDED option is set so that white space is ignored): - - \\( ( (?>[^()]+) | (?R) )* \\) - -First it matches an opening parenthesis. Then it matches any number of -substrings which can either be a sequence of non-parentheses, or a recursive -match of the pattern itself (that is a correctly parenthesized substring). -Finally there is a closing parenthesis. - -If this were part of a larger pattern, you would not want to recurse the entire -pattern, so instead you could use this: - - ( \\( ( (?>[^()]+) | (?1) )* \\) ) - -We have put the pattern into parentheses, and caused the recursion to refer to -them instead of the whole pattern. In a larger pattern, keeping track of -parenthesis numbers can be tricky. It may be more convenient to use named -parentheses instead. For this, PCRE uses (?P>name), which is an extension to -the Python syntax that PCRE uses for named parentheses (Perl does not provide -named parentheses). We could rewrite the above example as follows: - - (?P<pn> \\( ( (?>[^()]+) | (?P>pn) )* \\) ) - -This particular example pattern contains nested unlimited repeats, and so the -use of atomic grouping for matching strings of non-parentheses is important -when applying the pattern to strings that do not match. For example, when this -pattern is applied to - - (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa() - -it yields "no match" quickly. However, if atomic grouping is not used, -the match runs for a very long time indeed because there are so many different -ways the + and * repeats can carve up the subject, and all have to be tested -before failure can be reported. - -At the end of a match, the values set for any capturing subpatterns are those -from the outermost level of the recursion at which the subpattern value is set. -If you want to obtain intermediate values, a callout function can be used (see -below and the -.\" HREF -\fBpcrecallout\fR -.\" -documentation). If the pattern above is matched against - - (ab(cd)ef) - -the value for the capturing parentheses is "ef", which is the last value taken -on at the top level. If additional parentheses are added, giving - - \\( ( ( (?>[^()]+) | (?R) )* ) \\) - ^ ^ - ^ ^ - -the string they capture is "ab(cd)ef", the contents of the top level -parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE -has to obtain extra memory to store data during a recursion, which it does by -using \fBpcre_malloc\fR, freeing it via \fBpcre_free\fR afterwards. If no -memory can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error. - -Do not confuse the (?R) item with the condition (R), which tests for recursion. -Consider this pattern, which matches text in angle brackets, allowing for -arbitrary nesting. Only digits are allowed in nested brackets (that is, when -recursing), whereas any characters are permitted at the outer level. - - < (?: (?(R) \\d++ | [^<>]*+) | (?R)) * > - -In this pattern, (?(R) is the start of a conditional subpattern, with two -different alternatives for the recursive and non-recursive cases. The (?R) item -is the actual recursive call. - -.\" HTML <a name="subpatternsassubroutines"></a> -.SH SUBPATTERNS AS SUBROUTINES -.rs -.sp -If the syntax for a recursive subpattern reference (either by number or by -name) is used outside the parentheses to which it refers, it operates like a -subroutine in a programming language. An earlier example pointed out that the -pattern - - (sens|respons)e and \\1ibility - -matches "sense and sensibility" and "response and responsibility", but not -"sense and responsibility". If instead the pattern - - (sens|respons)e and (?1)ibility - -is used, it does match "sense and responsibility" as well as the other two -strings. Such references must, however, follow the subpattern to which they -refer. - -.SH CALLOUTS -.rs -.sp -Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl -code to be obeyed in the middle of matching a regular expression. This makes it -possible, amongst other things, to extract different substrings that match the -same pair of parentheses when there is a repetition. - -PCRE provides a similar feature, but of course it cannot obey arbitrary Perl -code. The feature is called "callout". The caller of PCRE provides an external -function by putting its entry point in the global variable \fIpcre_callout\fR. -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)\dabc(?C2)def - -During matching, when PCRE reaches a callout point (and \fIpcre_callout\fR is -set), the external function is called. It is provided with the number of the -callout, and, optionally, one item of data originally supplied by the caller of -\fBpcre_exec()\fR. The callout function may cause matching to backtrack, or to -fail altogether. A complete description of the interface to the callout -function is given in the -.\" HREF -\fBpcrecallout\fR -.\" -documentation. - -.in 0 -Last updated: 03 February 2003 -.br -Copyright (c) 1997-2003 University of Cambridge. |