@@ -60,51 +62,92 @@ published by O'Reilly, covers regular expressions in g description of PCRE's regular expressions is intended as reference material.
+This document discusses the patterns that are supported by PCRE when one its +main matching functions, pcre_exec() (8-bit) or pcre[16|32]_exec() +(16- or 32-bit), is used. PCRE also has alternative matching functions, +pcre_dfa_exec() and pcre[16|32_dfa_exec(), which match using a +different algorithm that is not Perl-compatible. Some of the features discussed +below are not available when DFA matching is used. The advantages and +disadvantages of the alternative functions, and how they differ from the normal +functions, are discussed in the +pcrematching +page. +
++A number of options that can be passed to pcre_compile() can also be set +by special items at the start of a pattern. These are not Perl-compatible, but +are provided to make these options accessible to pattern writers who are not +able to change the program that processes the pattern. Any number of these +items may appear, but they must all be together right at the start of the +pattern string, and the letters must be in upper case. +
+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, -PCRE must be built to include UTF-8 support, and you must call -pcre_compile() or pcre_compile2() with the PCRE_UTF8 option. There -is also a special sequence that can be given at the start of a pattern: +there is now also support for UTF-8 strings in the original library, an +extra library that supports 16-bit and UTF-16 character strings, and a +third library that supports 32-bit and UTF-32 character strings. To use these +features, PCRE must be built to include appropriate support. When using UTF +strings you must either call the compiling function with the PCRE_UTF8, +PCRE_UTF16, or PCRE_UTF32 option, or the pattern must start with one of +these special sequences:
(*UTF8) + (*UTF16) + (*UTF32) + (*UTF)-Starting a pattern with this sequence is equivalent to setting the PCRE_UTF8 -option. This feature is not Perl-compatible. How setting UTF-8 mode affects -pattern matching is mentioned in several places below. There is also a summary -of UTF-8 features in the +(*UTF) is a generic sequence that can be used with any of the libraries. +Starting a pattern with such a sequence is equivalent to setting the relevant +option. How setting a UTF mode affects pattern matching is mentioned in several +places below. There is also a summary of features in the pcreunicode page.
-Another special sequence that may appear at the start of a pattern or in -combination with (*UTF8) is: -
- (*UCP) -+Some applications that allow their users to supply patterns may wish to +restrict them to non-UTF data for security reasons. If the PCRE_NEVER_UTF +option is set at compile time, (*UTF) etc. are not allowed, and their +appearance causes an error. + +
+Another special sequence that may appear at the start of a pattern is (*UCP). This has the same effect as setting the PCRE_UCP option: it causes sequences such as \d and \w to use Unicode properties to determine character types, instead of recognizing only characters with codes less than 128 via a lookup table.
+-If a pattern starts with (*NO_START_OPT), it has the same effect as setting the -PCRE_NO_START_OPTIMIZE option either at compile or matching time. There are -also some more of these special sequences that are concerned with the handling -of newlines; they are described below. +If a pattern starts with (*NO_AUTO_POSSESS), it has the same effect as setting +the PCRE_NO_AUTO_POSSESS option at compile time. This stops PCRE from making +quantifiers possessive when what follows cannot match the repeated item. For +example, by default a+b is treated as a++b. For more details, see the +pcreapi +documentation.
+-The remainder of this document discusses the patterns that are supported by -PCRE when its main matching function, pcre_exec(), is used. -From release 6.0, PCRE offers a second matching function, -pcre_dfa_exec(), which matches using a different algorithm that is not -Perl-compatible. Some of the features discussed below are not available when -pcre_dfa_exec() is used. The advantages and disadvantages of the -alternative function, and how it differs from the normal function, are -discussed in the -pcrematching -page. +If a pattern starts with (*NO_START_OPT), it has the same effect as setting the +PCRE_NO_START_OPTIMIZE option either at compile or matching time. This disables +several optimizations for quickly reaching "no match" results. For more +details, see the +pcreapi +documentation.
-PCRE supports five different conventions for indicating line breaks in strings: a single CR (carriage return) character, a single LF (linefeed) @@ -126,30 +169,58 @@ string with one of the following five sequences: (*ANYCRLF) any of the three above (*ANY) all Unicode newline sequences -These override the default and the options given to pcre_compile() or -pcre_compile2(). For example, on a Unix system where LF is the default -newline sequence, the pattern +These override the default and the options given to the compiling function. For +example, on a Unix system where LF is the default newline sequence, the pattern
(*CR)a.bchanges the convention to CR. That pattern matches "a\nb" because LF is no -longer a newline. Note that these special settings, which are not -Perl-compatible, are recognized only at the very start of a pattern, and that -they must be in upper case. If more than one of them is present, the last one +longer a newline. If more than one of these settings is present, the last one is used.
-The newline convention affects the interpretation of the dot metacharacter when -PCRE_DOTALL is not set, and also the behaviour of \N. However, it does not -affect what the \R escape sequence matches. By default, this is any Unicode -newline sequence, for Perl compatibility. However, this can be changed; see the +The newline convention affects where the circumflex and dollar assertions are +true. It also affects the interpretation of the dot metacharacter when +PCRE_DOTALL is not set, and the behaviour of \N. However, it does not affect +what the \R escape sequence matches. By default, this is any Unicode newline +sequence, for Perl compatibility. However, this can be changed; see the description of \R in the section entitled "Newline sequences" below. A change of \R setting can be combined with a change of newline convention.
-+The caller of pcre_exec() can set a limit on the number of times the +internal match() function is called and on the maximum depth of +recursive calls. These facilities are provided to catch runaway matches that +are provoked by patterns with huge matching trees (a typical example is a +pattern with nested unlimited repeats) and to avoid running out of system stack +by too much recursion. When one of these limits is reached, pcre_exec() +gives an error return. The limits can also be set by items at the start of the +pattern of the form +
+ (*LIMIT_MATCH=d) + (*LIMIT_RECURSION=d) ++where d is any number of decimal digits. However, the value of the setting must +be less than the value set (or defaulted) by the caller of pcre_exec() +for it to have any effect. In other words, the pattern writer can lower the +limits set by the programmer, but not raise them. If there is more than one +setting of one of these limits, the lower value is used. + +
+PCRE can be compiled to run in an environment that uses EBCDIC as its character +code rather than ASCII or Unicode (typically a mainframe system). In the +sections below, character code values are ASCII or Unicode; in an EBCDIC +environment these characters may have different code values, and there are no +code points greater than 255. +
+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 @@ -158,13 +229,13 @@ corresponding characters in the subject. As a trivial matches a portion of a subject string that is identical to itself. When caseless matching is specified (the PCRE_CASELESS option), letters are matched -independently of case. In UTF-8 mode, PCRE always understands the concept of +independently of case. In a UTF mode, PCRE always understands the concept of case for characters whose values are less than 128, so caseless matching is always possible. For characters with higher values, the concept of case is supported if PCRE is compiled with Unicode property support, but not otherwise. If you want to use caseless matching for characters 128 and above, you must ensure that PCRE is compiled with Unicode property support as well as with -UTF-8 support. +UTF support.
The power of regular expressions comes from the ability to include alternatives @@ -205,7 +276,7 @@ a character class the only metacharacters are: The following sections describe the use of each of the metacharacters.
-The backslash character has several uses. Firstly, if it is followed by a character that is not a number or a letter, it takes away any special meaning @@ -220,15 +291,16 @@ non-alphanumeric with backslash to specify that it sta particular, if you want to match a backslash, you write \\.
-In UTF-8 mode, only ASCII numbers and letters have any special meaning after a +In a UTF mode, only ASCII numbers and letters have any special meaning after a backslash. All other characters (in particular, those whose codepoints are greater than 127) are treated as literals.
-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 are ignored. An escaping backslash can -be used to include a whitespace or # character as part of the pattern. +If a pattern is compiled with the PCRE_EXTENDED option, most white space in the +pattern (other than in a character class), and characters between a # outside a +character class and the next newline, inclusive, are ignored. An escaping +backslash can be used to include a white space or # character as part of the +pattern.
