@@ -60,27 +61,62 @@ 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 strings in the original library, and a -second library that supports 16-bit and UTF-16 character strings. To use these +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 or -PCRE_UTF16 option, or the pattern must start with one of these special -sequences: +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)+(*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. This feature is not Perl-compatible. How setting a UTF mode affects -pattern matching is mentioned in several places below. There is also a summary -of features in the +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) or (*UTF16) is: +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)@@ -89,25 +125,16 @@ such as \d and \w to use Unicode properties to determi 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. -
--The remainder of this document discusses the patterns that are supported by -PCRE when one its main matching functions, pcre_exec() (8-bit) or -pcre16_exec() (16-bit), is used. PCRE also has alternative matching -functions, pcre_dfa_exec() and pcre16_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. +PCRE_NO_START_OPTIMIZE option either at compile or matching time.
-PCRE supports five different conventions for indicating line breaks in strings: a single CR (carriage return) character, a single LF (linefeed) @@ -135,23 +162,52 @@ example, on a Unix system where LF is the default newl (*CR)a.b changes 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 by the caller of pcre_exec() for it to have +any effect. In other words, the pattern writer can lower the limit set by the +programmer, but not raise it. 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 @@ -207,7 +263,7 @@ 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 @@ -227,10 +283,10 @@ backslash. All other characters (in particular, those greater than 127) are treated as literals.
-If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the +If a pattern is compiled with the PCRE_EXTENDED option, white space 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. +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 @@ -264,7 +320,7 @@ 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) @@ -273,16 +329,24 @@ one of the following escape sequences than the binary \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 -all modes. (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.
+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. +
+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 character code is constrained as follows: @@ -291,9 +355,11 @@ between \x{ and }, but the character code is constrain 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 0x80000000 + 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). +"surrogate" codepoints), and 0xffef.
If characters other than hexadecimal digits appear between \x{ and }, or if @@ -307,6 +373,8 @@ as just described only when it is followed by two hexa 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. +Character codes specified by \u in JavaScript mode are constrained in the same +was as those specified by \x in non-JavaScript mode.
Characters whose value is less than 256 can be defined by either of the two @@ -339,7 +407,7 @@ subsequent digits stand for themselves. The value of t constrained in the same way as characters specified in hexadecimal. 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 @@ -406,12 +474,12 @@ Another use of backslash is for specifying generic chasequences of non-digits cannot be broken, and failure happens quickly. -\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@@ -473,7 +541,7 @@ release 5.10. In contrast to the other sequences, whic characters by default, these always match certain high-valued codepoints, 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 @@ -495,11 +563,11 @@ whether or not PCRE_UCP is set. The horizontal space cThe 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@@ -520,7 +588,7 @@ 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. @@ -551,10 +619,10 @@ change of newline convention; for example, a pattern c(*ANY)(*BSR_ANYCRLF)-They can also be combined with the (*UTF8), (*UTF16), 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.
Unicode character properties @@ -568,7 +636,7 @@ 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 clusterThe property names represented by xx above are limited to the Unicode script names, the general category properties, "Any", which matches any @@ -596,13 +664,16 @@ Armenian, Avestan, Balinese, Bamum, +Batak, Bengali, Bopomofo, +Brahmi, Braille, Buginese, Buhid, Canadian_Aboriginal, Carian, +Chakma, Cham, Cherokee, Common, @@ -645,7 +716,11 @@ Lisu, Lycian, Lydian, Malayalam, +Mandaic, Meetei_Mayek, +Meroitic_Cursive, +Meroitic_Hieroglyphs, +Miao, Mongolian, Myanmar, New_Tai_Lue, @@ -664,8 +739,10 @@ Rejang, Runic, Samaritan, Saurashtra, +Sharada, Shavian, Sinhala, +Sora_Sompeng, Sundanese, Syloti_Nagri, Syriac, @@ -674,6 +751,7 @@ Tagbanwa, Tai_Le, Tai_Tham, Tai_Viet, +Takri, Tamil, Telugu, Thaana, @@ -753,7 +831,8 @@ 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 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 and PCRE_NO_UTF16_CHECK in the +(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. @@ -769,42 +848,74 @@ 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). +
+
+Extended grapheme clusters +
++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 -8-bit 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 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.
PCRE's additional properties
-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 and POSIX character classes 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 @@ -812,10 +923,20 @@ 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. + ++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.)
