There are a number of features of PCRE2 regular expressions that are not -supported by the alternative matching algorithm. They are as follows: +supported or behave differently in the alternative matching function. Those +that are not supported cause an error if encountered.
1. Because the algorithm finds all possible matches, the greedy or ungreedy @@ -159,29 +160,32 @@ do this. This means that no captured substrings are available.
3. Because no substrings are captured, backreferences within the pattern are -not supported, and cause errors if encountered. +not supported.
4. For the same reason, conditional expressions that use a backreference as the condition or test for a specific group recursion are not supported.
-5. Because many paths through the tree may be active, the \K escape sequence, -which resets the start of the match when encountered (but may be on some paths -and not on others), is not supported. It causes an error if encountered. +5. Again for the same reason, script runs are not supported.
-6. Callouts are supported, but the value of the capture_top field is +6. Because many paths through the tree may be active, the \K escape sequence, +which resets the start of the match when encountered (but may be on some paths +and not on others), is not supported. +
++7. Callouts are supported, but the value of the capture_top field is always 1, and the value of the capture_last field is always 0.
-7. The \C escape sequence, which (in the standard algorithm) always matches a +8. The \C escape sequence, which (in the standard algorithm) always matches a single code unit, even in a UTF mode, is not supported in these modes, because the alternative algorithm moves through the subject string one character (not code unit) at a time, for all active paths through the tree.
-8. Except for (*FAIL), the backtracking control verbs such as (*PRUNE) are not +9. Except for (*FAIL), the backtracking control verbs such as (*PRUNE) are not supported. (*FAIL) is supported, and behaves like a failing negative assertion.
-2. Capturing parentheses and backreferences are not supported. +2. Capturing parentheses, backreferences, and script runs are not supported.
3. Although atomic groups are supported, their use does not provide the @@ -232,9 +236,9 @@ Cambridge, England.
-Last updated: 29 September 2014
+Last updated: 10 October 2018
-Copyright © 1997-2014 University of Cambridge.
+Copyright © 1997-2018 University of Cambridge.
Return to the PCRE2 index page. diff --git a/doc/html/pcre2pattern.html b/doc/html/pcre2pattern.html index 824d59b..e76035e 100644 --- a/doc/html/pcre2pattern.html +++ b/doc/html/pcre2pattern.html @@ -33,16 +33,17 @@ please consult the man page, in case the conversion went wrong.
@@ -756,7 +757,7 @@ sequences that match characters with specific properties are available. In 8-bit non-UTF-8 mode, these sequences are of course limited to testing characters whose code points are less than 256, but they do work in this mode. In 32-bit non-UTF mode, code points greater than 0x10ffff (the Unicode limit) -may be encountered. These are all treated as being in the Common script and +may be encountered. These are all treated as being in the Unknown script and with an unassigned type. The extra escape sequences are:
\p{xx} a character with the xx property
@@ -780,8 +781,10 @@ example:
\p{Greek}
\P{Han}
-Those that are not part of an identified script are lumped together as
-"Common". The current list of scripts is:
+Unassigned characters (and in non-UTF 32-bit mode, characters with code points
+greater than 0x10FFFF) are assigned the "Unknown" script. Others that are not
+part of an identified script are lumped together as "Common". The current list
+of scripts is:
Adlam, @@ -928,6 +931,7 @@ Tibetan, Tifinagh, Tirhuta, Ugaritic, +Unknown, Vai, Warang_Citi, Yi, @@ -2589,8 +2593,70 @@ preceded by "foo", while is another pattern that matches "foo" preceded by three digits and any three characters that are not "999". +
++In concept, a script run is a sequence of characters that are all from the same +Unicode script such as Latin or Greek. However, because some scripts are +commonly used together, and because some diacritical and other marks are used +with multiple scripts, it is not that simple. There is a full description of +the rules that PCRE2 uses in the section entitled +"Script Runs" +in the +pcre2unicode +documentation. +
++If part of a pattern is enclosed between (*script_run: or (*sr: and a closing +parenthesis, it fails if the sequence of characters that it matches are not a +script run. After a failure, normal backtracking occurs. Script runs can be +used to detect spoofing attacks using characters that look the same, but are +from different scripts. The string "paypal.com" is an infamous example, where +the letters could be a mixture of Latin and Cyrillic. This pattern ensures that +the matched characters in a sequence of non-spaces that follow white space are +a script run: +
+ \s+(*sr:\S+) ++To be sure that they are all from the Latin script (for example), a lookahead +can be used: +
+ \s+(?=\p{Latin})(*sr:\S+)
+
+This works as long as the first character is expected to be a character in that
+script, and not (for example) punctuation, which is allowed with any script. If
+this is not the case, a more creative lookahead is needed. For example, if
+digits, underscore, and dots are permitted at the start:
+
+ \s+(?=[0-9_.]*\p{Latin})(*sr:\S+)
+
+
+
++In many cases, backtracking into a script run pattern fragment is not +desirable. The script run can employ an atomic group to prevent this. Because +this is a common requirement, a shorthand notation is provided by +(*atomic_script_run: or (*asr: +
+ (*asr:...) is the same as (*sr:(?>...)) ++Note that the atomic group is inside the script run. Putting it outside would +not prevent backtracking into the script run pattern. + +
+Support for script runs is not available if PCRE2 is compiled without Unicode +support. A compile-time error is given if any of the above constructs is +encountered. Script runs are not supported by the alternate matching function, +pcre2_dfa_match() because they use the same mechanism as capturing +parentheses. +
++Warning: The (*ACCEPT) control verb +(see below) +should not be used within a script run subpattern, because it causes an +immediate exit from the subpattern, bypassing the script run checking.
-It is possible to cause the matching process to obey a subpattern conditionally or to choose between two alternative subpatterns, depending on @@ -2790,7 +2856,7 @@ positive and negative assertions, because matching always continues after the assertion, whether it succeeds or fails. (Compare non-conditional assertions, when captures are retained only for positive assertions that succeed.)
-There are two ways of including comments in patterns that are processed by PCRE2. In both cases, the start of the comment must not be in a character @@ -2820,7 +2886,7 @@ a newline in the pattern. The sequence \n is still literal at this stage, so it does not terminate the comment. Only an actual character with the code value 0x0a (the default newline) does so.
-Consider the problem of matching a string in parentheses, allowing for unlimited nested parentheses. Without the use of recursion, the best that can @@ -3008,7 +3074,7 @@ alternative matches "a" and then recurses. In the recursion, \1 does now match "b" and so the whole match succeeds. This match used to fail in Perl, but in later versions (I tried 5.024) it now works.
-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 a bit @@ -3057,7 +3123,7 @@ in subpatterns when called as subroutines is described in the section entitled "Backtracking verbs in subroutines" below.
-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 @@ -3075,7 +3141,7 @@ plus or a minus sign it is taken as a relative reference. For example: Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are not synonymous. The former is a backreference; the latter is a subroutine call.
-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 @@ -3151,7 +3217,7 @@ example: The doubling is removed before the string is passed to the callout function.
-There are a number of special "Backtracking Control Verbs" (to use Perl's terminology) that modify the behaviour of backtracking during matching. They @@ -3222,7 +3288,7 @@ documentation.
Experiments with Perl suggest that it too has similar optimizations, and like PCRE2, turning them off can change the result of a match. -
++Warning: (*ACCEPT) should not be used within a script run subpattern, +because it causes an immediate exit from the subpattern, bypassing the script +run checking.
(*FAIL) or (*FAIL:NAME)@@ -3644,12 +3715,12 @@ behaviour). However, if there is no such group within the subroutine subpattern, the subroutine match fails and there is a backtrack at the outer level. -
pcre2api(3), pcre2callout(3), pcre2matching(3), pcre2syntax(3), pcre2(3).
-
Philip Hazel
@@ -3658,9 +3729,9 @@ University Computing Service
Cambridge, England.
-Last updated: 24 September 2018
+Last updated: 12 October 2018
Copyright © 1997-2018 University of Cambridge.
diff --git a/doc/html/pcre2syntax.html b/doc/html/pcre2syntax.html
index d6e1e30..e7a5df3 100644
--- a/doc/html/pcre2syntax.html
+++ b/doc/html/pcre2syntax.html
@@ -32,14 +32,15 @@ please consult the man page, in case the conversion went wrong.
@@ -533,7 +534,17 @@ setting with a similar syntax. Each top-level branch of a lookbehind must be of a fixed length.
-+
+ (*script_run:...) ) script run, can be backtracked into + (*sr:...) ) + + (*atomic_script_run:...) ) atomic script run + (*asr:...) ) ++ +
\n reference by number (can be ambiguous) @@ -550,7 +561,7 @@ Each top-level branch of a lookbehind must be of a fixed length. (?P=name) reference by name (Python)-
(?R) recurse whole pattern @@ -569,7 +580,7 @@ Each top-level branch of a lookbehind must be of a fixed length. \g'-n' call subpattern by relative number (PCRE2 extension)-
(?(condition)yes-pattern) @@ -592,7 +603,7 @@ Note the ambiguity of (?(R) and (?(Rn) which might be named reference conditions or recursion tests. Such a condition is interpreted as a reference condition if the relevant named group exists. -
BACKTRACKING CONTROL
+
BACKTRACKING CONTROL
All backtracking control verbs may be in the form (*VERB:NAME). For (*MARK) the name is mandatory, for the others it is optional. (*SKIP) changes its behaviour @@ -619,7 +630,7 @@ pattern is not anchored. The effect of one of these verbs in a group called as a subroutine is confined to the subroutine call.
-
CALLOUTS
+
CALLOUTS
(?C) callout (assumed number 0) @@ -630,12 +641,12 @@ The allowed string delimiters are ` ' " ^ % # $ (which are the same for the start and the end), and the starting delimiter { matched with the ending delimiter }. To encode the ending delimiter within the string, double it. -
SEE ALSO
+
SEE ALSO
pcre2pattern(3), pcre2api(3), pcre2callout(3), pcre2matching(3), pcre2(3).
-
AUTHOR
+
AUTHOR
Philip Hazel
-
@@ -644,9 +655,9 @@ University Computing Service Cambridge, England.
REVISION
+
REVISION
-Last updated: 24 September 2018 +Last updated: 10 October 2018
+
Copyright © 1997-2018 University of Cambridge.
diff --git a/doc/html/pcre2unicode.html b/doc/html/pcre2unicode.html index 24f6d93..e446182 100644 --- a/doc/html/pcre2unicode.html +++ b/doc/html/pcre2unicode.html @@ -124,6 +124,116 @@ for characters whose code points are less than 128 and that have at most two case-equivalent values. For these, a direct table lookup is used for speed. A few Unicode characters such as Greek sigma have more than two code points that are case-equivalent, and these are treated as such. +
+SCRIPT RUNS +
++The pattern constructs (*script_run:...) and (*atomic_script_run:...), with +synonyms (*sr:...) and (*asr:...), verify that the string matched within the +parentheses is a script run. In concept, a script run is a sequence of +characters that are all from the same Unicode script. However, because some +scripts are commonly used together, and because some diacritical and other +marks are used with multiple scripts, it is not that simple. +
++Every Unicode character has a Script property, mostly with a value +corresponding to the name of a script, such as Latin, Greek, or Cyrillic. There +are also three special values: +
++"Unknown" is used for code points that have not been assigned, and also for the +surrogate code points. In the PCRE2 32-bit library, characters whose code +points are greater than the Unicode maximum (U+10FFFF), which are accessible +only in non-UTF mode, are assigned the Unknown script. +
++"Common" is used for characters that are used with many scripts. These include +punctuation, emoji, mathematical, musical, and currency symbols, and the ASCII +digits 0 to 9. +
++"Inherited" is used for characters such as diacritical marks that modify a +previous character. These are considered to take on the script of the character +that they modify. +
++Some Inherited characters are used with many scripts, but many of them are only +normally used with a small number of scripts. For example, U+102E0 (Coptic +Epact thousands mark) is used only with Arabic and Coptic. In order to make it +possible to check this, a Unicode property called Script Extension exists. Its +value is a list of scripts that apply to the character. For the majority of +characters, the list contains just one script, the same one as the Script +property. However, for characters such as U+102E0 more than one Script is +listed. There are also some Common characters that have a single, non-Common +script in their Script Extension list. +
++The next section describes the basic rules for deciding whether a given string +of characters is a script run. Note, however, that there are some special cases +involving the Chinese Han script, and an additional constraint for decimal +digits. These are covered in subsequent sections. +
+
+Basic script run rules +
++A string that is less than two characters long is a script run. This is the +only case in which an Unknown character can be part of a script run. Longer +strings are checked using only the Script Extensions property, not the basic +Script property. +
++If a character's Script Extension property is the single value "Inherited", it +is always accepted as part of a script run. This is also true for the property +"Common", subject to the checking of decimal digits described below. All the +remaining characters in a script run must have at least one script in common in +their Script Extension lists. In set-theoretic terminology, the intersection of +all the sets of scripts must not be empty. +
++A simple example is an Internet name such as "google.com". The letters are all +in the Latin script, and the dot is Common, so this string is a script run. +However, the Cyrillic letter "o" looks exactly the same as the Latin "o"; a +string that looks the same, but with Cyrillic "o"s is not a script run. +
++More interesting examples involve characters with more than one script in their +Script Extension. Consider the following characters: +
+ U+060C Arabic comma + U+06D4 Arabic full stop ++The first has the Script Extension list Arabic, Hanifi Rohingya, Syriac, and +Thaana; the second has just Arabic and Hanifi Rohingya. Both of them could +appear in script runs of either Arabic or Hanifi Rohingya. The first could also +appear in Syriac or Thaana script runs, but the second could not. + +
+The Chinese Han script +
++The Chinese Han script is commonly used in conjunction with other scripts for +writing certain languages. Japanese uses the Hiragana and Katakana scripts +together with Han; Korean uses Hangul and Han; Taiwanese Mandarin uses Bopomofo +and Han. These three combinations are treated as special cases when checking +script runs and are, in effect, "virtual scripts". Thus, a script run may +contain a mixture of Hiragana, Katakana, and Han, or a mixture of Hangul and +Han, or a mixture of Bopomofo and Han, but not, for example, a mixture of +Hangul and Bopomofo and Han. PCRE2 (like Perl) follows Unicode's Technical +Standard 39 ("Unicode Security Mechanisms", http://unicode.org/reports/tr39/) +in allowing such mixtures. +
+
+Decimal digits +
++Unicode contains many sets of 10 decimal digits in different scripts, and some +scripts (including the Common script) contain more than one set. Some of these +decimal digits them are visually indistinguishable from the common ASCII +digits. In addition to the script checking described above, if a script run +contains any decimal digits, they must all come from the same set of 10 +adjacent characters.