If you want to remove the special meaning from a sequence of characters, you @@ -262,52 +334,35 @@ one of the following escape sequences than the binary \a alarm, that is, the BEL character (hex 07) \cx "control-x", where x is any ASCII character \e escape (hex 1B) - \f formfeed (hex 0C) + \f form feed (hex 0C) \n linefeed (hex 0A) \r carriage return (hex 0D) \t tab (hex 09) + \0dd character with octal code 0dd \ddd character with octal code ddd, or back reference + \o{ddd..} character with octal code ddd.. \xhh character with hex code hh \x{hhh..} character with hex code hhh.. (non-JavaScript mode) \uhhhh character with hex code hhhh (JavaScript 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 (z is 7A), but \c{ becomes hex 3B ({ is 7B), while -\c; becomes hex 7B (; is 3B). If the byte following \c has a value greater -than 127, a compile-time error occurs. This locks out non-ASCII characters in -both byte mode and UTF-8 mode. (When PCRE is compiled in EBCDIC mode, all byte -values are valid. A lower case letter is converted to upper case, and then the -0xc0 bits are flipped.) +The precise effect of \cx on ASCII characters 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 \cA to \cZ become hex 01 to hex 1A (A is 41, Z is 5A), +but \c{ becomes hex 3B ({ is 7B), and \c; becomes hex 7B (; is 3B). If the +data item (byte or 16-bit value) following \c has a value greater than 127, a +compile-time error occurs. This locks out non-ASCII characters in all modes.
-By default, after \x, from zero to two hexadecimal digits are read (letters -can be in upper or lower case). Any number of hexadecimal digits may appear -between \x{ and }, but the value of the character code must be less than 256 -in non-UTF-8 mode, and less than 2**31 in UTF-8 mode. That is, the maximum -value in hexadecimal is 7FFFFFFF. Note that this is bigger than the largest -Unicode code point, which is 10FFFF. +The \c facility was designed for use with ASCII characters, but with the +extension to Unicode it is even less useful than it once was. It is, however, +recognized when PCRE is compiled in EBCDIC mode, where data items are always +bytes. In this mode, all values are valid after \c. If the next character is a +lower case letter, it is converted to upper case. Then the 0xc0 bits of the +byte are inverted. Thus \cA becomes hex 01, as in ASCII (A is C1), but because +the EBCDIC letters are disjoint, \cZ becomes hex 29 (Z is E9), and other +characters also generate different values.
-If characters other than hexadecimal digits appear between \x{ and }, or if -there is no terminating }, this form of escape is not recognized. Instead, the -initial \x will be interpreted as a basic hexadecimal escape, with no -following digits, giving a character whose value is zero. -
--If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \x is -as just described only when it is followed by two hexadecimal digits. -Otherwise, it matches a literal "x" character. In JavaScript mode, support for -code points greater than 256 is provided by \u, which must be followed by -four hexadecimal digits; otherwise it matches a literal "u" character. -
--Characters whose value is less than 256 can be defined by either of the two -syntaxes for \x (or by \u in JavaScript mode). There is no difference in the -way they are handled. For example, \xdc is exactly the same as \x{dc} (or -\u00dc in JavaScript mode). -
-After \0 up to two further octal digits are read. If there are fewer than two digits, just those that are present are used. Thus the sequence \0\x\07 specifies two binary zeros followed by a BEL character (code value 7). Make @@ -315,9 +370,23 @@ sure you supply two digits after the initial zero if t 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 +The escape \o must be followed by a sequence of octal digits, enclosed in +braces. An error occurs if this is not the case. This escape is a recent +addition to Perl; it provides way of specifying character code points as octal +numbers greater than 0777, and it also allows octal numbers and back references +to be unambiguously specified. +
++For greater clarity and unambiguity, it is best to avoid following \ by a +digit greater than zero. Instead, use \o{} or \x{} to specify character +numbers, and \g{} to specify back references. The following paragraphs +describe the old, ambiguous syntax. +
++The handling of a backslash followed by a digit other than 0 is complicated, +and Perl has changed in recent releases, causing PCRE also to change. Outside a +character class, PCRE reads the digit and any following digits as a decimal +number. If the number is less than 8, 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, @@ -325,27 +394,67 @@ 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 uses them to generate a data character. Any -subsequent digits stand for themselves. In non-UTF-8 mode, the value of a -character specified in octal must be less than \400. In UTF-8 mode, values up -to \777 are permitted. For example: +Inside a character class, or if the decimal number following \ is greater than +7 and there have not been that many capturing subpatterns, PCRE handles \8 and +\9 as the literal characters "8" and "9", and otherwise re-reads up to three +octal digits following the backslash, using them to generate a data character. +Any subsequent digits stand for themselves. For example:
- \040 is another way of writing a space + \040 is another way of writing an ASCII 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" + \377 might be a back reference, otherwise the value 255 (decimal) + \81 is either a back reference, or 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. +Note that octal values of 100 or greater that are specified using this syntax +must not be introduced by a leading zero, because no more than three octal +digits are ever read.
+By default, after \x that is not followed by {, from zero to two hexadecimal +digits are read (letters can be in upper or lower case). Any number of +hexadecimal digits may appear between \x{ and }. If a character other than +a hexadecimal digit appears between \x{ and }, or if there is no terminating +}, an error occurs. +
++If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \x is +as just described only when it is followed by two hexadecimal digits. +Otherwise, it matches a literal "x" character. In JavaScript mode, support for +code points greater than 256 is provided by \u, which must be followed by +four hexadecimal digits; otherwise it matches a literal "u" character. +
++Characters whose value is less than 256 can be defined by either of the two +syntaxes for \x (or by \u in JavaScript mode). There is no difference in the +way they are handled. For example, \xdc is exactly the same as \x{dc} (or +\u00dc in JavaScript mode). +
++Characters that are specified using octal or hexadecimal numbers are +limited to certain values, as follows: +
+ 8-bit non-UTF mode less than 0x100 + 8-bit UTF-8 mode less than 0x10ffff and a valid codepoint + 16-bit non-UTF mode less than 0x10000 + 16-bit UTF-16 mode less than 0x10ffff and a valid codepoint + 32-bit non-UTF mode less than 0x100000000 + 32-bit UTF-32 mode less than 0x10ffff and a valid codepoint ++Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-called +"surrogate" codepoints), and 0xffef. + +
All the sequences that define a single character value can be used both inside and outside character classes. In addition, inside a character class, \b is interpreted as the backspace character (hex 08). @@ -399,12 +508,12 @@ Another use of backslash is for specifying generic cha
\d any decimal digit \D any character that is not a decimal digit - \h any horizontal whitespace character - \H any character that is not a horizontal whitespace character - \s any whitespace character - \S any character that is not a whitespace character - \v any vertical whitespace character - \V any character that is not a vertical whitespace character + \h any horizontal white space character + \H any character that is not a horizontal white space character + \s any white space character + \S any character that is not a white space character + \v any vertical white space character + \V any character that is not a vertical white space character \w any "word" character \W any "non-word" character@@ -423,11 +532,14 @@ matching point is at the end of the subject string, al 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). If "use locale;" is -included in a Perl script, \s may match the VT character. In PCRE, it never -does. +For compatibility with Perl, \s did not used to match the VT character (code +11), which made it different from the the POSIX "space" class. However, Perl +added VT at release 5.18, and PCRE followed suit at release 8.34. The default +\s characters are now HT (9), LF (10), VT (11), FF (12), CR (13), and space +(32), which are defined as white space in the "C" locale. This list may vary if +locale-specific matching is taking place. For example, in some locales the +"non-breaking space" character (\xA0) is recognized as white space, and in +others the VT character is not.
A "word" character is an underscore or any character that is a letter or digit. @@ -438,21 +550,23 @@ place (see in the pcreapi page). For example, in a French locale such as "fr_FR" in Unix-like systems, -or "french" in Windows, some character codes greater than 128 are used for +or "french" in Windows, some character codes greater than 127 are used for accented letters, and these are then matched by \w. The use of locales with Unicode is discouraged.