Resetting the match start @@ -912,11 +1033,16 @@ If all the alternatives of a pattern begin with \G, th to the starting match position, and the "anchored" flag is set in the compiled regular expression. -
CIRCUMFLEX AND DOLLAR
+
CIRCUMFLEX AND DOLLAR
+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 @@ -932,12 +1058,12 @@ 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 @@ -966,7 +1092,7 @@ 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.
-
FULL STOP (PERIOD, DOT) AND \N
+
FULL STOP (PERIOD, DOT) AND \N
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 @@ -997,18 +1123,20 @@ 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.
-
MATCHING A SINGLE DATA UNIT
+
MATCHING A SINGLE DATA UNIT
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. Unlike a dot, \C always +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 option is used). +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 @@ -1035,7 +1163,7 @@ 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. @@ -1063,9 +1191,9 @@ string, and therefore it fails if the current pointer string.
-In UTF-8 (UTF-16) 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. +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 @@ -1145,7 +1273,7 @@ introducing a POSIX class name - see the next section) 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 @@ -1207,7 +1335,7 @@ Negated versions, such as [:^alpha:] use \P instead of classes are unchanged, and match only characters with code points less than 128.
-
VERTICAL BAR
+
VERTICAL BAR
Vertical bar characters are used to separate alternative patterns. For example, the pattern @@ -1222,7 +1350,7 @@ 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.
-
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 @@ -1278,11 +1406,14 @@ the pattern can contain special leading sequences such 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), (*UTF16), 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, 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: @@ -1338,7 +1469,7 @@ 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 @@ -1382,7 +1513,7 @@ 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, @@ -1457,7 +1588,7 @@ matching. For this reason, an error is given at compil 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.
-
REPETITION
+
REPETITION
Repetition is specified by quantifiers, which can follow any of the following items: @@ -1500,8 +1631,8 @@ quantifier, but a literal string of four characters. 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 sequences, each of which may be several -data units long (and they may be of different lengths). +\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 @@ -1590,7 +1721,7 @@ 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: @@ -1601,6 +1732,16 @@ 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
@@ -1615,7 +1756,7 @@ 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 @@ -1719,7 +1860,7 @@ 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 @@ -1818,7 +1959,7 @@ 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.
@@ -1847,7 +1988,7 @@ 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 @@ -1864,8 +2005,8 @@ 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 @@ -2037,7 +2178,7 @@ 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 @@ -2160,7 +2301,7 @@ 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) ) @@ -2192,7 +2333,7 @@ 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. -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: +
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, @@ -2221,7 +2362,7 @@ 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 @@ -2436,7 +2577,7 @@ 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 @@ -2477,7 +2618,7 @@ 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 @@ -2495,7 +2636,7 @@ 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 @@ -2506,8 +2647,8 @@ 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 -(8-bit library) or pcre16_callout (16-bit library). By default, this -variable contains NULL, which disables all calling out. +(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.
Within a regular expression, (?C) indicates the points at which the external @@ -2519,7 +2660,14 @@ For example, this pattern has two callout points:
+ (?(?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, the external function is @@ -2531,50 +2679,58 @@ the callout function is given in the pcrecallout documentation.
-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.
+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. +
+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, which use a backtracking algorithm. With the exception of -(*FAIL), which behaves like a failing negative assertion, they cause an error -if encountered by a DFA matching function. +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.
-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. -
+The behaviour of these verbs in +repeated groups, +assertions, +and in +subpatterns called as subroutines +(whether or not recursively) is documented below. + +-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. -
-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 @@ -2592,8 +2748,12 @@ 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)@@ -2627,8 +2787,9 @@ 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 as described in the section entitled +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 @@ -2649,23 +2810,29 @@ 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 = B
Note 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. +
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@@ -2699,6 +2871,11 @@ 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: @@ -2718,15 +2895,20 @@ starting points. (*PRUNE) or (*PRUNE:NAME) 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). +
++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)@@ -2747,31 +2929,41 @@ 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: @@ -2791,7 +2983,7 @@ 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: @@ -2815,24 +3007,98 @@ 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) cases +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), pcre16(3). +pcresyntax(3), pcre(3), pcre16(3), pcre32(3).
-
Philip Hazel
@@ -2841,11 +3107,11 @@ University Computing Service
Cambridge CB2 3QH, England.
-Last updated: 09 January 2012
+Last updated: 26 April 2013
-Copyright © 1997-2012 University of Cambridge.
+Copyright © 1997-2013 University of Cambridge.
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