VALIDITY OF UTF STRINGS @@ -300,7 +410,7 @@ Cambridge, England. REVISION
-Last updated: 02 September 2018 +Last updated: 12 October 2018
Copyright © 1997-2018 University of Cambridge.
diff --git a/doc/pcre2.txt b/doc/pcre2.txt index 2abc377..6ad6922 100644 --- a/doc/pcre2.txt +++ b/doc/pcre2.txt @@ -5391,8 +5391,8 @@ THE ALTERNATIVE MATCHING ALGORITHM SESS option when compiling. There are a number of features of PCRE2 regular expressions that are - not supported by the alternative matching algorithm. They are as fol- - lows: + not supported or behave differently in the alternative matching func- + tion. Those that are not supported cause an error if encountered. 1. Because the algorithm finds all possible matches, the greedy or ungreedy nature of repetition quantifiers is not relevant (though it @@ -5417,47 +5417,48 @@ THE ALTERNATIVE MATCHING ALGORITHM strings are available. 3. Because no substrings are captured, backreferences within the pat- - tern are not supported, and cause errors if encountered. + tern are not supported. 4. For the same reason, conditional expressions that use a backrefer- ence as the condition or test for a specific group recursion are not supported. - 5. Because many paths through the tree may be active, the \K escape - sequence, which resets the start of the match when encountered (but may - be on some paths and not on others), is not supported. It causes an - error if encountered. + 5. Again for the same reason, script runs are not supported. - 6. Callouts are supported, but the value of the capture_top field is + 6. Because many paths through the tree may be active, the \K escape + sequence, which resets the start of the match when encountered (but may + be on some paths and not on others), is not supported. + + 7. Callouts are supported, but the value of the capture_top field is always 1, and the value of the capture_last field is always 0. - 7. The \C escape sequence, which (in the standard algorithm) always - matches a single code unit, even in a UTF mode, is not supported in - these modes, because the alternative algorithm moves through the sub- - ject string one character (not code unit) at a time, for all active + 8. The \C escape sequence, which (in the standard algorithm) always + matches a single code unit, even in a UTF mode, is not supported in + these modes, because the alternative algorithm moves through the sub- + ject string one character (not code unit) at a time, for all active paths through the tree. - 8. Except for (*FAIL), the backtracking control verbs such as (*PRUNE) - are not supported. (*FAIL) is supported, and behaves like a failing + 9. Except for (*FAIL), the backtracking control verbs such as (*PRUNE) + are not supported. (*FAIL) is supported, and behaves like a failing negative assertion. ADVANTAGES OF THE ALTERNATIVE ALGORITHM - Using the alternative matching algorithm provides the following advan- + Using the alternative matching algorithm provides the following advan- tages: 1. All possible matches (at a single point in the subject) are automat- - ically found, and in particular, the longest match is found. To find + ically found, and in particular, the longest match is found. To find more than one match using the standard algorithm, you have to do kludgy things with callouts. - 2. Because the alternative algorithm scans the subject string just + 2. Because the alternative algorithm scans the subject string just once, and never needs to backtrack (except for lookbehinds), it is pos- - sible to pass very long subject strings to the matching function in + sible to pass very long subject strings to the matching function in several pieces, checking for partial matching each time. Although it is - also possible to do multi-segment matching using the standard algo- - rithm, by retaining partially matched substrings, it is more compli- + also possible to do multi-segment matching using the standard algo- + rithm, by retaining partially matched substrings, it is more compli- cated. The pcre2partial documentation gives details of partial matching and discusses multi-segment matching. @@ -5466,11 +5467,12 @@ DISADVANTAGES OF THE ALTERNATIVE ALGORITHM The alternative algorithm suffers from a number of disadvantages: - 1. It is substantially slower than the standard algorithm. This is - partly because it has to search for all possible matches, but is also + 1. It is substantially slower than the standard algorithm. This is + partly because it has to search for all possible matches, but is also because it is less susceptible to optimization. - 2. Capturing parentheses and backreferences are not supported. + 2. Capturing parentheses, backreferences, and script runs are not sup- + ported. 3. Although atomic groups are supported, their use does not provide the performance advantage that it does for the standard algorithm. @@ -5485,8 +5487,8 @@ AUTHOR REVISION - Last updated: 29 September 2014 - Copyright (c) 1997-2014 University of Cambridge. + Last updated: 10 October 2018 + Copyright (c) 1997-2018 University of Cambridge. ------------------------------------------------------------------------------ @@ -6586,7 +6588,7 @@ BACKSLASH limited to testing characters whose code points are less than 256, but they do work in this mode. In 32-bit non-UTF mode, code points greater than 0x10ffff (the Unicode limit) may be encountered. These are all - treated as being in the Common script and with an unassigned type. The + treated as being in the Unknown script and with an unassigned type. The extra escape sequences are: \p{xx} a character with the xx property @@ -6607,8 +6609,10 @@ BACKSLASH \p{Greek} \P{Han} - Those that are not part of an identified script are lumped together as - "Common". The current list of scripts is: + Unassigned characters (and in non-UTF 32-bit mode, characters with code + points greater than 0x10FFFF) are assigned the "Unknown" script. Others + that are not part of an identified script are lumped together as "Com- + mon". The current list of scripts is: Adlam, Ahom, Anatolian_Hieroglyphs, Arabic, Armenian, Avestan, Bali- nese, Bamum, Bassa_Vah, Batak, Bengali, Bhaiksuki, Bopomofo, Brahmi, @@ -6632,17 +6636,18 @@ BACKSLASH vian, Siddham, SignWriting, Sinhala, Sogdian, Sora_Sompeng, Soyombo, Sundanese, Syloti_Nagri, Syriac, Tagalog, Tagbanwa, Tai_Le, Tai_Tham, Tai_Viet, Takri, Tamil, Tangut, Telugu, Thaana, Thai, Tibetan, Tifi- - nagh, Tirhuta, Ugaritic, Vai, Warang_Citi, Yi, Zanabazar_Square. + nagh, Tirhuta, Ugaritic, Unknown, Vai, Warang_Citi, Yi, Zan- + abazar_Square. Each character has exactly one Unicode general category property, spec- - ified by a two-letter abbreviation. For compatibility with Perl, nega- - tion can be specified by including a circumflex between the opening - brace and the property name. For example, \p{^Lu} is the same as + ified by a two-letter abbreviation. For compatibility with Perl, nega- + tion can be specified by including a circumflex between the opening + brace and the property name. For example, \p{^Lu} is the same as \P{Lu}. If only one letter is specified with \p or \P, it includes all the gen- - eral category properties that start with that letter. In this case, in - the absence of negation, the curly brackets in the escape sequence are + eral category properties that start with that letter. In this case, in + the absence of negation, the curly brackets in the escape sequence are optional; these two examples have the same effect: \p{L} @@ -6694,86 +6699,86 @@ BACKSLASH Zp Paragraph separator Zs Space separator - The special property L& is also supported: it matches a character that - has the Lu, Ll, or Lt property, in other words, a letter that is not + The special property L& is also supported: it matches a character that + has the Lu, Ll, or Lt property, in other words, a letter that is not classified as a modifier or "other". - The 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 PCRE2, unless UTF validity checking has been - turned off (see the discussion of PCRE2_NO_UTF_CHECK in the pcre2api + 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 PCRE2, unless UTF validity checking has been + turned off (see the discussion of PCRE2_NO_UTF_CHECK in the pcre2api page). Perl does not support the Cs property. - The long synonyms for property names that Perl supports (such as - \p{Letter}) are not supported by PCRE2, nor is it permitted to prefix + The long synonyms for property names that Perl supports (such as + \p{Letter}) are not supported by PCRE2, nor is it permitted to prefix any of these properties with "Is". No character that is in the Unicode table has the Cn (unassigned) prop- erty. Instead, this property is assumed for any code point that is not in the Unicode table. - Specifying caseless matching does not affect these escape sequences. - For example, \p{Lu} always matches only upper case letters. This is + Specifying caseless matching does not affect these escape sequences. + For example, \p{Lu} always matches only upper case letters. This is different from the behaviour of current versions of Perl. - Matching characters by Unicode property is not fast, because PCRE2 has - to do a multistage table lookup in order to find a character's prop- + Matching characters by Unicode property is not fast, because PCRE2 has + to do a multistage table lookup in order to find a character's prop- erty. That is why the traditional escape sequences such as \d and \w do - not use Unicode properties in PCRE2 by default, though you can make - them do so by setting the PCRE2_UCP option or by starting the pattern + not use Unicode properties in PCRE2 by default, though you can make + them do so by setting the PCRE2_UCP option or by starting the pattern with (*UCP). Extended grapheme clusters - The \X escape matches any number of Unicode characters that form an + 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). Unicode supports various kinds of composite character by - giving each character a grapheme breaking property, and having rules + (see below). Unicode supports various kinds of composite character by + giving each character a grapheme breaking property, and having rules that use these properties to define the boundaries of extended grapheme - clusters. The rules are defined in Unicode Standard Annex 29, "Unicode - Text Segmentation". Unicode 11.0.0 abandoned the use of some previous - properties that had been used for emojis. Instead it introduced vari- - ous emoji-specific properties. PCRE2 uses only the Extended Picto- + clusters. The rules are defined in Unicode Standard Annex 29, "Unicode + Text Segmentation". Unicode 11.0.0 abandoned the use of some previous + properties that had been used for emojis. Instead it introduced vari- + ous emoji-specific properties. PCRE2 uses only the Extended Picto- graphic property. - \X always matches at least one character. Then it decides whether to + \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 char- + 2. Do not end between CR and LF; otherwise end after any control char- acter. - 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 + 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 or the - "zero-width joiner" character. Characters with the "mark" property + 4. Do not end before extending characters or spacing marks or the + "zero-width joiner" character. Characters with the "mark" property always have the "extend" grapheme breaking property. 5. Do not end after prepend characters. 6. Do not break within emoji modifier sequences or emoji zwj sequences. That is, do not break between characters with the Extended_Pictographic - property. Extend and ZWJ characters are allowed between the charac- + property. Extend and ZWJ characters are allowed between the charac- ters. - 7. Do not break within emoji flag sequences. That is, do not break - between regional indicator (RI) characters if there are an odd number + 7. Do not break within emoji flag sequences. That is, do not break + between regional indicator (RI) characters if there are an odd number of RI characters before the break point. 8. Otherwise, end the cluster. PCRE2's additional properties - As well as the standard Unicode properties described above, PCRE2 sup- - ports four more that make it possible to convert traditional escape + As well as the standard Unicode properties described above, PCRE2 sup- + ports four more that make it possible to convert traditional escape sequences such as \w and \s to use Unicode properties. PCRE2 uses these - non-standard, non-Perl properties internally when PCRE2_UCP is set. + non-standard, non-Perl properties internally when PCRE2_UCP is set. However, they may also be used explicitly. These properties are: Xan Any alphanumeric character @@ -6781,69 +6786,69 @@ BACKSLASH Xsp Any Perl space character Xwd Any Perl "word" character - Xan matches characters that have either the L (letter) or the N (num- - ber) 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; in PCRE1 it used to - exclude vertical tab, for Perl compatibility, but Perl changed. Xwd + Xan matches characters that have either the L (letter) or the N (num- + ber) 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; in PCRE1 it used to + exclude vertical tab, for Perl compatibility, but Perl changed. Xwd matches the same characters as Xan, plus underscore. - There is another non-standard property, Xuc, which matches any charac- - ter 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. + There is another non-standard property, Xuc, which matches any charac- + ter 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 char- acters that they represent.) Resetting the match start - In normal use, the escape sequence \K causes any previously matched - characters not to be included in the final matched sequence that is + In normal use, the escape sequence \K causes any previously matched + characters not to be included in the final matched sequence that is returned. For example, the pattern: foo\Kbar - matches "foobar", but reports that it has matched "bar". \K does not + matches "foobar", but reports that it has matched "bar". \K does not interact with anchoring in any way. The pattern: ^foo\Kbar - matches only when the subject begins with "foobar" (in single line - mode), though it again reports the matched string as "bar". This fea- - ture is similar to a lookbehind assertion (described below). However, - in this case, the part of the subject before the real match does not - have to be of fixed length, as lookbehind assertions do. The use of \K - does not interfere with the setting of captured substrings. For exam- + matches only when the subject begins with "foobar" (in single line + mode), though it again reports the matched string as "bar". This fea- + ture is similar to a lookbehind assertion (described below). However, + in this case, the part of the subject before the real match does not + have to be of fixed length, as lookbehind assertions do. The use of \K + does not interfere with the setting of captured substrings. For exam- ple, when the pattern (foo)\Kbar matches "foobar", the first substring is still set to "foo". - Perl documents that the use of \K within assertions is "not well - defined". In PCRE2, \K is acted upon when it occurs inside positive - assertions, but is ignored in negative assertions. Note that when a - pattern such as (?=ab\K) matches, the reported start of the match can - be greater than the end of the match. Using \K in a lookbehind asser- - tion at the start of a pattern can also lead to odd effects. For exam- + Perl documents that the use of \K within assertions is "not well + defined". In PCRE2, \K is acted upon when it occurs inside positive + assertions, but is ignored in negative assertions. Note that when a + pattern such as (?=ab\K) matches, the reported start of the match can + be greater than the end of the match. Using \K in a lookbehind asser- + tion at the start of a pattern can also lead to odd effects. For exam- ple, consider this pattern: (?