-By default, in UTF-8 mode, characters with values greater than 128 never match -\d, \s, or \w, and always match \D, \S, and \W. These sequences retain -their original meanings from before UTF-8 support was available, mainly for -efficiency reasons. However, if PCRE is compiled with Unicode property support, -and the PCRE_UCP option is set, the behaviour is changed so that Unicode -properties are used to determine character types, as follows: +By default, characters whose code points are greater than 127 never match \d, +\s, or \w, and always match \D, \S, and \W, although this may vary for +characters in the range 128-255 when locale-specific matching is happening. +These escape sequences retain their original meanings from before Unicode +support was available, mainly for efficiency reasons. If PCRE is compiled with +Unicode property support, and the PCRE_UCP option is set, the behaviour is +changed so that Unicode properties are used to determine character types, as +follows:
- \d any character that \p{Nd} matches (decimal digit)
- \s any character that \p{Z} matches, plus HT, LF, FF, CR
- \w any character that \p{L} or \p{N} matches, plus underscore
+ \d any character that matches \p{Nd} (decimal digit)
+ \s any character that matches \p{Z} or \h or \v
+ \w any character that matches \p{L} or \p{N}, plus underscore
The upper case escapes match the inverse sets of characters. Note that \d
matches only decimal digits, whereas \w matches any Unicode digit, as well as
@@ -463,11 +577,10 @@ is noticeably slower when PCRE_UCP is set.
The sequences \h, \H, \v, and \V are features that were added to Perl at release 5.10. In contrast to the other sequences, which match only ASCII -characters by default, these always match certain high-valued codepoints in -UTF-8 mode, whether or not PCRE_UCP is set. The horizontal space characters -are: +characters by default, these always match certain high-valued code points, +whether or not PCRE_UCP is set. The horizontal space characters are:
- U+0009 Horizontal tab + U+0009 Horizontal tab (HT) U+0020 Space U+00A0 Non-break space U+1680 Ogham space mark @@ -489,21 +602,24 @@ are:The vertical space characters are:
- U+000A Linefeed - U+000B Vertical tab - U+000C Formfeed - U+000D Carriage return - U+0085 Next line + U+000A Linefeed (LF) + U+000B Vertical tab (VT) + U+000C Form feed (FF) + U+000D Carriage return (CR) + U+0085 Next line (NEL) U+2028 Line separator U+2029 Paragraph separator -- + +In 8-bit, non-UTF-8 mode, only the characters with codepoints less than 256 are +relevant. +
Outside a character class, by default, the escape sequence \R matches any -Unicode newline sequence. In non-UTF-8 mode \R is equivalent to the following: +Unicode newline sequence. In 8-bit non-UTF-8 mode \R is equivalent to the +following:
(?>\r\n|\n|\x0b|\f|\r|\x85)@@ -511,12 +627,12 @@ This is an example of an "atomic group", details of wh below. This particular group matches either the two-character sequence CR followed by LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab, -U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next +U+000B), FF (form feed, U+000C), CR (carriage return, U+000D), or NEL (next line, U+0085). The two-character sequence is treated as a single unit that cannot be split.
-In UTF-8 mode, two additional characters whose codepoints are greater than 255 +In other modes, two additional characters whose codepoints are greater than 255 are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029). Unicode character property support is not needed for these characters to be recognized. @@ -533,19 +649,19 @@ one of the following sequences: (*BSR_ANYCRLF) CR, LF, or CRLF only (*BSR_UNICODE) any Unicode newline sequence -These override the default and the options given to pcre_compile() or -pcre_compile2(), but they can be overridden by options given to -pcre_exec() or pcre_dfa_exec(). Note that these special settings, -which are not Perl-compatible, are recognized only at the very start of a -pattern, and that they must be in upper case. If more than one of them is -present, the last one is used. They can be combined with a change of newline -convention; for example, a pattern can start with: +These override the default and the options given to the compiling function, but +they can themselves be overridden by options given to a matching function. Note +that these special settings, which are not Perl-compatible, are recognized only +at the very start of a pattern, and that they must be in upper case. If more +than one of them is present, the last one is used. They can be combined with a +change of newline convention; for example, a pattern can start with:
(*ANY)(*BSR_ANYCRLF)-They can also be combined with the (*UTF8) or (*UCP) special sequences. Inside -a character class, \R is treated as an unrecognized escape sequence, and so -matches the letter "R" by default, but causes an error if PCRE_EXTRA is set. +They can also be combined with the (*UTF8), (*UTF16), (*UTF32), (*UTF) or +(*UCP) special sequences. Inside a character class, \R is treated as an +unrecognized escape sequence, and so matches the letter "R" by default, but +causes an error if PCRE_EXTRA is set.
When PCRE is built with Unicode character property support, three additional escape sequences that match characters with specific properties are available. -When not in UTF-8 mode, these sequences are of course limited to testing +When in 8-bit non-UTF-8 mode, these sequences are of course limited to testing characters whose codepoints are less than 256, but they do work in this mode. The extra escape sequences are:
\p{xx} a character with the xx property
\P{xx} a character without the xx property
- \X an extended Unicode sequence
+ \X a Unicode extended grapheme cluster
The property names represented by xx above are limited to the Unicode
script names, the general category properties, "Any", which matches any
@@ -587,13 +703,16 @@ Armenian,
Avestan,
Balinese,
Bamum,
+Batak,
Bengali,
Bopomofo,
+Brahmi,
Braille,
Buginese,
Buhid,
Canadian_Aboriginal,
Carian,
+Chakma,
Cham,
Cherokee,
Common,
@@ -636,7 +755,11 @@ Lisu,
Lycian,
Lydian,
Malayalam,
+Mandaic,
Meetei_Mayek,
+Meroitic_Cursive,
+Meroitic_Hieroglyphs,
+Miao,
Mongolian,
Myanmar,
New_Tai_Lue,
@@ -655,8 +778,10 @@ Rejang,
Runic,
Samaritan,
Saurashtra,
+Sharada,
Shavian,
Sinhala,
+Sora_Sompeng,
Sundanese,
Syloti_Nagri,
Syriac,
@@ -665,6 +790,7 @@ Tagbanwa,
Tai_Le,
Tai_Tham,
Tai_Viet,
+Takri,
Tamil,
Telugu,
Thaana,
@@ -742,9 +868,10 @@ a modifier or "other".
The Cs (Surrogate) property applies only to characters in the range U+D800 to -U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so -cannot be tested by PCRE, unless UTF-8 validity checking has been turned off -(see the discussion of PCRE_NO_UTF8_CHECK in the +U+DFFF. Such characters are not valid in Unicode strings and so +cannot be tested by PCRE, unless UTF validity checking has been turned off +(see the discussion of PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK and +PCRE_NO_UTF32_CHECK in the pcreapi page). Perl does not support the Cs property.
@@ -760,43 +887,74 @@ Unicode table.Specifying caseless matching does not affect these escape sequences. For -example, \p{Lu} always matches only upper case letters. +example, \p{Lu} always matches only upper case letters. This is different from +the behaviour of current versions of Perl.
-The \X escape matches any number of Unicode characters that form an extended -Unicode sequence. \X is equivalent to +Matching characters by Unicode property is not fast, because PCRE has to do a +multistage table lookup in order to find a character's property. That is why +the traditional escape sequences such as \d and \w do not use Unicode +properties in PCRE by default, though you can make them do so by setting the +PCRE_UCP option or by starting the pattern with (*UCP). +
++The \X escape matches any number of Unicode characters that form an "extended +grapheme cluster", and treats the sequence as an atomic group +(see below). +Up to and including release 8.31, PCRE matched an earlier, simpler definition +that was 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. None of them have codepoints less than 256, so in -non-UTF-8 mode \X matches any one character. +That is, it matched a character without the "mark" property, followed by zero +or more characters with the "mark" property. Characters with the "mark" +property are typically non-spacing accents that affect the preceding character.
-Note that recent versions of Perl have changed \X to match what Unicode calls -an "extended grapheme cluster", which has a more complicated definition. +This simple definition was extended in Unicode to include more complicated +kinds of composite character by giving each character a grapheme breaking +property, and creating rules that use these properties to define the boundaries +of extended grapheme clusters. In releases of PCRE later than 8.31, \X matches +one of these clusters.
-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 by default, though you can make them do so by setting the -PCRE_UCP option for pcre_compile() or by starting the pattern with -(*UCP). +\X always matches at least one character. Then it decides whether to add +additional characters according to the following rules for ending a cluster: +
++1. End at the end of the subject string. +
++2. Do not end between CR and LF; otherwise end after any control character. +
++3. Do not break Hangul (a Korean script) syllable sequences. Hangul characters +are of five types: L, V, T, LV, and LVT. An L character may be followed by an +L, V, LV, or LVT character; an LV or V character may be followed by a V or T +character; an LVT or T character may be follwed only by a T character. +
++4. Do not end before extending characters or spacing marks. Characters with +the "mark" property always have the "extend" grapheme breaking property. +
++5. Do not end after prepend characters. +
++6. Otherwise, end the cluster.
-As well as the standard Unicode properties described in the previous -section, PCRE supports four more that make it possible to convert traditional -escape sequences such as \w and \s and POSIX character classes to use Unicode -properties. PCRE uses these non-standard, non-Perl properties internally when -PCRE_UCP is set. They are: +As well as the standard Unicode properties described above, PCRE supports four +more that make it possible to convert traditional escape sequences such as \w +and \s to use Unicode properties. PCRE uses these non-standard, non-Perl +properties internally when PCRE_UCP is set. However, they may also be used +explicitly. These properties are:
Xan Any alphanumeric character Xps Any POSIX space character @@ -804,10 +962,21 @@ PCRE_UCP is set. They are: Xwd Any Perl "word" characterXan matches characters that have either the L (letter) or the N (number) -property. Xps matches the characters tab, linefeed, vertical tab, formfeed, or +property. Xps matches the characters tab, linefeed, vertical tab, form feed, or carriage return, and any other character that has the Z (separator) property. -Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the -same characters as Xan, plus underscore. +Xsp is the same as Xps; it used to exclude vertical tab, for Perl +compatibility, but Perl changed, and so PCRE followed at release 8.34. Xwd +matches the same characters as Xan, plus underscore. + +
+There is another non-standard property, Xuc, which matches any character that +can be represented by a Universal Character Name in C++ and other programming +languages. These are the characters $, @, ` (grave accent), and all characters +with Unicode code points greater than or equal to U+00A0, except for the +surrogates U+D800 to U+DFFF. Note that most base (ASCII) characters are +excluded. (Universal Character Names are of the form \uHHHH or \UHHHHHHHH +where H is a hexadecimal digit. Note that the Xuc property does not match these +sequences but the characters that they represent.)
+The circumflex and dollar metacharacters are zero-width assertions. That is, +they test for a particular condition being true without consuming any +characters from the subject string. +
+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 argument of +character is an assertion that is true only if the current matching point is at +the start of the subject string. If the startoffset argument of pcre_exec() is non-zero, circumflex can never match if the PCRE_MULTILINE option is unset. Inside a character class, circumflex has an entirely different meaning @@ -924,12 +1098,12 @@ constrained to match only at the start of the subject, 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 -at the end of the string (by default). Dollar need not be the last character of -the pattern if a number of alternatives are involved, but it should be the last -item in any branch in which it appears. Dollar has no special meaning in a -character class. +The dollar character is an assertion that is true only if the current matching +point is at the end of the subject string, or immediately before a newline at +the end of the string (by default). Note, however, that it does not actually +match the newline. 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 @@ -958,11 +1132,11 @@ Note that the sequences \A, \Z, and \z can be used to end of the subject in both modes, and if all branches of a pattern start with \A it is always anchored, whether or not PCRE_MULTILINE is set.
-Outside a character class, a dot in the pattern matches any one character in the subject string except (by default) a character that signifies the end of a -line. In UTF-8 mode, the matched character may be more than one byte long. +line.
When a line ending is defined as a single character, dot never matches that @@ -989,29 +1163,32 @@ the PCRE_DOTALL option. In other words, it matches any that signifies the end of a line. Perl also uses \N to match characters by name; PCRE does not support this.
--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 line-ending -characters. The feature is provided in Perl in order to match individual bytes -in UTF-8 mode, but it is unclear how it can usefully be used. Because \C -breaks up characters into individual bytes, matching one byte with \C in UTF-8 -mode means that the rest of the string may start with a malformed UTF-8 -character. This has undefined results, because PCRE assumes that it is dealing -with valid UTF-8 strings (and by default it checks this at the start of -processing unless the PCRE_NO_UTF8_CHECK option is used). +Outside a character class, the escape sequence \C matches any one data unit, +whether or not a UTF mode is set. In the 8-bit library, one data unit is one +byte; in the 16-bit library it is a 16-bit unit; in the 32-bit library it is +a 32-bit unit. Unlike a dot, \C always +matches line-ending characters. The feature is provided in Perl in order to +match individual bytes in UTF-8 mode, but it is unclear how it can usefully be +used. Because \C breaks up characters into individual data units, matching one +unit with \C in a UTF mode means that the rest of the string may start with a +malformed UTF character. This has undefined results, because PCRE assumes that +it is dealing with valid UTF strings (and by default it checks this at the +start of processing unless the PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK or +PCRE_NO_UTF32_CHECK option is used).
PCRE does not allow \C to appear in lookbehind assertions (described below) -in UTF-8 mode, because this would make it impossible to calculate the length of +in a UTF mode, because this would make it impossible to calculate the length of the lookbehind.
-In general, the \C escape sequence is best avoided in UTF-8 mode. However, one -way of using it that avoids the problem of malformed UTF-8 characters is to -use a lookahead to check the length of the next character, as in this pattern -(ignore white space and line breaks): +In general, the \C escape sequence is best avoided. However, one +way of using it that avoids the problem of malformed UTF characters is to use a +lookahead to check the length of the next character, as in this pattern, which +could be used with a UTF-8 string (ignore white space and line breaks):
(?| (?=[\x00-\x7f])(\C) |
(?=[\x80-\x{7ff}])(\C)(\C) |
@@ -1026,7 +1203,7 @@ character for values whose encoding uses 1, 2, 3, or 4
character's individual bytes are then captured by the appropriate number of
groups.
-
SQUARE BRACKETS AND CHARACTER CLASSES
+
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 special by default.
@@ -1036,12 +1213,12 @@ a member of the class, it should be the first data cha
(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 be more than one byte long. 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
+A character class matches a single character in the subject. In a UTF mode, the
+character may be more than one data unit long. 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.
@@ -1054,20 +1231,21 @@ string, and therefore it fails if the current pointer
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.
+In UTF-8 (UTF-16, UTF-32) mode, characters with values greater than 255 (0xffff)
+can be included in a class as a literal string of data units, 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. In UTF-8 mode, PCRE always understands the concept of
+caseful version would. In a UTF mode, PCRE always understands the concept of
case for characters whose values are less than 128, so caseless matching is
always possible. For characters with higher values, the concept of case is
supported if PCRE is compiled with Unicode property support, but not otherwise.
-If you want to use caseless matching in UTF8-mode for characters 128 and above,
-you must ensure that PCRE is compiled with Unicode property support as well as
-with UTF-8 support.
+If you want to use caseless matching in a UTF mode for characters 128 and
+above, you must ensure that PCRE is compiled with Unicode property support as
+well as with UTF support.
Characters that might indicate line breaks are never treated in any special way
@@ -1080,7 +1258,9 @@ The minus (hyphen) character can be used to specify 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.
+indicating a range, typically as the first or last character in the class, or
+immediately after a range. For example, [b-d-z] matches letters in the range b
+to d, a hyphen character, or z.
It is not possible to have the literal character "]" as the end character of a
@@ -1092,17 +1272,22 @@ followed by two other characters. The octal or hexadec
"]" can also be used to end a range.
+An error is generated if a POSIX character class (see below) or an escape
+sequence other than one that defines a single character appears at a point
+where a range ending character is expected. For example, [z-\xff] is valid,
+but [A-\d] and [A-[:digit:]] are not.
+
+
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}].
+used for characters specified numerically, for example [\000-\037]. Ranges
+can include any characters that are valid for the current mode.
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
+[][\\^_`wxyzabc], matched caselessly, and in a non-UTF mode, if character
tables for a French 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 in both cases. In UTF modes, PCRE supports the concept of case for
characters with values greater than 128 only when it is compiled with Unicode
property support.
@@ -1110,7 +1295,7 @@ property support.