<=\Kfoo)bar - If the subject is "foobar", a call to pcre2_match() with a starting - offset of 3 succeeds and reports the matching string as "foobar", that - is, the start of the reported match is earlier than where the match + If the subject is "foobar", a call to pcre2_match() with a starting + offset of 3 succeeds and reports the matching string as "foobar", that + is, the start of the reported match is earlier than where the match started. Simple assertions - The final use of backslash is for certain simple assertions. An asser- - tion specifies a condition that has to be met at a particular point in - a match, without consuming any characters from the subject string. The - use of subpatterns for more complicated assertions is described below. + The final use of backslash is for certain simple assertions. An asser- + tion specifies a condition that has to be met at a particular point in + a match, without consuming any characters from the subject string. The + use of subpatterns for more complicated assertions is described below. The backslashed assertions are: \b matches at a word boundary @@ -6854,190 +6859,190 @@ BACKSLASH \z matches only at the end of the subject \G matches at the first matching position in the subject - Inside a character class, \b has a different meaning; it matches the - backspace character. If any other of these assertions appears in a + Inside a character class, \b has a different meaning; it matches the + backspace character. If any other of these assertions appears in a character class, an "invalid escape sequence" error is generated. - A word boundary is a position in the subject string where the current - character and the previous character do not both match \w or \W (i.e. - one matches \w and the other matches \W), or the start or end of the - string if the first or last character matches \w, respectively. In a - UTF mode, the meanings of \w and \W can be changed by setting the + A word boundary is a position in the subject string where the current + character and the previous character do not both match \w or \W (i.e. + one matches \w and the other matches \W), or the start or end of the + string if the first or last character matches \w, respectively. In a + UTF mode, the meanings of \w and \W can be changed by setting the PCRE2_UCP option. When this is done, it also affects \b and \B. Neither - PCRE2 nor Perl has a separate "start of word" or "end of word" metase- - quence. However, whatever follows \b normally determines which it is. + PCRE2 nor Perl has a separate "start of word" or "end of word" metase- + quence. However, whatever follows \b normally determines which it is. For example, the fragment \ba matches "a" at the start of a word. - The \A, \Z, and \z assertions differ from the traditional circumflex + The \A, \Z, and \z assertions differ from the traditional circumflex and dollar (described in the next section) in that they only ever match - at the very start and end of the subject string, whatever options are - set. Thus, they are independent of multiline mode. These three asser- - tions are not affected by the PCRE2_NOTBOL or PCRE2_NOTEOL options, - which affect only the behaviour of the circumflex and dollar metachar- - acters. However, if the startoffset argument of pcre2_match() is non- - zero, indicating that matching is to start at a point other than the - beginning of the subject, \A can never match. The difference between - \Z and \z is that \Z matches before a newline at the end of the string + at the very start and end of the subject string, whatever options are + set. Thus, they are independent of multiline mode. These three asser- + tions are not affected by the PCRE2_NOTBOL or PCRE2_NOTEOL options, + which affect only the behaviour of the circumflex and dollar metachar- + acters. However, if the startoffset argument of pcre2_match() is non- + zero, indicating that matching is to start at a point other than the + beginning of the subject, \A can never match. The difference between + \Z and \z is that \Z matches before a newline at the end of the string as well as at the very end, whereas \z matches only at the end. - The \G assertion is true only when the current matching position is at - the start point of the matching process, as specified by the startoff- - set argument of pcre2_match(). It differs from \A when the value of - startoffset is non-zero. By calling pcre2_match() multiple times with - appropriate arguments, you can mimic Perl's /g option, and it is in + The \G assertion is true only when the current matching position is at + the start point of the matching process, as specified by the startoff- + set argument of pcre2_match(). It differs from \A when the value of + startoffset is non-zero. By calling pcre2_match() multiple times with + appropriate arguments, you can mimic Perl's /g option, and it is in this kind of implementation where \G can be useful. - Note, however, that PCRE2's implementation of \G, being true at the - starting character of the matching process, is subtly different from - Perl's, which defines it as true at the end of the previous match. In - Perl, these can be different when the previously matched string was + Note, however, that PCRE2's implementation of \G, being true at the + starting character of the matching process, is subtly different from + Perl's, which defines it as true at the end of the previous match. In + Perl, these can be different when the previously matched string was empty. Because PCRE2 does just one match at a time, it cannot reproduce this behaviour. - If all the alternatives of a pattern begin with \G, the expression is + If all the alternatives of a pattern begin with \G, the expression is anchored to the starting match position, and the "anchored" flag is set in the compiled regular expression. CIRCUMFLEX AND DOLLAR - The circumflex and dollar metacharacters are zero-width assertions. - That is, they test for a particular condition being true without con- + The circumflex and dollar metacharacters are zero-width assertions. + That is, they test for a particular condition being true without con- suming any characters from the subject string. These two metacharacters - are concerned with matching the starts and ends of lines. If the new- - line convention is set so that only the two-character sequence CRLF is - recognized as a newline, isolated CR and LF characters are treated as + are concerned with matching the starts and ends of lines. If the new- + line convention is set so that only the two-character sequence CRLF is + recognized as a newline, isolated CR and LF characters are treated as ordinary data characters, and are not recognized as newlines. Outside a character class, in the default matching mode, the circumflex - character is an assertion that is true only if the current matching - point is at the start of the subject string. If the startoffset argu- - ment of pcre2_match() is non-zero, or if PCRE2_NOTBOL is set, circum- - flex can never match if the PCRE2_MULTILINE option is unset. Inside a - character class, circumflex has an entirely different meaning (see + character is an assertion that is true only if the current matching + point is at the start of the subject string. If the startoffset argu- + ment of pcre2_match() is non-zero, or if PCRE2_NOTBOL is set, circum- + flex can never match if the PCRE2_MULTILINE option is unset. Inside a + character class, circumflex has an entirely different meaning (see below). - Circumflex need not be the first character of the pattern if a number - of alternatives are involved, but it should be the first thing in each - alternative in which it appears if the pattern is ever to match that - branch. If all possible alternatives start with a circumflex, that is, - if the pattern is constrained to match only at the start of the sub- - ject, it is said to be an "anchored" pattern. (There are also other + Circumflex need not be the first character of the pattern if a number + of alternatives are involved, but it should be the first thing in each + alternative in which it appears if the pattern is ever to match that + branch. If all possible alternatives start with a circumflex, that is, + if the pattern is constrained to match only at the start of the sub- + ject, it is said to be an "anchored" pattern. (There are also other constructs that can cause a pattern to be anchored.) - 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), unless + 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), unless PCRE2_NOTEOL is set. Note, however, that it does not actually match the newline. Dollar need not be the last character of the pattern if a num- ber 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 charac- + branch in which it appears. Dollar has no special meaning in a charac- ter class. - The meaning of dollar can be changed so that it matches only at the - very end of the string, by setting the PCRE2_DOLLAR_ENDONLY option at + The meaning of dollar can be changed so that it matches only at the + very end of the string, by setting the PCRE2_DOLLAR_ENDONLY option at compile time. This does not affect the \Z assertion. The meanings of the circumflex and dollar metacharacters are changed if - the PCRE2_MULTILINE option is set. When this is the case, a dollar - character matches before any newlines in the string, as well as at the - very end, and a circumflex matches immediately after internal newlines - as well as at the start of the subject string. It does not match after - a newline that ends the string, for compatibility with Perl. However, + the PCRE2_MULTILINE option is set. When this is the case, a dollar + character matches before any newlines in the string, as well as at the + very end, and a circumflex matches immediately after internal newlines + as well as at the start of the subject string. It does not match after + a newline that ends the string, for compatibility with Perl. However, this can be changed by setting the PCRE2_ALT_CIRCUMFLEX option. - For example, the pattern /^abc$/ matches the subject string "def\nabc" - (where \n represents a newline) in multiline mode, but not otherwise. - Consequently, patterns that are anchored in single line mode because - all branches start with ^ are not anchored in multiline mode, and a - match for circumflex is possible when the startoffset argument of - pcre2_match() is non-zero. The PCRE2_DOLLAR_ENDONLY option is ignored + For example, the pattern /^abc$/ matches the subject string "def\nabc" + (where \n represents a newline) in multiline mode, but not otherwise. + Consequently, patterns that are anchored in single line mode because + all branches start with ^ are not anchored in multiline mode, and a + match for circumflex is possible when the startoffset argument of + pcre2_match() is non-zero. The PCRE2_DOLLAR_ENDONLY option is ignored if PCRE2_MULTILINE is set. - When the newline convention (see "Newline conventions" below) recog- - nizes the two-character sequence CRLF as a newline, this is preferred, - even if the single characters CR and LF are also recognized as new- - lines. For example, if the newline convention is "any", a multiline - mode circumflex matches before "xyz" in the string "abc\r\nxyz" rather - than after CR, even though CR on its own is a valid newline. (It also + When the newline convention (see "Newline conventions" below) recog- + nizes the two-character sequence CRLF as a newline, this is preferred, + even if the single characters CR and LF are also recognized as new- + lines. For example, if the newline convention is "any", a multiline + mode circumflex matches before "xyz" in the string "abc\r\nxyz" rather + than after CR, even though CR on its own is a valid newline. (It also matches at the very start of the string, of course.) - Note that the sequences \A, \Z, and \z can be used to match the start - and end of the subject in both modes, and if all branches of a pattern - start with \A it is always anchored, whether or not PCRE2_MULTILINE is + Note that the sequences \A, \Z, and \z can be used to match the start + and end of the subject in both modes, and if all branches of a pattern + start with \A it is always anchored, whether or not PCRE2_MULTILINE is set. FULL STOP (PERIOD, DOT) AND \N Outside a character class, a dot in the pattern matches any one charac- - ter in the subject string except (by default) a character that signi- + ter in the subject string except (by default) a character that signi- fies the end of a line. - When a line ending is defined as a single character, dot never matches - that character; when the two-character sequence CRLF is used, dot does - not match CR if it is immediately followed by LF, but otherwise it - matches all characters (including isolated CRs and LFs). When any Uni- - code line endings are being recognized, dot does not match CR or LF or + When a line ending is defined as a single character, dot never matches + that character; when the two-character sequence CRLF is used, dot does + not match CR if it is immediately followed by LF, but otherwise it + matches all characters (including isolated CRs and LFs). When any Uni- + code line endings are being recognized, dot does not match CR or LF or any of the other line ending characters. - The behaviour of dot with regard to newlines can be changed. If the - PCRE2_DOTALL option is set, a dot matches any one character, without - exception. If the two-character sequence CRLF is present in the sub- + The behaviour of dot with regard to newlines can be changed. If the + PCRE2_DOTALL option is set, a dot matches any one character, without + exception. If the two-character sequence CRLF is present in the sub- ject string, it takes two dots to match it. - The handling of dot is entirely independent of the handling of circum- - flex and dollar, the only relationship being that they both involve + The handling of dot is entirely independent of the handling of circum- + flex and dollar, the only relationship being that they both involve newlines. Dot has no special meaning in a character class. - The escape sequence \N when not followed by an opening brace behaves - like a dot, except that it is not affected by the PCRE2_DOTALL option. - In other words, it matches any character except one that signifies the + The escape sequence \N when not followed by an opening brace behaves + like a dot, except that it is not affected by the PCRE2_DOTALL option. + In other words, it matches any character except one that signifies the end of a line. When \N is followed by an opening brace it has a different meaning. See - the section entitled "Non-printing characters" above for details. Perl - also uses \N{name} to specify characters by Unicode name; PCRE2 does + the section entitled "Non-printing characters" above for details. Perl + also uses \N{name} to specify characters by Unicode name; PCRE2 does not support this. MATCHING A SINGLE CODE UNIT - Outside a character class, the escape sequence \C matches any one code - unit, whether or not a UTF mode is set. In the 8-bit library, one code - 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 + Outside a character class, the escape sequence \C matches any one code + unit, whether or not a UTF mode is set. In the 8-bit library, one code + 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 use- fully be used. - Because \C breaks up characters into individual code units, matching - one unit with \C in UTF-8 or UTF-16 mode means that the rest of the - string may start with a malformed UTF character. This has undefined + Because \C breaks up characters into individual code units, matching + one unit with \C in UTF-8 or UTF-16 mode means that the rest of the + string may start with a malformed UTF character. This has undefined results, because PCRE2 assumes that it is matching character by charac- - ter in a valid UTF string (by default it checks the subject string's - validity at the start of processing unless the PCRE2_NO_UTF_CHECK + ter in a valid UTF string (by default it checks the subject string's + validity at the start of processing unless the PCRE2_NO_UTF_CHECK option is used). - An application can lock out the use of \C by setting the - PCRE2_NEVER_BACKSLASH_C option when compiling a pattern. It is also + An application can lock out the use of \C by setting the + PCRE2_NEVER_BACKSLASH_C option when compiling a pattern. It is also possible to build PCRE2 with the use of \C permanently disabled. - PCRE2 does not allow \C to appear in lookbehind assertions (described - below) in UTF-8 or UTF-16 modes, because this would make it impossible - to calculate the length of the lookbehind. Neither the alternative + PCRE2 does not allow \C to appear in lookbehind assertions (described + below) in UTF-8 or UTF-16 modes, because this would make it impossible + to calculate the length of the lookbehind. Neither the alternative matching function pcre2_dfa_match() nor the JIT optimizer support \C in these UTF modes. The former gives a match-time error; the latter fails to optimize and so the match is always run using the interpreter. - In the 32-bit library, however, \C is always supported (when not - explicitly locked out) because it always matches a single code unit, + In the 32-bit library, however, \C is always supported (when not + explicitly locked out) because it always matches a single code unit, whether or not UTF-32 is specified. In general, the \C escape sequence is best avoided. However, one way of - using it that avoids the problem of malformed UTF-8 or UTF-16 charac- - ters 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 + using it that avoids the problem of malformed UTF-8 or UTF-16 charac- + ters 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) | @@ -7045,10 +7050,10 @@ MATCHING A SINGLE CODE UNIT (?=[\x{800}-\x{ffff}])(\C)(\C)(\C) | (?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C)) - In this example, a group that starts with (?| resets the capturing + In this example, a group that starts with (?| resets the capturing parentheses numbers in each alternative (see "Duplicate Subpattern Num- bers" below). The assertions at the start of each branch check the next - UTF-8 character for values whose encoding uses 1, 2, 3, or 4 bytes, + UTF-8 character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The character's individual bytes are then captured by the appropriate number of \C groups. @@ -7057,115 +7062,115 @@ SQUARE BRACKETS AND CHARACTER CLASSES An opening square bracket introduces a character class, terminated by a closing square bracket. A closing square bracket on its own is not spe- - cial by default. If a closing square bracket is required as a member + cial by default. If a closing square bracket is required as a member of the class, it should be the first data character in the class (after - an initial circumflex, if present) or escaped with a backslash. This - means that, by default, an empty class cannot be defined. However, if - the PCRE2_ALLOW_EMPTY_CLASS option is set, a closing square bracket at + an initial circumflex, if present) or escaped with a backslash. This + means that, by default, an empty class cannot be defined. However, if + the PCRE2_ALLOW_EMPTY_CLASS option is set, a closing square bracket at the start does end the (empty) class. - A character class matches a single character in the subject. A matched + A character class matches a single character in the subject. 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 + 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 + If a circumflex is actually required as a member of the class, ensure it is not the first character, or escape it with a backslash. - For example, the character class [aeiou] matches any lower case vowel, - while [^aeiou] matches any character that is not a lower case vowel. + For example, the character class [aeiou] matches any lower case vowel, + while [^aeiou] matches any character that is not a lower case vowel. Note that a circumflex is just a convenient notation for specifying the - characters that are in the class by enumerating those that are not. A - class that starts with a circumflex is not an assertion; it still con- - sumes a character from the subject string, and therefore it fails if + characters that are in the class by enumerating those that are not. A + class that starts with a circumflex is not an assertion; it still con- + sumes a character from the subject string, and therefore it fails if the current pointer is at the end of the string. - Characters in a class may be specified by their code points using \o, - \x, or \N{U+hh..} in the usual way. When caseless matching is set, any - letters in a class represent both their upper case and lower case ver- - sions, so for example, a caseless [aeiou] matches "A" as well as "a", - and a caseless [^aeiou] does not match "A", whereas a caseful version + Characters in a class may be specified by their code points using \o, + \x, or \N{U+hh..} in the usual way. When caseless matching is set, any + letters in a class represent both their upper case and lower case ver- + sions, 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. - Characters that might indicate line breaks are never treated in any - special way when matching character classes, whatever line-ending - sequence is in use, and whatever setting of the PCRE2_DOTALL and - PCRE2_MULTILINE options is used. A class such as [^a] always matches + Characters that might indicate line breaks are never treated in any + special way when matching character classes, whatever line-ending + sequence is in use, and whatever setting of the PCRE2_DOTALL and + PCRE2_MULTILINE options is used. A class such as [^a] always matches one of these characters. The generic character type 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 modes, the PCRE2_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" above. The escape - sequence \b has a different meaning inside a character class; it - matches the backspace character. The sequences \B, \R, and \X are not - special inside a character class. Like any other unrecognized escape - sequences, they cause an error. The same is true for \N when not fol- + \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 modes, the PCRE2_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" above. The escape + sequence \b has a different meaning inside a character class; it + matches the backspace character. The sequences \B, \R, and \X are not + special inside a character class. Like any other unrecognized escape + sequences, they cause an error. The same is true for \N when not fol- lowed by an opening brace. - The minus (hyphen) character can be used to specify a range of charac- - ters in a character class. For example, [d-m] matches any letter - between d and m, inclusive. If a minus character is required in a - class, it must be escaped with a backslash or appear in a position - where it cannot be interpreted as indicating a range, typically as the + The minus (hyphen) character can be used to specify a range of charac- + ters in a character class. For example, [d-m] matches any letter + between d and m, inclusive. If a minus character is required in a + class, it must be escaped with a backslash or appear in a position + where it cannot be interpreted as indicating a range, typically as the first or last character in the class, or immediately after a range. For - example, [b-d-z] matches letters in the range b to d, a hyphen charac- + example, [b-d-z] matches letters in the range b to d, a hyphen charac- ter, or z. Perl treats a hyphen as a literal if it appears before or after a POSIX class (see below) or before or after a character type escape such as as - \d or \H. However, unless the hyphen is the last character in the - class, Perl outputs a warning in its warning mode, as this is most - likely a user error. As PCRE2 has no facility for warning, an error is + \d or \H. However, unless the hyphen is the last character in the + class, Perl outputs a warning in its warning mode, as this is most + likely a user error. As PCRE2 has no facility for warning, an error is given in these cases. It is not possible to have the literal character "]" as the end charac- - ter of a range. A pattern such as [W-]46] is interpreted as a class of - two characters ("W" and "-") followed by a literal string "46]", so it - would match "W46]" or "-46]". However, if the "]" is escaped with a - backslash it is interpreted as the end of range, so [W-\]46] is inter- - preted as a class containing a range followed by two other characters. - The octal or hexadecimal representation of "]" can also be used to end + ter of a range. A pattern such as [W-]46] is interpreted as a class of + two characters ("W" and "-") followed by a literal string "46]", so it + would match "W46]" or "-46]". However, if the "]" is escaped with a + backslash it is interpreted as the end of range, so [W-\]46] is inter- + preted as a class containing a range followed by two other characters. + The octal or hexadecimal representation of "]" can also be used to end a range. Ranges normally include all code points between the start and end char- - acters, inclusive. They can also be used for code points specified + acters, inclusive. They can also be used for code points specified numerically, for example [\000-\037]. Ranges can include any characters - that are valid for the current mode. In any UTF mode, the so-called - "surrogate" characters (those whose code points lie between 0xd800 and - 0xdfff inclusive) may not be specified explicitly by default (the - PCRE2_EXTRA_ALLOW_SURROGATE_ESCAPES option disables this check). How- + that are valid for the current mode. In any UTF mode, the so-called + "surrogate" characters (those whose code points lie between 0xd800 and + 0xdfff inclusive) may not be specified explicitly by default (the + PCRE2_EXTRA_ALLOW_SURROGATE_ESCAPES option disables this check). How- ever, ranges such as [\x{d7ff}-\x{e000}], which include the surrogates, are always permitted. - There is a special case in EBCDIC environments for ranges whose end + There is a special case in EBCDIC environments for ranges whose end points are both specified as literal letters in the same case. For com- - patibility with Perl, EBCDIC code points within the range that are not - letters are omitted. For example, [h-k] matches only four characters, + patibility with Perl, EBCDIC code points within the range that are not + letters are omitted. For example, [h-k] matches only four characters, even though the codes for h and k are 0x88 and 0x92, a range of 11 code - points. However, if the range is specified numerically, for example, + points. However, if the range is specified numerically, for example, [\x88-\x92] or [h-\x92], all code points are included. 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 a non-UTF mode, if - character tables for a French locale are in use, [\xc8-\xcb] matches + to [][\\^_`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. - A circumflex can conveniently be used with the upper case character - types to specify a more restricted set of characters than the matching - lower case type. For example, the class [^\W_] matches any letter or + A circumflex can conveniently be used with the upper case character + types to specify a more restricted set of characters than the matching + lower case type. For example, the class [^\W_] matches any letter or digit, but not underscore, whereas [\w] includes underscore. A positive character class should be read as "something OR something OR ..." and a negative class as "NOT something AND NOT 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, or for a - special compatibility feature - see the next two sections), and the + 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, 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. @@ -7173,7 +7178,7 @@ SQUARE BRACKETS AND CHARACTER CLASSES POSIX CHARACTER CLASSES Perl supports the POSIX notation for character classes. This uses names - enclosed by [: and :] within the enclosing square brackets. PCRE2 also + enclosed by [: and :] within the enclosing square brackets. PCRE2 also supports this notation. For example, [01[:alpha:]%] @@ -7196,13 +7201,13 @@ POSIX CHARACTER CLASSES word "word" characters (same as \w) xdigit hexadecimal digits - 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 + 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" and \s match the same set of characters. - The name "word" is a Perl extension, and "blank" is a GNU extension - from Perl 5.8. Another Perl extension is negation, which is indicated + The name "word" is a Perl extension, and "blank" is a GNU extension + from Perl 5.8. Another Perl extension is negation, which is indicated by a ^ character after the colon. For example, [12[:^digit:]] @@ -7213,9 +7218,9 @@ POSIX CHARACTER CLASSES By default, characters with values greater than 127 do not match any of the POSIX character classes, although this may be different for charac- - ters in the range 128-255 when locale-specific matching is happening. - However, if the PCRE2_UCP option is passed to pcre2_compile(), some of - the classes are changed so that Unicode character properties are used. + ters in the range 128-255 when locale-specific matching is happening. + However, if the PCRE2_UCP option is passed to pcre2_compile(), some of + the classes are changed so that Unicode character properties are used. This is achieved by replacing certain POSIX classes with other sequences, as follows: @@ -7229,10 +7234,10 @@ POSIX CHARACTER CLASSES [:upper:] becomes \p{Lu} [:word:] becomes \p{Xwd} - Negated versions, such as [:^alpha:] use \P instead of \p. Three other + 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 + [:graph:] This matches characters that have glyphs that mark the page when printed. In Unicode property terms, it matches all char- acters with the L, M, N, P, S, or Cf properties, except for: @@ -7241,60 +7246,60 @@ POSIX CHARACTER CLASSES 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 + [: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 (punctua- - tion) property, plus those characters with code points less + tion) property, plus those characters with code points less than 256 that have the S (Symbol) property. - The other POSIX classes are unchanged, and match only characters with + The other POSIX classes are unchanged, and match only characters with code points less than 256. COMPATIBILITY FEATURE FOR WORD BOUNDARIES - In the POSIX.2 compliant library that was included in 4.4BSD Unix, the - ugly syntax [[:<:]] and [[:>:]] is used for matching "start of word" + 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". PCRE2 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 + [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 asser- - tions" 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 be- + that \b matches at the start and the end of a word (see "Simple asser- + tions" 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 be- haviour. VERTICAL BAR - Vertical bar characters are used to separate alternative patterns. For + Vertical bar characters are used to separate alternative patterns. For example, the pattern gilbert|sullivan - matches either "gilbert" or "sullivan". Any number of alternatives may - appear, and an empty alternative is permitted (matching the empty + matches either "gilbert" or "sullivan". Any number of alternatives may + appear, and an empty alternative is permitted (matching the empty string). The matching process tries each alternative in turn, from left - to right, and the first one that succeeds is used. If the alternatives - are within a subpattern (defined below), "succeeds" means matching the + to right, and the first one that succeeds is used. If the alternatives + are within a subpattern (defined below), "succeeds" means matching the rest of the main pattern as well as the alternative in the subpattern. INTERNAL OPTION SETTING - The settings of the PCRE2_CASELESS, PCRE2_MULTILINE, PCRE2_DOTALL, - PCRE2_EXTENDED, PCRE2_EXTENDED_MORE, and PCRE2_NO_AUTO_CAPTURE options - can be changed from within the pattern by a sequence of letters - enclosed between "(?" and ")". These options are Perl-compatible, and - are described in detail in the pcre2api documentation. The option let- + The settings of the PCRE2_CASELESS, PCRE2_MULTILINE, PCRE2_DOTALL, + PCRE2_EXTENDED, PCRE2_EXTENDED_MORE, and PCRE2_NO_AUTO_CAPTURE options + can be changed from within the pattern by a sequence of letters + enclosed between "(?" and ")". These options are Perl-compatible, and + are described in detail in the pcre2api documentation. The option let- ters are: i for PCRE2_CASELESS @@ -7305,49 +7310,49 @@ INTERNAL OPTION SETTING xx for PCRE2_EXTENDED_MORE For example, (?im) sets caseless, multiline matching. It is also possi- - ble to unset these options by preceding the relevant letters with a + ble to unset these options by preceding the relevant letters with a hyphen, for example (?-im). The two "extended" options are not indepen- dent; unsetting either one cancels the effects of both of them. - A combined setting and unsetting such as (?im-sx), which sets - PCRE2_CASELESS and PCRE2_MULTILINE while unsetting PCRE2_DOTALL and - PCRE2_EXTENDED, is also permitted. Only one hyphen may appear in the - options string. If a letter appears both before and after the hyphen, - the option is unset. An empty options setting "(?)" is allowed. Need- + A combined setting and unsetting such as (?im-sx), which sets + PCRE2_CASELESS and PCRE2_MULTILINE while unsetting PCRE2_DOTALL and + PCRE2_EXTENDED, is also permitted. Only one hyphen may appear in the + options string. If a letter appears both before and after the hyphen, + the option is unset. An empty options setting "(?)" is allowed. Need- less to say, it has no effect. - If the first character following (? is a circumflex, it causes all of - the above options to be unset. Thus, (?^) is equivalent to (?-imnsx). - Letters may follow the circumflex to cause some options to be re- + If the first character following (? is a circumflex, it causes all of + the above options to be unset. Thus, (?^) is equivalent to (?