The character escape sequences \d, \D, \h, \H, \p, \P, \s, \S, \v,
\V, \w, and \W may appear in a character class, and add the characters that
they match to the class. For example, [\dABCDEF] matches any hexadecimal
-digit. In UTF-8 mode, the PCRE_UCP option affects the meanings of \d, \s, \w
+digit. In UTF modes, the PCRE_UCP option affects the meanings of \d, \s, \w
and their upper case partners, just as it does when they appear outside a
character class, as described in the section entitled
"Generic character types"
@@ -1132,11 +1317,11 @@ something AND NOT ...".
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.
+introducing a POSIX class name, or for a special compatibility feature - see
+the next two sections), and the terminating closing square bracket. However,
+escaping other non-alphanumeric characters does no harm.
-
POSIX CHARACTER CLASSES
+
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
@@ -1157,15 +1342,17 @@ are:
lower lower case letters
print printing characters, including space
punct printing characters, excluding letters and digits and space
- space white space (not quite the same as \s)
+ space white space (the same as \s from PCRE 8.34)
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 default "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13),
+and space (32). If locale-specific matching is taking place, the list of space
+characters may be different; there may be fewer or more of them. "Space" used
+to be different to \s, which did not include VT, for Perl compatibility.
+However, Perl changed at release 5.18, and PCRE followed at release 8.34.
+"Space" and \s now match the same set of characters.
The name "word" is a Perl extension, and "blank" is a GNU extension from Perl @@ -1179,11 +1366,11 @@ syntax [.ch.] and [=ch=] where "ch" is a "collating el supported, and an error is given if they are encountered.
-By default, in UTF-8 mode, characters with values greater than 128 do not match -any of the POSIX character classes. However, if the PCRE_UCP option is passed -to pcre_compile(), some of the classes are changed so that Unicode -character properties are used. This is achieved by replacing the POSIX classes -by other sequences, as follows: +By default, characters with values greater than 128 do not match any of the +POSIX character classes. However, if the PCRE_UCP option is passed to +pcre_compile(), some of the classes are changed so that Unicode character +properties are used. This is achieved by replacing certain POSIX classes by +other sequences, as follows:
[:alnum:] becomes \p{Xan}
[:alpha:] becomes \p{L}
@@ -1194,12 +1381,57 @@ by other sequences, as follows:
[:upper:] becomes \p{Lu}
[:word:] becomes \p{Xwd}
-Negated versions, such as [:^alpha:] use \P instead of \p. The other POSIX
-classes are unchanged, and match only characters with code points less than
-128.
+Negated versions, such as [:^alpha:] use \P instead of \p. Three other POSIX
+classes are handled specially in UCP mode:
-+[:graph:] +This matches characters that have glyphs that mark the page when printed. In +Unicode property terms, it matches all characters with the L, M, N, P, S, or Cf +properties, except for: +
+ U+061C Arabic Letter Mark + U+180E Mongolian Vowel Separator + U+2066 - U+2069 Various "isolate"s + ++ +
+[:print:] +This matches the same characters as [:graph:] plus space characters that are +not controls, that is, characters with the Zs property. +
++[:punct:] +This matches all characters that have the Unicode P (punctuation) property, +plus those characters whose code points are less than 128 that have the S +(Symbol) property. +
++The other POSIX classes are unchanged, and match only characters with code +points less than 128. +
++In the POSIX.2 compliant library that was included in 4.4BSD Unix, the ugly +syntax [[:<:]] and [[:>:]] is used for matching "start of word" and "end of +word". PCRE treats these items as follows: +
+ [[:<:]] is converted to \b(?=\w) + [[:>:]] is converted to \b(?<=\w) ++Only these exact character sequences are recognized. A sequence such as +[a[:<:]b] provokes error for an unrecognized POSIX class name. This support is +not compatible with Perl. It is provided to help migrations from other +environments, and is best not used in any new patterns. Note that \b matches +at the start and the end of a word (see +"Simple assertions" +above), and in a Perl-style pattern the preceding or following character +normally shows which is wanted, without the need for the assertions that are +used above in order to give exactly the POSIX behaviour. + +
Vertical bar characters are used to separate alternative patterns. For example, the pattern
@@ -1213,7 +1445,7 @@ that succeeds is used. If the alternatives are within "succeeds" means matching the rest of the main pattern as well as the alternative in the subpattern. -sequences of non-digits cannot be broken, and failure happens quickly. -
INTERNAL OPTION SETTING
+
INTERNAL OPTION SETTING
The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and PCRE_EXTENDED options (which are Perl-compatible) can be changed from within @@ -1264,16 +1496,19 @@ behaviour otherwise.
Note: There are other PCRE-specific options that can be set by the -application when the compile or match functions are called. In some cases the -pattern can contain special leading sequences such as (*CRLF) to override what -the application has set or what has been defaulted. Details are given in the -section entitled +application when the compiling or matching functions are called. In some cases +the pattern can contain special leading sequences such as (*CRLF) to override +what the application has set or what has been defaulted. Details are given in +the section entitled "Newline sequences" -above. There are also the (*UTF8) and (*UCP) leading sequences that can be used -to set UTF-8 and Unicode property modes; they are equivalent to setting the -PCRE_UTF8 and the PCRE_UCP options, respectively. +above. There are also the (*UTF8), (*UTF16),(*UTF32), and (*UCP) leading +sequences that can be used to set UTF and Unicode property modes; they are +equivalent to setting the PCRE_UTF8, PCRE_UTF16, PCRE_UTF32 and the PCRE_UCP +options, respectively. The (*UTF) sequence is a generic version that can be +used with any of the libraries. However, the application can set the +PCRE_NEVER_UTF option, which locks out the use of the (*UTF) sequences.
-
SUBPATTERNS
+
SUBPATTERNS
Subpatterns are delimited by parentheses (round brackets), which can be nested. Turning part of a pattern into a subpattern does two things: @@ -1289,10 +1524,14 @@ match "cataract", "erpillar" or an 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 pattern +subpattern is passed back to the caller via the ovector argument of the +matching function. (This applies only to the traditional matching functions; +the DFA matching functions do not support capturing.) ++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 pattern
the ((red|white) (king|queen))@@ -1325,7 +1564,7 @@ from left to right, and options are not reset until th is reached, an option setting in one branch does affect subsequent branches, so the above patterns match "SUNDAY" as well as "Saturday". -
DUPLICATE SUBPATTERN NUMBERS
+
DUPLICATE SUBPATTERN NUMBERS
Perl 5.10 introduced a feature whereby each alternative in a subpattern uses the same numbers for its capturing parentheses. Such a subpattern starts with @@ -1369,7 +1608,7 @@ true if any of the subpatterns of that number have mat An alternative approach to using this "branch reset" feature is to use duplicate named subpatterns, as described in the next section.
-
NAMED SUBPATTERNS
+
NAMED SUBPATTERNS
Identifying capturing parentheses by number is simple, but it can be very hard to keep track of the numbers in complicated regular expressions. Furthermore, @@ -1391,11 +1630,12 @@ and can be made by name as well as by number.
-Names consist of up to 32 alphanumeric characters and underscores. Named -capturing parentheses are still allocated numbers as well as names, exactly as -if the names were not present. The PCRE API provides function calls for -extracting the name-to-number translation table from a compiled pattern. There -is also a convenience function for extracting a captured substring by name. +Names consist of up to 32 alphanumeric characters and underscores, but must +start with a non-digit. Named capturing parentheses are still allocated numbers +as well as names, exactly as if the names were not present. The PCRE API +provides function calls for extracting the name-to-number translation table +from a compiled pattern. There is also a convenience function for extracting a +captured substring by name.
By default, a name must be unique within a pattern, but it is possible to relax @@ -1424,9 +1664,23 @@ matched. This saves searching to find which numbered s
If you make a back reference to a non-unique named subpattern from elsewhere in -the pattern, the one that corresponds to the first occurrence of the name is -used. In the absence of duplicate numbers (see the previous section) this is -the one with the lowest number. If you use a named reference in a condition +the pattern, the subpatterns to which the name refers are checked in the order +in which they appear in the overall pattern. The first one that is set is used +for the reference. For example, this pattern matches both "foofoo" and +"barbar" but not "foobar" or "barfoo": +
+ (?:(?<n>foo)|(?<n>bar))\k<n> + ++ ++If you make a subroutine call to a non-unique named subpattern, the one that +corresponds to the first occurrence of the name is used. In the absence of +duplicate numbers (see the previous section) this is the one with the lowest +number. +
++If you use a named reference in a condition test (see the section about conditions below), either to check whether a subpattern has matched, or to check for @@ -1441,10 +1695,11 @@ documentation. Warning: You cannot use different names to distinguish between two subpatterns with the same number because PCRE uses only the numbers when matching. For this reason, an error is given at compile time if different names -are given to subpatterns with the same number. However, you can give the same -name to subpatterns with the same number, even when PCRE_DUPNAMES is not set. +are given to subpatterns with the same number. However, you can always give the +same name to subpatterns with the same number, even when PCRE_DUPNAMES is not +set.