-imnsx). + Letters may follow the circumflex to cause some options to be re- instated, but a hyphen may not appear. - The PCRE2-specific options PCRE2_DUPNAMES and PCRE2_UNGREEDY can be - changed in the same way as the Perl-compatible options by using the + The PCRE2-specific options PCRE2_DUPNAMES and PCRE2_UNGREEDY can be + changed in the same way as the Perl-compatible options by using the characters J and U respectively. However, these are not unset by (?^). - When one of these option changes occurs at top level (that is, not - inside subpattern parentheses), the change applies to the remainder of - the pattern that follows. An option change within a subpattern (see - below for a description of subpatterns) affects only that part of the + When one of these option changes occurs at top level (that is, not + inside subpattern parentheses), the change applies to the remainder of + the pattern that follows. An option change within a subpattern (see + below for a description of subpatterns) affects only that part of the subpattern that follows it, so (a(?i)b)c - matches abc and aBc and no other strings (assuming PCRE2_CASELESS is - not used). By this means, options can be made to have different set- + matches abc and aBc and no other strings (assuming PCRE2_CASELESS is + not used). By this means, options can be made to have different set- tings in different parts of the pattern. Any changes made in one alter- native do carry on into subsequent branches within the same subpattern. For example, (a(?i)b|c) - matches "ab", "aB", "c", and "C", even though when matching "C" the - first branch is abandoned before the option setting. This is because - the effects of option settings happen at compile time. There would be + matches "ab", "aB", "c", and "C", even though when matching "C" the + first branch is abandoned before the option setting. This is because + the effects of option settings happen at compile time. There would be some very weird behaviour otherwise. - As a convenient shorthand, if any option settings are required at the - start of a non-capturing subpattern (see the next section), the option + As a convenient shorthand, if any option settings are required at the + start of a non-capturing subpattern (see the next section), the option letters may appear between the "?" and the ":". Thus the two patterns (?i:saturday|sunday) @@ -7355,14 +7360,14 @@ INTERNAL OPTION SETTING match exactly the same set of strings. - Note: There are other PCRE2-specific options that can be set by the + Note: There are other PCRE2-specific options that can be set by the application when the compiling function is called. The pattern can con- - tain 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 - (*UTF) and (*UCP) leading sequences that can be used to set UTF and - Unicode property modes; they are equivalent to setting the PCRE2_UTF - and PCRE2_UCP options, respectively. However, the application can set + tain 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 + (*UTF) and (*UCP) leading sequences that can be used to set UTF and + Unicode property modes; they are equivalent to setting the PCRE2_UTF + and PCRE2_UCP options, respectively. However, the application can set the PCRE2_NEVER_UTF and PCRE2_NEVER_UCP options, which lock out the use of the (*UTF) and (*UCP) sequences. @@ -7376,18 +7381,18 @@ SUBPATTERNS cat(aract|erpillar|) - matches "cataract", "caterpillar", or "cat". Without the parentheses, + matches "cataract", "caterpillar", or "cat". Without the parentheses, it would match "cataract", "erpillar" or an empty string. - 2. It sets up the subpattern as a capturing subpattern. This means + 2. It sets up the subpattern as a capturing subpattern. This means that, when the whole pattern matches, the portion of the subject string - that matched the subpattern is passed back to the caller, separately - from the portion that matched the whole pattern. (This applies only to - the traditional matching function; the DFA matching function does not + that matched the subpattern is passed back to the caller, separately + from the portion that matched the whole pattern. (This applies only to + the traditional matching function; the DFA matching function does 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 + obtain numbers for the capturing subpatterns. For example, if the string "the red king" is matched against the pattern the ((red|white) (king|queen)) @@ -7395,12 +7400,12 @@ SUBPATTERNS the captured substrings are "red king", "red", and "king", and are num- bered 1, 2, and 3, respectively. - The fact that plain parentheses fulfil two functions is not always - helpful. There are often times when a grouping subpattern is required - without a capturing requirement. If an opening parenthesis is followed - by a question mark and a colon, the subpattern does not do any captur- - ing, and is not counted when computing the number of any subsequent - capturing subpatterns. For example, if the string "the white queen" is + The fact that plain parentheses fulfil two functions is not always + helpful. There are often times when a grouping subpattern is required + without a capturing requirement. If an opening parenthesis is followed + by a question mark and a colon, the subpattern does not do any captur- + ing, and is not counted when computing the number of any subsequent + capturing subpatterns. For example, if the string "the white queen" is matched against the pattern the ((?:red|white) (king|queen)) @@ -7408,37 +7413,37 @@ SUBPATTERNS the captured substrings are "white queen" and "queen", and are numbered 1 and 2. The maximum number of capturing subpatterns is 65535. - As a convenient shorthand, if any option settings are required at the - start of a non-capturing subpattern, the option letters may appear + As a convenient shorthand, if any option settings are required at the + start of a non-capturing subpattern, the option letters may appear between the "?" and the ":". Thus the two patterns (?i:saturday|sunday) (?:(?i)saturday|sunday) match exactly the same set of strings. Because alternative branches are - tried from left to right, and options are not reset until the end of - the subpattern is reached, an option setting in one branch does affect - subsequent branches, so the above patterns match "SUNDAY" as well as + tried from left to right, and options are not reset until the end of + the subpattern is reached, an option setting in one branch does affect + subsequent branches, so the above patterns match "SUNDAY" as well as "Saturday". 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 (?| and is itself a non-capturing subpattern. For example, + uses the same numbers for its capturing parentheses. Such a subpattern + starts with (?| and is itself a non-capturing subpattern. For example, consider this pattern: (?|(Sat)ur|(Sun))day - Because the two alternatives are inside a (?| group, both sets of cap- - turing parentheses are numbered one. Thus, when the pattern matches, - you can look at captured substring number one, whichever alternative - matched. This construct is useful when you want to capture part, but + Because the two alternatives are inside a (?| group, both sets of cap- + turing parentheses are numbered one. Thus, when the pattern matches, + you can look at captured substring number one, whichever alternative + matched. This construct is useful when you want to capture part, but not all, of one of a number of alternatives. Inside a (?| group, paren- - theses are numbered as usual, but the number is reset at the start of - each branch. The numbers of any capturing parentheses that follow the - subpattern start after the highest number used in any branch. The fol- + theses are numbered as usual, but the number is reset at the start of + each branch. The numbers of any capturing parentheses that follow the + subpattern start after the highest number used in any branch. The fol- lowing example is taken from the Perl documentation. The numbers under- neath show in which buffer the captured content will be stored. @@ -7446,14 +7451,14 @@ DUPLICATE SUBPATTERN NUMBERS / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x # 1 2 2 3 2 3 4 - A backreference to a numbered subpattern uses the most recent value - that is set for that number by any subpattern. The following pattern + A backreference to a numbered subpattern uses the most recent value + that is set for that number by any subpattern. The following pattern matches "abcabc" or "defdef": /(?|(abc)|(def))\1/ - In contrast, a subroutine call to a numbered subpattern always refers - to the first one in the pattern with the given number. The following + In contrast, a subroutine call to a numbered subpattern always refers + to the first one in the pattern with the given number. The following pattern matches "abcabc" or "defabc": /(?|(abc)|(def))(?1)/ @@ -7461,75 +7466,75 @@ DUPLICATE SUBPATTERN NUMBERS A relative reference such as (?-1) is no different: it is just a conve- nient way of computing an absolute group number. - If a condition test for a subpattern's having matched refers to a non- - unique number, the test is true if any of the subpatterns of that num- + If a condition test for a subpattern's having matched refers to a non- + unique number, the test is true if any of the subpatterns of that num- ber have matched. - An alternative approach to using this "branch reset" feature is to use + An alternative approach to using this "branch reset" feature is to use duplicate named subpatterns, as described in the next section. NAMED SUBPATTERNS - Identifying capturing parentheses by number is simple, but it can be - very hard to keep track of the numbers in complicated patterns. Fur- + Identifying capturing parentheses by number is simple, but it can be + very hard to keep track of the numbers in complicated patterns. Fur- thermore, if an expression is modified, the numbers may change. To help - with this difficulty, PCRE2 supports the naming of capturing subpat- - terns. This feature was not added to Perl until release 5.10. Python - had the feature earlier, and PCRE1 introduced it at release 4.0, using + with this difficulty, PCRE2 supports the naming of capturing subpat- + terns. This feature was not added to Perl until release 5.10. Python + had the feature earlier, and PCRE1 introduced it at release 4.0, using the Python syntax. PCRE2 supports both the Perl and the Python syntax. - In PCRE2, a capturing subpattern can be named in one of three ways: + In PCRE2, a capturing subpattern can be named in one of three ways: (?...) or (?'name'...) as in Perl, or (?P ...) as in Python. - Names consist of up to 32 alphanumeric characters and underscores, but - must start with a non-digit. References to capturing parentheses from + Names consist of up to 32 alphanumeric characters and underscores, but + must start with a non-digit. References to capturing parentheses from other parts of the pattern, such as backreferences, recursion, and con- ditions, can all be made by name as well as by number. - Named capturing parentheses are allocated numbers as well as names, - exactly as if the names were not present. In both PCRE2 and Perl, cap- - turing subpatterns are primarily identified by numbers; any names are - just aliases for these numbers. The PCRE2 API provides function calls - for extracting the complete name-to-number translation table from a - compiled pattern, as well as convenience functions for extracting cap- + Named capturing parentheses are allocated numbers as well as names, + exactly as if the names were not present. In both PCRE2 and Perl, cap- + turing subpatterns are primarily identified by numbers; any names are + just aliases for these numbers. The PCRE2 API provides function calls + for extracting the complete name-to-number translation table from a + compiled pattern, as well as convenience functions for extracting cap- tured substrings by name. - Warning: When more than one subpattern has the same number, as - described in the previous section, a name given to one of them applies - to all of them. Perl allows identically numbered subpatterns to have - different names. Consider this pattern, where there are two capturing + Warning: When more than one subpattern has the same number, as + described in the previous section, a name given to one of them applies + to all of them. Perl allows identically numbered subpatterns to have + different names. Consider this pattern, where there are two capturing subpatterns, both numbered 1: (?|(? aa)|(? bb)) - Perl allows this, with both names AA and BB as aliases of group 1. + Perl allows this, with both names AA and BB as aliases of group 1. Thus, after a successful match, both names yield the same value (either "aa" or "bb"). - In an attempt to reduce confusion, PCRE2 does not allow the same group + In an attempt to reduce confusion, PCRE2 does not allow the same group number to be associated with more than one name. The example above pro- - vokes a compile-time error. However, there is still scope for confu- + vokes a compile-time error. However, there is still scope for confu- sion. Consider this pattern: (?|(? aa)|(bb)) - Although the second subpattern number 1 is not explicitly named, the + Although the second subpattern number 1 is not explicitly named, the name AA is still an alias for subpattern 1. Whether the pattern matches - "aa" or "bb", a reference by name to group AA yields the matched + "aa" or "bb", a reference by name to group AA yields the matched string. - By default, a name must be unique within a pattern, except that dupli- - cate names are permitted for subpatterns with the same number, for + By default, a name must be unique within a pattern, except that dupli- + cate names are permitted for subpatterns with the same number, for example: (?|(? aa)|(? bb)) The duplicate name constraint can be disabled by setting the PCRE2_DUP- NAMES option at compile time, or by the use of (?J) within the pattern. - Duplicate names can be useful for patterns where only one instance of - the named parentheses can match. Suppose you want to match the name of - a weekday, either as a 3-letter abbreviation or as the full name, and - in both cases you want to extract the abbreviation. This pattern + Duplicate names can be useful for patterns where only one instance of + the named parentheses can match. Suppose you want to match the name of + a weekday, either as a 3-letter abbreviation or as the full name, and + in both cases you want to extract the abbreviation. This pattern (ignoring the line breaks) does the job: (? Mon|Fri|Sun)(?:day)?| @@ -7538,17 +7543,17 @@ NAMED SUBPATTERNS (? Thu)(?:rsday)?