-
REPETITION
+
REPETITION
Repetition is specified by quantifiers, which can follow any of the following items: @@ -1452,7 +1707,7 @@ items: a literal data character the dot metacharacter the \C escape sequence - the \X escape sequence (in UTF-8 mode with Unicode properties) + the \X escape sequence the \R escape sequence an escape such as \d or \pL that matches a single character a character class @@ -1484,11 +1739,11 @@ quantifier, is taken as a literal character. For examp 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. 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). +In UTF modes, quantifiers apply to characters rather than to individual data +units. Thus, for example, \x{100}{2} matches two characters, each of +which is represented by a two-byte sequence in a UTF-8 string. Similarly, +\X{3} matches three Unicode extended grapheme clusters, each of which may be +several data units long (and they may be of different lengths).
The quantifier {0} is permitted, causing the expression to behave as if the @@ -1577,7 +1832,7 @@ worth setting PCRE_DOTALL in order to obtain this opti alternatively using ^ to indicate anchoring explicitly.
-However, there is one situation where the optimization cannot be used. When .* +However, there are some cases where the optimization cannot be used. When .* is inside capturing parentheses that are the subject of a back reference elsewhere in the pattern, a match at the start may fail where a later one succeeds. Consider, for example: @@ -1588,6 +1843,16 @@ If the subject is "xyz123abc123" the match point is th this reason, such a pattern is not implicitly anchored.
+Another case where implicit anchoring is not applied is when the leading .* is +inside an atomic group. Once again, a match at the start may fail where a later +one succeeds. Consider this pattern: +
+ (?>.*?a)b ++It matches "ab" in the subject "aab". The use of the backtracking control verbs +(*PRUNE) and (*SKIP) also disable this optimization. + +When a capturing subpattern is repeated, the value captured is the substring that matched the final iteration. For example, after
@@ -1602,7 +1867,7 @@ example, aftermatches "aba" the value of the second captured substring is "b". -
ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS
+
ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS
With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy") repetition, failure of what follows normally causes the repeated item to be @@ -1706,7 +1971,7 @@ an atomic group, like this:
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 @@ -1805,7 +2070,7 @@ Because there may be many capturing parentheses in a p following a 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, the \g{ syntax or an empty comment (see +white space. Otherwise, the \g{ syntax or an empty comment (see "Comments" below) can be used.
@@ -1834,7 +2099,7 @@ as an Once the whole group has been matched, a subsequent matching failure cannot cause backtracking into the middle of the group. -An assertion is a test on the characters following or preceding the current matching point that does not actually consume any characters. The simple @@ -1851,8 +2116,8 @@ except that it does not cause the current matching pos Assertion subpatterns are not capturing subpatterns. If such an 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. +capturing is carried out only for positive assertions. (Perl sometimes, but not +always, does do capturing in negative assertions.)
For compatibility with Perl, assertion subpatterns may be repeated; though @@ -1950,10 +2215,11 @@ match. If there are insufficient characters before the assertion fails.
-In UTF-8 mode, PCRE does not allow the \C escape (which matches a single byte, -even in UTF-8 mode) to appear in lookbehind assertions, because it makes it -impossible to calculate the length of the lookbehind. The \X and \R escapes, -which can match different numbers of bytes, are also not permitted. +In a UTF mode, PCRE does not allow the \C escape (which matches a single data +unit even in a UTF mode) to appear in lookbehind assertions, because it makes +it impossible to calculate the length of the lookbehind. The \X and \R +escapes, which can match different numbers of data units, are also not +permitted.
"Subroutine" @@ -2023,7 +2289,7 @@ preceded by "foo", while is another pattern that matches "foo" preceded by three digits and any three characters that are not "999".
-It is possible to cause the matching process to obey a subpattern conditionally or to choose between two alternative subpatterns, depending on @@ -2097,12 +2363,7 @@ Checking for a used subpattern by name
Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used subpattern by name. For compatibility with earlier versions of PCRE, which had -this facility before Perl, the syntax (?(name)...) is also recognized. However, -there is a possible ambiguity with this syntax, because subpattern names may -consist entirely of digits. PCRE looks first for a named subpattern; if it -cannot find one and the name consists entirely of digits, PCRE looks for a -subpattern of that number, which must be greater than zero. Using subpattern -names that consist entirely of digits is not recommended. +this facility before Perl, the syntax (?(name)...) is also recognized.
Rewriting the above example to use a named subpattern gives this: @@ -2146,7 +2407,7 @@ point in the pattern; the idea of DEFINE is that it ca subroutines that can be referenced from elsewhere. (The use of subroutines is described below.) For example, a pattern to match an IPv4 address such as -"192.168.23.245" could be written like this (ignore whitespace and line +"192.168.23.245" could be written like this (ignore white space and line breaks):
(?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) ) @@ -2178,7 +2439,7 @@ subject is matched against the first alternative; othe 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. -This verb causes the match to fail at the current starting position in the -subject if the rest of the pattern does not match. If the pattern is -unanchored, the normal "bumpalong" advance to the next starting character then -happens. Backtracking can occur as usual to the left of (*PRUNE), before it is -reached, or when matching to the right of (*PRUNE), but if there is no match to -the right, backtracking cannot cross (*PRUNE). In simple cases, the use of -(*PRUNE) is just an alternative to an atomic group or possessive quantifier, -but there are some uses of (*PRUNE) that cannot be expressed in any other way. -The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE). In an -anchored pattern (*PRUNE) has the same effect as (*COMMIT). +subject if there is a later matching failure that causes backtracking to reach +it. If the pattern is unanchored, the normal "bumpalong" advance to the next +starting character then happens. Backtracking can occur as usual to the left of +(*PRUNE), before it is reached, or when matching to the right of (*PRUNE), but +if there is no match to the right, backtracking cannot cross (*PRUNE). In +simple cases, the use of (*PRUNE) is just an alternative to an atomic group or +possessive quantifier, but there are some uses of (*PRUNE) that cannot be +expressed in any other way. In an anchored pattern (*PRUNE) has the same effect +as (*COMMIT). + +
COMMENTS
+
COMMENTS
There are two ways of including comments in patterns that are processed by PCRE. In both cases, the start of the comment must not be in a character class, @@ -2192,7 +2453,7 @@ closing parenthesis. Nested parentheses are not permit option is set, an unescaped # character also introduces a comment, which in this case continues to immediately after the next newline character or character sequence in the pattern. Which characters are interpreted as newlines -is controlled by the options passed to pcre_compile() or by a special +is controlled by the options passed to a compiling function or by a special sequence at the start of the pattern, as described in the section entitled "Newline conventions" above. Note that the end of this type of comment is a literal newline sequence @@ -2207,7 +2468,7 @@ a newline in the pattern. The sequence \n is still lit it does not terminate the comment. Only an actual character with the code value 0x0a (the default newline) does so.
-
RECURSIVE PATTERNS
+
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 @@ -2422,7 +2683,7 @@ now match "b" and so the whole match succeeds. In Perl match because inside the recursive call \1 cannot access the externally set value.
-
SUBPATTERNS AS SUBROUTINES
+
SUBPATTERNS AS SUBROUTINES
If the syntax for a recursive subpattern call (either by number or by name) is used outside the parentheses to which it refers, it operates like a @@ -2463,7 +2724,7 @@ different calls. For example, consider this pattern: It matches "abcabc". It does not match "abcABC" because the change of processing option does not affect the called subpattern.
-
ONIGURUMA SUBROUTINE SYNTAX
+
ONIGURUMA SUBROUTINE SYNTAX
For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or a number enclosed either in angle brackets or single quotes, is an alternative @@ -2481,7 +2742,7 @@ plus or a minus sign it is taken as a relative referen Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are not synonymous. The former is a back reference; the latter is a subroutine call.
-
CALLOUTS
+
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 @@ -2491,7 +2752,8 @@ same pair of parentheses when there is a repetition.
PCRE provides a similar feature, but of course it cannot obey arbitrary Perl code. The feature is called "callout". The caller of PCRE provides an external -function by putting its entry point in the global variable pcre_callout. +function by putting its entry point in the global variable pcre_callout +(8-bit library) or pcre[16|32]_callout (16-bit or 32-bit library). By default, this variable contains NULL, which disables all calling out.
@@ -2502,64 +2764,85 @@ 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 +If the PCRE_AUTO_CALLOUT flag is passed to a compiling function, callouts are automatically installed before each item in the pattern. They are all numbered -255. +255. If there is a conditional group in the pattern whose condition is an +assertion, an additional callout is inserted just before the condition. An +explicit callout may also be set at this position, as in this example: ++ (?(?C9)(?=a)abc|def) ++Note that this applies only to assertion conditions, not to other types of +condition.-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 altogether. A complete -description of the interface to the callout function is given in the +During matching, when PCRE reaches a callout point, 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 +the matching function. The callout function may cause matching to proceed, to +backtrack, or to fail altogether. +
++By default, PCRE implements a number of optimizations at compile time and +matching time, and one side-effect is that sometimes callouts are skipped. If +you need all possible callouts to happen, you need to set options that disable +the relevant optimizations. More details, and a complete description of the +interface to the callout function, are given in the pcrecallout documentation.
-
BACKTRACKING CONTROL
+
BACKTRACKING CONTROL
Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which -are described in the Perl documentation as "experimental and subject to change -or removal in a future version of Perl". It goes on to say: "Their usage in -production code should be noted to avoid problems during upgrades." The same +are still described in the Perl documentation as "experimental and subject to +change or removal in a future version of Perl". It goes on to say: "Their usage +in production code should be noted to avoid problems during upgrades." The same remarks apply to the PCRE features described in this section.
-Since these verbs are specifically related to backtracking, most of them can be -used only when the pattern is to be matched using pcre_exec(), which uses -a backtracking algorithm. With the exception of (*FAIL), which behaves like a -failing negative assertion, they cause an error if encountered by -pcre_dfa_exec(). +The new verbs make use of what was previously invalid syntax: an opening +parenthesis followed by an asterisk. They are generally of the form +(*VERB) or (*VERB:NAME). Some may take either form, possibly behaving +differently depending on whether or not a name is present. A name is any +sequence of characters that does not include a closing parenthesis. The maximum +length of name is 255 in the 8-bit library and 65535 in the 16-bit and 32-bit +libraries. If the name is empty, that is, if the closing parenthesis +immediately follows the colon, the effect is as if the colon were not there. +Any number of these verbs may occur in a pattern.
-If any of these verbs are used in an assertion or in a subpattern that is -called as a subroutine (whether or not recursively), their effect is confined -to that subpattern; it does not extend to the surrounding pattern, with one -exception: the name from a *(MARK), (*PRUNE), or (*THEN) that is encountered in -a successful positive assertion is passed back when a match succeeds -(compare capturing parentheses in assertions). Note that such subpatterns are -processed as anchored at the point where they are tested. Note also that Perl's -treatment of subroutines is different in some cases. +Since these verbs are specifically related to backtracking, most of them can be +used only when the pattern is to be matched using one of the traditional +matching functions, because these use a backtracking algorithm. With the +exception of (*FAIL), which behaves like a failing negative assertion, the +backtracking control verbs cause an error if encountered by a DFA matching +function.
-The new verbs make use of what was previously invalid syntax: an opening -parenthesis followed by an asterisk. They are generally of the form -(*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour, -depending on whether or not an argument is present. A name is any sequence of -characters that does not include a closing parenthesis. If the name is empty, -that is, if the closing parenthesis immediately follows the colon, the effect -is as if the colon were not there. Any number of these verbs may occur in a -pattern. -
+The behaviour of these verbs in +repeated groups, +assertions, +and in +subpatterns called as subroutines +(whether or not recursively) is documented below. + +
+Optimizations that affect backtracking verbs +
PCRE contains some optimizations that are used to speed up matching by running some checks at the start of each match attempt. For example, it may know the minimum length of matching subject, or that a particular character must be -present. When one of these optimizations suppresses the running of a match, any +present. When one of these optimizations bypasses the running of a match, any included backtracking verbs will not, of course, be processed. You can suppress the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option when calling pcre_compile() or pcre_exec(), or by starting the -pattern with (*NO_START_OPT). +pattern with (*NO_START_OPT). There is more discussion of this option in the +section entitled +"Option bits for pcre_exec()" +in the +pcreapi +documentation.
Experiments with Perl suggest that it too has similar optimizations, sometimes @@ -2577,8 +2860,12 @@ followed by a name. This verb causes the match to end successfully, skipping the remainder of the pattern. However, when it is inside a subpattern that is called as a subroutine, only that subpattern is ended successfully. Matching then continues -at the outer level. If (*ACCEPT) is inside capturing parentheses, the data so -far is captured. For example: +at the outer level. If (*ACCEPT) in triggered in a positive assertion, the +assertion succeeds; in a negative assertion, the assertion fails. +
++If (*ACCEPT) is inside capturing parentheses, the data so far is captured. For +example:
A((?:A|B(*ACCEPT)|C)D)@@ -2612,10 +2899,10 @@ A name is always required with this verb. There may be (*MARK) as you like in a pattern, and their names do not have to be unique.-When a match succeeds, the name of the last-encountered (*MARK) on the matching -path is passed back to the caller via the pcre_extra data structure, as -described in the -section on pcre_extra +When a match succeeds, the name of the last-encountered (*MARK:NAME), +(*PRUNE:NAME), or (*THEN:NAME) on the matching path is passed back to the +caller as described in the section entitled +"Extra data for pcre_exec()" in the pcreapi documentation. Here is an example of pcretest output, where the /K @@ -2635,23 +2922,29 @@ of obtaining this information than putting each altern capturing parentheses.
-If (*MARK) is encountered in a positive assertion, its name is recorded and -passed back if it is the last-encountered. This does not happen for negative -assertions. +If a verb with a name is encountered in a positive assertion that is true, the +name is recorded and passed back if it is the last-encountered. This does not +happen for negative assertions or failing positive assertions.
-After a partial match or a failed match, the name of the last encountered -(*MARK) in the entire match process is returned. For example: +After a partial match or a failed match, the last encountered name in the +entire match process is returned. For example:
re> /X(*MARK:A)Y|X(*MARK:B)Z/K data> XP No match, mark = BNote that in this unanchored example the mark is retained from the match -attempt that started at the letter "X". Subsequent match attempts starting at -"P" and then with an empty string do not get as far as the (*MARK) item, but -nevertheless do not reset it. +attempt that started at the letter "X" in the subject. Subsequent match +attempts starting at "P" and then with an empty string do not get as far as the +(*MARK) item, but nevertheless do not reset it. ++If you are interested in (*MARK) values after failed matches, you should +probably set the PCRE_NO_START_OPTIMIZE option +(see above) +to ensure that the match is always attempted. +
Verbs that act after backtracking
@@ -2659,23 +2952,28 @@ Verbs that act after backtracking The following verbs do nothing when they are encountered. Matching continues with what follows, but if there is no subsequent match, causing a backtrack to the verb, a failure is forced. That is, backtracking cannot pass to the left of -the verb. However, when one of these verbs appears inside an atomic group, its -effect is confined to that group, because once the group has been matched, -there is never any backtracking into it. In this situation, backtracking can -"jump back" to the left of the entire atomic group. (Remember also, as stated -above, that this localization also applies in subroutine calls and assertions.) +the verb. However, when one of these verbs appears inside an atomic group or an +assertion that is true, its effect is confined to that group, because once the +group has been matched, there is never any backtracking into it. In this +situation, backtracking can "jump back" to the left of the entire atomic group +or assertion. (Remember also, as stated above, that this localization also +applies in subroutine calls.)These verbs differ in exactly what kind of failure occurs when backtracking -reaches them. +reaches them. The behaviour described below is what happens when the verb is +not in a subroutine or an assertion. Subsequent sections cover these special +cases.