| (? Sat)(?:urday)? - There are five capturing substrings, but only one is ever set after a - match. The convenience functions for extracting the data by name - returns the substring for the first (and in this example, the only) - subpattern of that name that matched. This saves searching to find - which numbered subpattern it was. (An alternative way of solving this + There are five capturing substrings, but only one is ever set after a + match. The convenience functions for extracting the data by name + returns the substring for the first (and in this example, the only) + subpattern of that name that matched. This saves searching to find + which numbered subpattern it was. (An alternative way of solving this problem is to use a "branch reset" subpattern, as described in the pre- vious section.) If you make a backreference to a non-unique named subpattern from else- - where in the pattern, the subpatterns to which the name refers are - checked in the order in which they appear in the overall pattern. The + where in 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 pat- tern matches both "foofoo" and "barbar" but not "foobar" or "barfoo": @@ -7556,21 +7561,21 @@ NAMED SUBPATTERNS 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 + that corresponds to the first occurrence of the name is used. In the absence of duplicate numbers 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 recursion, all subpatterns with the same name are tested. - If the condition is true for any one of them, the overall condition is - true. This is the same behaviour as testing by number. For further - details of the interfaces for handling named subpatterns, see the + to check for recursion, all subpatterns with the same name are tested. + If the condition is true for any one of them, the overall condition is + true. This is the same behaviour as testing by number. For further + details of the interfaces for handling named subpatterns, see the pcre2api documentation. REPETITION - Repetition is specified by quantifiers, which can follow any of the + Repetition is specified by quantifiers, which can follow any of the following items: a literal data character @@ -7584,17 +7589,17 @@ REPETITION a parenthesized subpattern (including most assertions) a subroutine call to a subpattern (recursive or otherwise) - The general repetition quantifier specifies a minimum and maximum num- - ber of permitted matches, by giving the two numbers in curly brackets - (braces), separated by a comma. The numbers must be less than 65536, + The general repetition quantifier specifies a minimum and maximum num- + ber of permitted matches, by giving the two numbers in curly brackets + (braces), separated by a comma. The numbers must be less than 65536, and the first must be less than or equal to the second. For example: z{2,4} - matches "zz", "zzz", or "zzzz". A closing brace on its own is not a - special character. If the second number is omitted, but the comma is - present, there is no upper limit; if the second number and the comma - are both omitted, the quantifier specifies an exact number of required + matches "zz", "zzz", or "zzzz". A closing brace on its own is not a + special character. If the second number is omitted, but the comma is + present, there is no upper limit; if the second number and the comma + are both omitted, the quantifier specifies an exact number of required matches. Thus [aeiou]{3,} @@ -7603,50 +7608,50 @@ REPETITION \d{8} - matches exactly 8 digits. An opening curly bracket that appears in a - position where a quantifier is not allowed, or one that does not match - the syntax of a quantifier, is taken as a literal character. For exam- + matches exactly 8 digits. An opening curly bracket that appears in a + position where a quantifier is not allowed, or one that does not match + the syntax of a quantifier, is taken as a literal character. For exam- ple, {,6} is not a quantifier, but a literal string of four characters. In UTF modes, quantifiers apply to characters rather than to individual - code units. Thus, for example, \x{100}{2} matches two characters, each + code 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. Simi- - larly, \X{3} matches three Unicode extended grapheme clusters, each of - which may be several code units long (and they may be of different + larly, \X{3} matches three Unicode extended grapheme clusters, each of + which may be several code units long (and they may be of different lengths). The quantifier {0} is permitted, causing the expression to behave as if the previous item and the quantifier were not present. This may be use- - ful for subpatterns that are referenced as subroutines from elsewhere + ful for subpatterns that are referenced as subroutines from elsewhere in the pattern (but see also the section entitled "Defining subpatterns - for use by reference only" below). Items other than subpatterns that + for use by reference only" below). Items other than subpatterns that have a {0} quantifier are omitted from the compiled pattern. - For convenience, the three most common quantifiers have single-charac- + For convenience, the three most common quantifiers have single-charac- ter abbreviations: * is equivalent to {0,} + is equivalent to {1,} ? is equivalent to {0,1} - It is possible to construct infinite loops by following a subpattern + It is possible to construct infinite loops by following a subpattern that can match no characters with a quantifier that has no upper limit, for example: (a?)* - Earlier versions of Perl and PCRE1 used to give an error at compile + Earlier versions of Perl and PCRE1 used to give an error at compile time for such patterns. However, because there are cases where this can be useful, such patterns are now accepted, but if any repetition of the - subpattern does in fact match no characters, the loop is forcibly bro- + subpattern does in fact match no characters, the loop is forcibly bro- ken. - By default, the quantifiers are "greedy", that is, they match as much - as possible (up to the maximum number of permitted times), without - causing the rest of the pattern to fail. The classic example of where + By default, the quantifiers are "greedy", that is, they match as much + as possible (up to the maximum number of permitted times), without + causing the rest of the pattern to fail. The classic example of where this gives problems is in trying to match comments in C programs. These - appear between /* and */ and within the comment, individual * and / - characters may appear. An attempt to match C comments by applying the + appear between /* and */ and within the comment, individual * and / + characters may appear. An attempt to match C comments by applying the pattern /\*.*\*/ @@ -7655,19 +7660,19 @@ REPETITION /* first comment */ not comment /* second comment */ - fails, because it matches the entire string owing to the greediness of + fails, because it matches the entire string owing to the greediness of the .* item. If a quantifier is followed by a question mark, it ceases to be greedy, - and instead matches the minimum number of times possible, so the pat- + and instead matches the minimum number of times possible, so the pat- tern /\*.*?\*/ - does the right thing with the C comments. The meaning of the various - quantifiers is not otherwise changed, just the preferred number of - matches. Do not confuse this use of question mark with its use as a - quantifier in its own right. Because it has two uses, it can sometimes + does the right thing with the C comments. The meaning of the various + quantifiers is not otherwise changed, just the preferred number of + matches. Do not confuse this use of question mark with its use as a + quantifier in its own right. Because it has two uses, it can sometimes appear doubled, as in \d??\d @@ -7676,45 +7681,45 @@ REPETITION only way the rest of the pattern matches. If the PCRE2_UNGREEDY option is set (an option that is not available in - Perl), the quantifiers are not greedy by default, but individual ones - can be made greedy by following them with a question mark. In other + Perl), the quantifiers are not greedy by default, but individual ones + can be made greedy by following them with a question mark. In other words, it inverts the default behaviour. - When a parenthesized subpattern is quantified with a minimum repeat - count that is greater than 1 or with a limited maximum, more memory is - required for the compiled pattern, in proportion to the size of the + When a parenthesized subpattern is quantified with a minimum repeat + count that is greater than 1 or with a limited maximum, more memory is + required for the compiled pattern, in proportion to the size of the minimum or maximum. - If a pattern starts with .* or .{0,} and the PCRE2_DOTALL option - (equivalent to Perl's /s) is set, thus allowing the dot to match new- - lines, the pattern is implicitly anchored, because whatever follows - will be tried against every character position in the subject string, - so there is no point in retrying the overall match at any position + If a pattern starts with .* or .{0,} and the PCRE2_DOTALL option + (equivalent to Perl's /s) is set, thus allowing the dot to match new- + lines, the pattern is implicitly anchored, because whatever follows + will be tried against every character position in the subject string, + so there is no point in retrying the overall match at any position after the first. PCRE2 normally treats such a pattern as though it were preceded by \A. - In cases where it is known that the subject string contains no new- - lines, it is worth setting PCRE2_DOTALL in order to obtain this opti- + In cases where it is known that the subject string contains no new- + lines, it is worth setting PCRE2_DOTALL in order to obtain this opti- mization, or alternatively, using ^ to indicate anchoring explicitly. - However, there are some cases where the optimization cannot be used. - When .* is inside capturing parentheses that are the subject of a - backreference elsewhere in the pattern, a match at the start may fail + However, there are some cases where the optimization cannot be used. + When .* is inside capturing parentheses that are the subject of a + backreference elsewhere in the pattern, a match at the start may fail where a later one succeeds. Consider, for example: (.*)abc\1 - If the subject is "xyz123abc123" the match point is the fourth charac- + If the subject is "xyz123abc123" the match point is the fourth charac- ter. For this reason, such a pattern is not implicitly anchored. - Another case where implicit anchoring is not applied is when the lead- - ing .* is inside an atomic group. Once again, a match at the start may + Another case where implicit anchoring is not applied is when the lead- + ing .* 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 con- - trol verbs (*PRUNE) and (*SKIP) also disable this optimization, and + It matches "ab" in the subject "aab". The use of the backtracking con- + trol verbs (*PRUNE) and (*SKIP) also disable this optimization, and there is an option, PCRE2_NO_DOTSTAR_ANCHOR, to do so explicitly. When a capturing subpattern is repeated, the value captured is the sub- @@ -7723,8 +7728,8 @@ REPETITION (tweedle[dume]{3}\s*)+ has matched "tweedledum tweedledee" the value of the captured substring - is "tweedledee". However, if there are nested capturing subpatterns, - the corresponding captured values may have been set in previous itera- + is "tweedledee". However, if there are nested capturing subpatterns, + the corresponding captured values may have been set in previous itera- tions. For example, after (a|(b))+ @@ -7734,58 +7739,58 @@ REPETITION 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 re-evaluated to see if a different number of repeats allows the - rest of the pattern to match. Sometimes it is useful to prevent this, - either to change the nature of the match, or to cause it fail earlier - than it otherwise might, when the author of the pattern knows there is + With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy") + repetition, failure of what follows normally causes the repeated item + to be re-evaluated to see if a different number of repeats allows the + rest of the pattern to match. Sometimes it is useful to prevent this, + either to change the nature of the match, or to cause it fail earlier + than it otherwise might, when the author of the pattern knows there is no point in carrying on. - Consider, for example, the pattern \d+foo when applied to the subject + Consider, for example, the pattern \d+foo when applied to the subject line 123456bar After matching all 6 digits and then failing to match "foo", the normal - action of the matcher is to try again with only 5 digits matching the - \d+ item, and then with 4, and so on, before ultimately failing. - "Atomic grouping" (a term taken from Jeffrey Friedl's book) provides - the means for specifying that once a subpattern has matched, it is not + action of the matcher is to try again with only 5 digits matching the + \d+ item, and then with 4, and so on, before ultimately failing. + "Atomic grouping" (a term taken from Jeffrey Friedl's book) provides + the means for specifying that once a subpattern has matched, it is not to be re-evaluated in this way. - If we use atomic grouping for the previous example, the matcher gives - up immediately on failing to match "foo" the first time. The notation + If we use atomic grouping for the previous example, the matcher gives + up immediately on failing to match "foo" the first time. The notation is a kind of special parenthesis, starting with (?> as in this example: (?>\d+)foo - Perl 5.28 introduced an experimental alphabetic form starting with (* + Perl 5.28 introduced an experimental alphabetic form starting with (* which may be easier to remember: (*atomic:\d+)foo - This kind of parenthesis "locks up" the part of the pattern it con- - tains once it has matched, and a failure further into the pattern is - prevented from backtracking into it. Backtracking past it to previous + This kind of parenthesis "locks up" the part of the pattern it con- + tains once it has matched, and a failure further into the pattern is + prevented from backtracking into it. Backtracking past it to previous items, however, works as normal. - An alternative description is that a subpattern of this type matches - exactly the string of characters that an identical standalone pattern + An alternative description is that a subpattern of this type matches + exactly the string of characters that an identical standalone pattern would match, if anchored at the current point in the subject string. Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as the above example can be thought of as a maximizing repeat that - must swallow everything it can. So, while both \d+ and \d+? are pre- - pared to adjust the number of digits they match in order to make the + must swallow everything it can. So, while both \d+ and \d+? are pre- + pared to adjust the number of digits they match in order to make the rest of the pattern match, (?>\d+) can only match an entire sequence of digits. - Atomic groups in general can of course contain arbitrarily complicated - subpatterns, and can be nested. However, when the subpattern for an + Atomic groups in general can of course contain arbitrarily complicated + subpatterns, and can be nested. However, when the subpattern for an atomic group is just a single repeated item, as in the example above, a - simpler notation, called a "possessive quantifier" can be used. This - consists of an additional + character following a quantifier. Using + simpler notation, called a "possessive quantifier" can be used. This + consists of an additional + character following a quantifier. Using this notation, the previous example can be rewritten as \d++foo @@ -7795,46 +7800,46 @@ ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS (abc|xyz){2,3}+ - Possessive quantifiers are always greedy; the setting of the - PCRE2_UNGREEDY option is ignored. They are a convenient notation for - the simpler forms of atomic group. However, there is no difference in + Possessive quantifiers are always greedy; the setting of the + PCRE2_UNGREEDY option is ignored. They are a convenient notation for + the simpler forms of atomic group. However, there is no difference in the meaning of a possessive quantifier and the equivalent atomic group, - though there may be a performance difference; possessive quantifiers + though there may be a performance difference; possessive quantifiers should be slightly faster. - The possessive quantifier syntax is an extension to the Perl 5.8 syn- - tax. Jeffrey Friedl originated the idea (and the name) in the first + The possessive quantifier syntax is an extension to the Perl 5.8 syn- + tax. Jeffrey Friedl originated the idea (and the name) in the first edition of his book. Mike McCloskey liked it, so implemented it when he built Sun's Java package, and PCRE1 copied it from there. It ultimately found its way into Perl at release 5.10. - PCRE2 has an optimization that automatically "possessifies" certain - simple pattern constructs. For example, the sequence A+B is treated as - A++B because there is no point in backtracking into a sequence of A's + PCRE2 has an optimization that automatically "possessifies" certain + simple pattern constructs. For example, the sequence A+B is treated as + A++B because there is no point in backtracking into a sequence of A's when B must follow. This feature can be disabled by the PCRE2_NO_AUTO- POSSESS option, or starting the pattern with (*NO_AUTO_POSSESS). - When a pattern contains an unlimited repeat inside a subpattern that - can itself be repeated an unlimited number of times, the use of an - atomic group is the only way to avoid some failing matches taking a + When a pattern contains an unlimited repeat inside a subpattern that + can itself be repeated an unlimited number of times, the use of an + atomic group is the only way to avoid some failing matches taking a very long time indeed. The pattern (\D+|<\d+>)*[!?] - matches an unlimited number of substrings that either consist of non- - digits, or digits enclosed in <>, followed by either ! or ?. When it + matches an unlimited number of substrings that either consist of non- + digits, or digits enclosed in <>, followed by either ! or ?. When it matches, it runs quickly. However, if it is applied to aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa - it takes a long time before reporting failure. This is because the - string can be divided between the internal \D+ repeat and the external - * repeat in a large number of ways, and all have to be tried. (The - example uses [!?] rather than a single character at the end, because - both PCRE2 and Perl have an optimization that allows for fast failure - when a single character is used. They remember the last single charac- - ter that is required for a match, and fail early if it is not present - in the string.) If the pattern is changed so that it uses an atomic + it takes a long time before reporting failure. This is because the + string can be divided between the internal \D+ repeat and the external + * repeat in a large number of ways, and all have to be tried. (The + example uses [!?] rather than a single character at the end, because + both PCRE2 and Perl have an optimization that allows for fast failure + when a single character is used. They remember the last single charac- + ter that is required for a match, and fail early if it is not present + in the string.) If the pattern is changed so that it uses an atomic group, like this: ((?