(*COMMIT)This verb, which may not be followed by a name, causes the whole match to fail -outright if the rest of the pattern does not match. Even if the pattern is -unanchored, no further attempts to find a match by advancing the starting point -take place. Once (*COMMIT) has been passed, pcre_exec() is committed to -finding a match at the current starting point, or not at all. For example: +outright if there is a later matching failure that causes backtracking to reach +it. Even if the pattern is unanchored, no further attempts to find a match by +advancing the starting point take place. If (*COMMIT) is the only backtracking +verb that is encountered, once it has been passed pcre_exec() is +committed to finding a match at the current starting point, or not at all. For +example:a+(*COMMIT)b@@ -2685,6 +2983,11 @@ recently passed (*MARK) in the path is passed back whe match failure.+If there is more than one backtracking verb in a pattern, a different one that +follows (*COMMIT) may be triggered first, so merely passing (*COMMIT) during a +match does not always guarantee that a match must be at this starting point. +
+Note that (*COMMIT) at the start of a pattern is not the same as an anchor, unless PCRE's start-of-match optimizations are turned off, as shown in this pcretest example: @@ -2704,15 +3007,20 @@ starting points. (*PRUNE) or (*PRUNE:NAME)
+The behaviour of (*PRUNE:NAME) is the not the same as (*MARK:NAME)(*PRUNE). +It is like (*MARK:NAME) in that the name is remembered for passing back to the +caller. However, (*SKIP:NAME) searches only for names set with (*MARK).
(*SKIP)@@ -2733,31 +3041,41 @@ instead of skipping on to "c".
(*SKIP:NAME)-When (*SKIP) has an associated name, its behaviour is modified. If the -following pattern fails to match, the previous path through the pattern is -searched for the most recent (*MARK) that has the same name. If one is found, -the "bumpalong" advance is to the subject position that corresponds to that -(*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a -matching name is found, the (*SKIP) is ignored. +When (*SKIP) has an associated name, its behaviour is modified. When it is +triggered, the previous path through the pattern is searched for the most +recent (*MARK) that has the same name. If one is found, the "bumpalong" advance +is to the subject position that corresponds to that (*MARK) instead of to where +(*SKIP) was encountered. If no (*MARK) with a matching name is found, the +(*SKIP) is ignored. + +
+Note that (*SKIP:NAME) searches only for names set by (*MARK:NAME). It ignores +names that are set by (*PRUNE:NAME) or (*THEN:NAME).
(*THEN) or (*THEN:NAME)-This verb causes a skip to the next innermost alternative if the rest of the -pattern does not match. That is, it cancels pending backtracking, but only -within the current alternative. Its name comes from the observation that it can -be used for a pattern-based if-then-else block: +This verb causes a skip to the next innermost alternative when backtracking +reaches it. That is, it cancels any further backtracking within the current +alternative. Its name comes from the observation that it can be used for a +pattern-based if-then-else block:
( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...If the COND1 pattern matches, FOO is tried (and possibly further items after the end of the group if FOO succeeds); on failure, the matcher skips to the -second alternative and tries COND2, without backtracking into COND1. The -behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN). -If (*THEN) is not inside an alternation, it acts like (*PRUNE). +second alternative and tries COND2, without backtracking into COND1. If that +succeeds and BAR fails, COND3 is tried. If subsequently BAZ fails, there are no +more alternatives, so there is a backtrack to whatever came before the entire +group. If (*THEN) is not inside an alternation, it acts like (*PRUNE).
-Note that a subpattern that does not contain a | character is just a part of -the enclosing alternative; it is not a nested alternation with only one +The behaviour of (*THEN:NAME) is the not the same as (*MARK:NAME)(*THEN). +It is like (*MARK:NAME) in that the name is remembered for passing back to the +caller. However, (*SKIP:NAME) searches only for names set with (*MARK). +
++A subpattern that does not contain a | character is just a part of the +enclosing alternative; it is not a nested alternation with only one alternative. The effect of (*THEN) extends beyond such a subpattern to the enclosing alternative. Consider this pattern, where A, B, etc. are complex pattern fragments that do not contain any | characters at this level: @@ -2777,7 +3095,7 @@ because there are no more alternatives to try. In this backtrack into A.
-Note also that a conditional subpattern is not considered as having two +Note that a conditional subpattern is not considered as having two alternatives, because only one is ever used. In other words, the | character in a conditional subpattern has a different meaning. Ignoring white space, consider: @@ -2801,24 +3119,98 @@ unanchored pattern). (*SKIP) is similar, except that t than one character. (*COMMIT) is the strongest, causing the entire match to fail.
+-If more than one such verb is present in a pattern, the "strongest" one wins. -For example, consider this pattern, where A, B, etc. are complex pattern -fragments: +If more than one backtracking verb is present in a pattern, the one that is +backtracked onto first acts. For example, consider this pattern, where A, B, +etc. are complex pattern fragments:
- (A(*COMMIT)B(*THEN)C|D) + (A(*COMMIT)B(*THEN)C|ABD)-Once A has matched, PCRE is committed to this match, at the current starting -position. If subsequently B matches, but C does not, the normal (*THEN) action -of trying the next alternative (that is, D) does not happen because (*COMMIT) -overrides. +If A matches but B fails, the backtrack to (*COMMIT) causes the entire match to +fail. However, if A and B match, but C fails, the backtrack to (*THEN) causes +the next alternative (ABD) to be tried. This behaviour is consistent, but is +not always the same as Perl's. It means that if two or more backtracking verbs +appear in succession, all the the last of them has no effect. Consider this +example: +
+ ...(*COMMIT)(*PRUNE)... ++If there is a matching failure to the right, backtracking onto (*PRUNE) causes +it to be triggered, and its action is taken. There can never be a backtrack +onto (*COMMIT). + +
+PCRE differs from Perl in its handling of backtracking verbs in repeated +groups. For example, consider: +
+ /(a(*COMMIT)b)+ac/ ++If the subject is "abac", Perl matches, but PCRE fails because the (*COMMIT) in +the second repeat of the group acts. + +
+(*FAIL) in an assertion has its normal effect: it forces an immediate backtrack.
-+(*ACCEPT) in a positive assertion causes the assertion to succeed without any +further processing. In a negative assertion, (*ACCEPT) causes the assertion to +fail without any further processing. +
++The other backtracking verbs are not treated specially if they appear in a +positive assertion. In particular, (*THEN) skips to the next alternative in the +innermost enclosing group that has alternations, whether or not this is within +the assertion. +
++Negative assertions are, however, different, in order to ensure that changing a +positive assertion into a negative assertion changes its result. Backtracking +into (*COMMIT), (*SKIP), or (*PRUNE) causes a negative assertion to be true, +without considering any further alternative branches in the assertion. +Backtracking into (*THEN) causes it to skip to the next enclosing alternative +within the assertion (the normal behaviour), but if the assertion does not have +such an alternative, (*THEN) behaves like (*PRUNE). +
++These behaviours occur whether or not the subpattern is called recursively. +Perl's treatment of subroutines is different in some cases. +
++(*FAIL) in a subpattern called as a subroutine has its normal effect: it forces +an immediate backtrack. +
++(*ACCEPT) in a subpattern called as a subroutine causes the subroutine match to +succeed without any further processing. Matching then continues after the +subroutine call. +
++(*COMMIT), (*SKIP), and (*PRUNE) in a subpattern called as a subroutine cause +the subroutine match to fail. +
++(*THEN) skips to the next alternative in the innermost enclosing group within +the subpattern that has alternatives. If there is no such group within the +subpattern, (*THEN) causes the subroutine match to fail. +
+pcreapi(3), pcrecallout(3), pcrematching(3), -pcresyntax(3), pcre(3). +pcresyntax(3), pcre(3), pcre16(3), pcre32(3).
-
Philip Hazel
@@ -2827,11 +3219,11 @@ University Computing Service
Cambridge CB2 3QH, England.
-Last updated: 29 November 2011
+Last updated: 03 December 2013
-Copyright © 1997-2011 University of Cambridge.
+Copyright © 1997-2013 University of Cambridge.
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