>\D+)|<\d+>)*[!?] @@ -7845,29 +7850,29 @@ ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS BACKREFERENCES Outside a character class, a backslash followed by a digit greater than - 0 (and possibly further digits) is a backreference to a capturing sub- - pattern earlier (that is, to its left) in the pattern, provided there + 0 (and possibly further digits) is a backreference to a capturing sub- + pattern earlier (that is, to its left) in the pattern, provided there have been that many previous capturing left parentheses. - However, if the decimal number following the backslash is less than 8, - it is always taken as a backreference, and causes an error only if - there are not that many capturing left parentheses in the entire pat- - tern. In other words, the parentheses that are referenced need not be - to the left of the reference for numbers less than 8. A "forward back- - reference" of this type can make sense when a repetition is involved - and the subpattern to the right has participated in an earlier itera- + However, if the decimal number following the backslash is less than 8, + it is always taken as a backreference, and causes an error only if + there are not that many capturing left parentheses in the entire pat- + tern. In other words, the parentheses that are referenced need not be + to the left of the reference for numbers less than 8. A "forward back- + reference" of this type can make sense when a repetition is involved + and the subpattern to the right has participated in an earlier itera- tion. - It is not possible to have a numerical "forward backreference" to a - subpattern whose number is 8 or more using this syntax because a - sequence such as \50 is interpreted as a character defined in octal. + It is not possible to have a numerical "forward backreference" to a + subpattern whose number is 8 or more using this syntax because a + sequence such as \50 is interpreted as a character defined in octal. See the subsection entitled "Non-printing characters" above for further - details of the handling of digits following a backslash. There is no - such problem when named parentheses are used. A backreference to any + details of the handling of digits following a backslash. There is no + such problem when named parentheses are used. A backreference to any subpattern is possible using named parentheses (see below). - Another way of avoiding the ambiguity inherent in the use of digits - following a backslash is to use the \g escape sequence. This escape + Another way of avoiding the ambiguity inherent in the use of digits + following a backslash is to use the \g escape sequence. This escape must be followed by a signed or unsigned number, optionally enclosed in braces. These examples are all identical: @@ -7875,46 +7880,46 @@ BACKREFERENCES (ring), \g1 (ring), \g{1} - An unsigned number specifies an absolute reference without the ambigu- + An unsigned number specifies an absolute reference without the ambigu- ity that is present in the older syntax. It is also useful when literal - digits follow the reference. A signed number is a relative reference. + digits follow the reference. A signed number is a relative reference. Consider this example: (abc(def)ghi)\g{-1} The sequence \g{-1} is a reference to the most recently started captur- ing subpattern before \g, that is, is it equivalent to \2 in this exam- - ple. Similarly, \g{-2} would be equivalent to \1. The use of relative - references can be helpful in long patterns, and also in patterns that - are created by joining together fragments that contain references + ple. Similarly, \g{-2} would be equivalent to \1. The use of relative + references can be helpful in long patterns, and also in patterns that + are created by joining together fragments that contain references within themselves. - The sequence \g{+1} is a reference to the next capturing subpattern. - This kind of forward reference can be useful it patterns that repeat. + The sequence \g{+1} is a reference to the next capturing subpattern. + This kind of forward reference can be useful it patterns that repeat. Perl does not support the use of + in this way. A backreference matches whatever actually matched the capturing subpat- - tern in the current subject string, rather than anything matching the - subpattern itself (see "Subpatterns as subroutines" below for a way of + tern in the current subject string, rather than anything matching the + subpattern itself (see "Subpatterns as subroutines" below for a way of doing that). So the pattern (sens|respons)e and \1ibility - matches "sense and sensibility" and "response and responsibility", but - not "sense and responsibility". If caseful matching is in force at the - time of the backreference, the case of letters is relevant. For exam- + matches "sense and sensibility" and "response and responsibility", but + not "sense and responsibility". If caseful matching is in force at the + time of the backreference, the case of letters is relevant. For exam- ple, ((?i)rah)\s+\1 - matches "rah rah" and "RAH RAH", but not "RAH rah", even though the + matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original capturing subpattern is matched caselessly. - There are several different ways of writing backreferences to named - subpatterns. The .NET syntax \k{name} and the Perl syntax \k or - \k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's - unified backreference syntax, in which \g can be used for both numeric - and named references, is also supported. We could rewrite the above + There are several different ways of writing backreferences to named + subpatterns. The .NET syntax \k{name} and the Perl syntax \k or + \k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's + unified backreference syntax, in which \g can be used for both numeric + and named references, is also supported. We could rewrite the above example in any of the following ways: (? (?i)rah)\s+\k @@ -7922,32 +7927,32 @@ BACKREFERENCES (?P (?i)rah)\s+(?P=p1) (? (?i)rah)\s+\g{p1} - A subpattern that is referenced by name may appear in the pattern + A subpattern that is referenced by name may appear in the pattern before or after the reference. - There may be more than one backreference to the same subpattern. If a - subpattern has not actually been used in a particular match, any back- + There may be more than one backreference to the same subpattern. If a + subpattern has not actually been used in a particular match, any back- references to it always fail by default. For example, the pattern (a|(bc))\2 - always fails if it starts to match "a" rather than "bc". However, if + always fails if it starts to match "a" rather than "bc". However, if the PCRE2_MATCH_UNSET_BACKREF option is set at compile time, a backref- erence to an unset value matches an empty string. - Because there may be many capturing parentheses in a pattern, all dig- - its following a backslash are taken as part of a potential backrefer- - ence number. If the pattern continues with a digit character, some - delimiter must be used to terminate the backreference. If the - PCRE2_EXTENDED or PCRE2_EXTENDED_MORE option is set, this can be white - space. Otherwise, the \g{ syntax or an empty comment (see "Comments" + Because there may be many capturing parentheses in a pattern, all dig- + its following a backslash are taken as part of a potential backrefer- + ence number. If the pattern continues with a digit character, some + delimiter must be used to terminate the backreference. If the + PCRE2_EXTENDED or PCRE2_EXTENDED_MORE option is set, this can be white + space. Otherwise, the \g{ syntax or an empty comment (see "Comments" below) can be used. Recursive backreferences - A backreference that occurs inside the parentheses to which it refers - fails when the subpattern is first used, so, for example, (a\1) never - matches. However, such references can be useful inside repeated sub- + A backreference that occurs inside the parentheses to which it refers + fails when the subpattern is first used, so, for example, (a\1) never + matches. However, such references can be useful inside repeated sub- patterns. For example, the pattern (a|b\1)+ @@ -7955,86 +7960,86 @@ BACKREFERENCES matches any number of "a"s and also "aba", "ababbaa" etc. At each iter- ation of the subpattern, the backreference matches the character string corresponding to the previous iteration. In order for this to work, the - pattern must be such that the first iteration does not need to match - the backreference. This can be done using alternation, as in the exam- + pattern must be such that the first iteration does not need to match + the backreference. This can be done using alternation, as in the exam- ple above, or by a quantifier with a minimum of zero. - Backreferences of this type cause the group that they reference to be - treated as an atomic group. Once the whole group has been matched, a - subsequent matching failure cannot cause backtracking into the middle + Backreferences of this type cause the group that they reference to be + treated as an atomic group. Once the whole group has been matched, a + subsequent matching failure cannot cause backtracking into the middle of the group. ASSERTIONS - An assertion is a test on the characters following or preceding the + An assertion is a test on the characters following or preceding the current matching point that does not consume any characters. The simple - assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are described + assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are described above. - More complicated assertions are coded as subpatterns. There are two - kinds: those that look ahead of the current position in the subject - string, and those that look behind it, and in each case an assertion - may be positive (must match for the assertion to be true) or negative - (must not match for the assertion to be true). An assertion subpattern - is matched in the normal way, and if it is true, matching continues - after it, but with the matching position in the subject string is was + More complicated assertions are coded as subpatterns. There are two + kinds: those that look ahead of the current position in the subject + string, and those that look behind it, and in each case an assertion + may be positive (must match for the assertion to be true) or negative + (must not match for the assertion to be true). An assertion subpattern + is matched in the normal way, and if it is true, matching continues + after it, but with the matching position in the subject string is was it was before the assertion was processed. - A lookaround assertion may also appear as the condition in a condi- + A lookaround assertion may also appear as the condition in a condi- tional subpattern (see below). In this case, the result of matching the assertion determines which branch of the condition is followed. - Assertion subpatterns are not capturing subpatterns. If an assertion - contains capturing subpatterns within it, these are counted for the - purposes of numbering the capturing subpatterns in the whole pattern. - Within each branch of an assertion, locally captured substrings may be + Assertion subpatterns are not capturing subpatterns. If an assertion + contains capturing subpatterns within it, these are counted for the + purposes of numbering the capturing subpatterns in the whole pattern. + Within each branch of an assertion, locally captured substrings may be referenced in the usual way. For example, a sequence such as (.)\g{-1} can be used to check that two adjacent characters are the same. - When a branch within an assertion fails to match, any substrings that - were captured are discarded (as happens with any pattern branch that - fails to match). A negative assertion is true only when all its + When a branch within an assertion fails to match, any substrings that + were captured are discarded (as happens with any pattern branch that + fails to match). A negative assertion is true only when all its branches fail to match; this means that no captured substrings are ever - retained after a successful negative assertion. When an assertion con- + retained after a successful negative assertion. When an assertion con- tains a matching branch, what happens depends on the type of assertion. - For a positive assertion, internally captured substrings in the suc- - cessful branch are retained, and matching continues with the next pat- - tern item after the assertion. For a negative assertion, a matching - branch means that the assertion is not true. If such an assertion is + For a positive assertion, internally captured substrings in the suc- + cessful branch are retained, and matching continues with the next pat- + tern item after the assertion. For a negative assertion, a matching + branch means that the assertion is not true. If such an assertion is being used as a condition in a conditional subpattern (see below), cap- tured substrings are retained, because matching continues with the "no" branch of the condition. For other failing negative assertions, control passes to the previous backtracking point, thus discarding any captured strings within the assertion. - For compatibility with Perl, most assertion subpatterns may be - repeated; though it makes no sense to assert the same thing several - times, the side effect of capturing parentheses may occasionally be - useful. However, an assertion that forms the condition for a condi- - tional subpattern may not be quantified. In practice, for other asser- + For compatibility with Perl, most assertion subpatterns may be + repeated; though it makes no sense to assert the same thing several + times, the side effect of capturing parentheses may occasionally be + useful. However, an assertion that forms the condition for a condi- + tional subpattern may not be quantified. In practice, for other asser- tions, there only three cases: - (1) If the quantifier is {0}, the assertion is never obeyed during - matching. However, it may contain internal capturing parenthesized + (1) If the quantifier is {0}, the assertion is never obeyed during + matching. However, it may contain internal capturing parenthesized groups that are called from elsewhere via the subroutine mechanism. - (2) If quantifier is {0,n} where n is greater than zero, it is treated - as if it were {0,1}. At run time, the rest of the pattern match is + (2) If quantifier is {0,n} where n is greater than zero, it is treated + as if it were {0,1}. At run time, the rest of the pattern match is tried with and without the assertion, the order depending on the greed- iness of the quantifier. - (3) If the minimum repetition is greater than zero, the quantifier is - ignored. The assertion is obeyed just once when encountered during + (3) If the minimum repetition is greater than zero, the quantifier is + ignored. The assertion is obeyed just once when encountered during matching. Alphabetic assertion names - Traditionally, symbolic sequences such as (?= and (?<= have been used - to specify lookaround assertions. Perl 5.28 introduced some experimen- + Traditionally, symbolic sequences such as (?= and (?<= have been used + to specify lookaround assertions. Perl 5.28 introduced some experimen- tal alphabetic alternatives which might be easier to remember. They all - start with (* instead of (? and must be written using lower case let- + start with (* instead of (? and must be written using lower case let- ters. PCRE2 supports the following synonyms: (*positive_lookahead: or (*pla: is the same as (?= @@ -8042,8 +8047,8 @@ ASSERTIONS (*positive_lookbehind: or (*plb: is the same as (?<= (*negative_lookbehind: or (*nlb: is the same as (?...)) + + Note that the atomic group is inside the script run. Putting it outside + would not prevent backtracking into the script run pattern. + + Support for script runs is not available if PCRE2 is compiled without + Unicode support. A compile-time error is given if any of the above con- + structs is encountered. Script runs are not supported by the alternate + matching function, pcre2_dfa_match() because they use the same mecha- + nism as capturing parentheses. + + Warning: The (*ACCEPT) control verb (see below) should not be used + within a script run subpattern, because it causes an immediate exit + from the subpattern, bypassing the script run checking. + + CONDITIONAL SUBPATTERNS - It is possible to cause the matching process to obey a subpattern con- - ditionally or to choose between two alternative subpatterns, depending - on the result of an assertion, or whether a specific capturing subpat- - tern has already been matched. The two possible forms of conditional + It is possible to cause the matching process to obey a subpattern con- + ditionally or to choose between two alternative subpatterns, depending + on the result of an assertion, or whether a specific capturing subpat- + tern has already been matched. The two possible forms of conditional subpattern are: (?(condition)yes-pattern) (?(condition)yes-pattern|no-pattern) - If the condition is satisfied, the yes-pattern is used; otherwise the - no-pattern (if present) is used. An absent no-pattern is equivalent to - an empty string (it always matches). If there are more than two alter- + If the condition is satisfied, the yes-pattern is used; otherwise the + no-pattern (if present) is used. An absent no-pattern is equivalent to + an empty string (it always matches). If there are more than two alter- natives in the subpattern, a compile-time error occurs. Each of the two alternatives may itself contain nested subpatterns of any form, includ- ing conditional subpatterns; the restriction to two alternatives @@ -8220,88 +8281,88 @@ CONDITIONAL SUBPATTERNS (?(1) (A|B|C) | (D | (?(2)E|F) | E) ) - There are five kinds of condition: references to subpatterns, refer- - ences to recursion, two pseudo-conditions called DEFINE and VERSION, + There are five kinds of condition: references to subpatterns, refer- + ences to recursion, two pseudo-conditions called DEFINE and VERSION, and assertions. Checking for a used subpattern by number - If the text between the parentheses consists of a sequence of digits, + If the text between the parentheses consists of a sequence of digits, the condition is true if a capturing subpattern of that number has pre- - viously matched. If there is more than one capturing subpattern with - the same number (see the earlier section about duplicate subpattern - numbers), the condition is true if any of them have matched. An alter- - native notation is to precede the digits with a plus or minus sign. In - this case, the subpattern number is relative rather than absolute. The - most recently opened parentheses can be referenced by (?(-1), the next - most recent by (?(-2), and so on. Inside loops it can also make sense + viously matched. If there is more than one capturing subpattern with + the same number (see the earlier section about duplicate subpattern + numbers), the condition is true if any of them have matched. An alter- + native notation is to precede the digits with a plus or minus sign. In + this case, the subpattern number is relative rather than absolute. The + most recently opened parentheses can be referenced by (?(-1), the next + most recent by (?(-2), and so on. Inside loops it can also make sense to refer to subsequent groups. The next parentheses to be opened can be - referenced as (?(+1), and so on. (The value zero in any of these forms + referenced as (?(+1), and so on. (The value zero in any of these forms is not used; it provokes a compile-time error.) - Consider the following pattern, which contains non-significant white - space to make it more readable (assume the PCRE2_EXTENDED option) and + Consider the following pattern, which contains non-significant white + space to make it more readable (assume the PCRE2_EXTENDED option) and to divide it into three parts for ease of discussion: ( \( )? [^()]+ (?(1) \) ) - The first part matches an optional opening parenthesis, and if that + The first part matches an optional opening parenthesis, and if that character is present, sets it as the first captured substring. The sec- - ond part matches one or more characters that are not parentheses. The - third part is a conditional subpattern that tests whether or not the - first set of parentheses matched. If they did, that is, if subject - started with an opening parenthesis, the condition is true, and so the - yes-pattern is executed and a closing parenthesis is required. Other- - wise, since no-pattern is not present, the subpattern matches nothing. - In other words, this pattern matches a sequence of non-parentheses, + ond part matches one or more characters that are not parentheses. The + third part is a conditional subpattern that tests whether or not the + first set of parentheses matched. If they did, that is, if subject + started with an opening parenthesis, the condition is true, and so the + yes-pattern is executed and a closing parenthesis is required. Other- + wise, since no-pattern is not present, the subpattern matches nothing. + In other words, this pattern matches a sequence of non-parentheses, optionally enclosed in parentheses. - If you were embedding this pattern in a larger one, you could use a + If you were embedding this pattern in a larger one, you could use a relative reference: ...other stuff... ( \( )? [^()]+ (?(-1) \) ) ... - This makes the fragment independent of the parentheses in the larger + This makes the fragment independent of the parentheses in the larger pattern. Checking for a used subpattern by name - Perl uses the syntax (?( )...) or (?('name')...) to test for a - used subpattern by name. For compatibility with earlier versions of - PCRE1, which had this facility before Perl, the syntax (?(name)...) is - also recognized. Note, however, that undelimited names consisting of - the letter R followed by digits are ambiguous (see the following sec- + Perl uses the syntax (?( )...) or (?('name')...) to test for a + used subpattern by name. For compatibility with earlier versions of + PCRE1, which had this facility before Perl, the syntax (?(name)...) is + also recognized. Note, however, that undelimited names consisting of + the letter R followed by digits are ambiguous (see the following sec- tion). Rewriting the above example to use a named subpattern gives this: (? \( )? [^()]+ (?( ) \) ) - If the name used in a condition of this kind is a duplicate, the test - is applied to all subpatterns of the same name, and is true if any one + If the name used in a condition of this kind is a duplicate, the test + is applied to all subpatterns of the same name, and is true if any one of them has matched. Checking for pattern recursion - "Recursion" in this sense refers to any subroutine-like call from one - part of the pattern to another, whether or not it is actually recur- - sive. See the sections entitled "Recursive patterns" and "Subpatterns + "Recursion" in this sense refers to any subroutine-like call from one + part of the pattern to another, whether or not it is actually recur- + sive. See the sections entitled "Recursive patterns" and "Subpatterns as subroutines" below for details of recursion and subpattern calls. - If a condition is the string (R), and there is no subpattern with the - name R, the condition is true if matching is currently in a recursion - or subroutine call to the whole pattern or any subpattern. If digits - follow the letter R, and there is no subpattern with that name, the + If a condition is the string (R), and there is no subpattern with the + name R, the condition is true if matching is currently in a recursion + or subroutine call to the whole pattern or any subpattern. If digits + follow the letter R, and there is no subpattern with that name, the condition is true if the most recent call is into a subpattern with the - given number, which must exist somewhere in the overall pattern. This + given number, which must exist somewhere in the overall pattern. This is a contrived example that is equivalent to a+b: ((?(R1)a+|(?1)b)) - However, in both cases, if there is a subpattern with a matching name, - the condition tests for its being set, as described in the section - above, instead of testing for recursion. For example, creating a group - with the name R1 by adding (? ) to the above pattern completely + However, in both cases, if there is a subpattern with a matching name, + the condition tests for its being set, as described in the section + above, instead of testing for recursion. For example, creating a group + with the name R1 by adding (? ) to the above pattern completely changes its meaning. If a name preceded by ampersand follows the letter R, for example: @@ -8312,7 +8373,7 @@ CONDITIONAL SUBPATTERNS of that name (which must exist within the pattern). This condition does not check the entire recursion stack. It tests only - the current level. If the name used in a condition of this kind is a + the current level. If the name used in a condition of this kind is a duplicate, the test is applied to all subpatterns of the same name, and is true if any one of them is the most recent recursion. @@ -8321,10 +8382,10 @@ CONDITIONAL SUBPATTERNS Defining subpatterns for use by reference only If the condition is the string (DEFINE), the condition is always false, - even if there is a group with the name DEFINE. In this case, there may + even if there is a group with the name DEFINE. In this case, there may be only one alternative in the subpattern. It is always skipped if con- - trol reaches this point in the pattern; the idea of DEFINE is that it - can be used to define subroutines that can be referenced from else- + trol reaches this point in the pattern; the idea of DEFINE is that it + can be used to define subroutines that can be referenced from else- where. (The use of subroutines is described below.) For example, a pat- tern to match an IPv4 address such as "192.168.23.245" could be written like this (ignore white space and line breaks): @@ -8332,97 +8393,97 @@ CONDITIONAL SUBPATTERNS (?(DEFINE) (? 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) ) \b (?&byte) (\.(?&byte)){3} \b - The first part of the pattern is a DEFINE group inside which a another - group named "byte" is defined. This matches an individual component of - an IPv4 address (a number less than 256). When matching takes place, - this part of the pattern is skipped because DEFINE acts like a false - condition. The rest of the pattern uses references to the named group - to match the four dot-separated components of an IPv4 address, insist- + The first part of the pattern is a DEFINE group inside which a another + group named "byte" is defined. This matches an individual component of + an IPv4 address (a number less than 256). When matching takes place, + this part of the pattern is skipped because DEFINE acts like a false + condition. The rest of the pattern uses references to the named group + to match the four dot-separated components of an IPv4 address, insist- ing on a word boundary at each end. Checking the PCRE2 version - Programs that link with a PCRE2 library can check the version by call- - ing pcre2_config() with appropriate arguments. Users of applications - that do not have access to the underlying code cannot do this. A spe- - cial "condition" called VERSION exists to allow such users to discover + Programs that link with a PCRE2 library can check the version by call- + ing pcre2_config() with appropriate arguments. Users of applications + that do not have access to the underlying code cannot do this. A spe- + cial "condition" called VERSION exists to allow such users to discover which version of PCRE2 they are dealing with by using this condition to - match a string such as "yesno". VERSION must be followed either by "=" + match a string such as "yesno". VERSION must be followed either by "=" or ">=" and a version number. For example: (?(VERSION>=10.4)yes|no) - This pattern matches "yes" if the PCRE2 version is greater or equal to - 10.4, or "no" otherwise. The fractional part of the version number may + This pattern matches "yes" if the PCRE2 version is greater or equal to + 10.4, or "no" otherwise. The fractional part of the version number may not contain more than two digits. Assertion conditions - If the condition is not in any of the above formats, it must be an - assertion. This may be a positive or negative lookahead or lookbehind - assertion. Consider this pattern, again containing non-significant + If the condition is not in any of the above formats, it must be an + assertion. This may be a positive or negative lookahead or lookbehind + assertion. Consider this pattern, again containing non-significant white space, and with the two alternatives on the second line: (?(?=[^a-z]*[a-z]) \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} ) - The condition is a positive lookahead assertion that matches an - optional sequence of non-letters followed by a letter. In other words, - it tests for the presence of at least one letter in the subject. If a - letter is found, the subject is matched against the first alternative; - otherwise it is matched against the second. This pattern matches - strings in one of the two forms dd-aaa-dd or dd-dd-dd, where aaa are + The condition is a positive lookahead assertion that matches an + optional sequence of non-letters followed by a letter. In other words, + it tests for the presence of at least one letter in the subject. If a + letter is found, the subject is matched against the first alternative; + otherwise it is matched against the second. This pattern matches + strings in one of the two forms dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits. - When an assertion that is a condition contains capturing subpatterns, - any capturing that occurs in a matching branch is retained afterwards, + When an assertion that is a condition contains capturing subpatterns, + any capturing that occurs in a matching branch is retained afterwards, for both positive and negative assertions, because matching always con- tinues after the assertion, whether it succeeds or fails. (Compare non- - conditional assertions, when captures are retained only for positive + conditional assertions, when captures are retained only for positive assertions that succeed.) COMMENTS There are two ways of including comments in patterns that are processed - by PCRE2. In both cases, the start of the comment must not be in a - character class, nor in the middle of any other sequence of related - characters such as (?: or a subpattern name or number. The characters + by PCRE2. In both cases, the start of the comment must not be in a + character class, nor in the middle of any other sequence of related + characters such as (?: or a subpattern name or number. The characters that make up a comment play no part in the pattern matching. - The sequence (?# marks the start of a comment that continues up to the - next closing parenthesis. Nested parentheses are not permitted. If the - PCRE2_EXTENDED or PCRE2_EXTENDED_MORE 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 sequence (?# marks the start of a comment that continues up to the + next closing parenthesis. Nested parentheses are not permitted. If the + PCRE2_EXTENDED or PCRE2_EXTENDED_MORE 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 an option passed to the compiling function or by a special sequence + by an option passed to the compiling function or by a special sequence at the start of the pattern, as described in the section entitled "New- line conventions" above. Note that the end of this type of comment is a - literal newline sequence in the pattern; escape sequences that happen + literal newline sequence in the pattern; escape sequences that happen to represent a newline do not count. For example, consider this pattern - when PCRE2_EXTENDED is set, and the default newline convention (a sin- + when PCRE2_EXTENDED is set, and the default newline convention (a sin- gle linefeed character) is in force: abc #comment \n still comment - On encountering the # character, pcre2_compile() skips along, looking - for a newline in the pattern. The sequence \n is still literal at this - stage, so it does not terminate the comment. Only an actual character + On encountering the # character, pcre2_compile() skips along, looking + for a newline in the pattern. The sequence \n is still literal at this + stage, so it does not terminate the comment. Only an actual character with the code value 0x0a (the default newline) does so. RECURSIVE PATTERNS - Consider the problem of matching a string in parentheses, allowing for - unlimited nested parentheses. Without the use of recursion, the best - that can be done is to use a pattern that matches up to some fixed - depth of nesting. It is not possible to handle an arbitrary nesting + Consider the problem of matching a string in parentheses, allowing for + unlimited nested parentheses. Without the use of recursion, the best + that can be done is to use a pattern that matches up to some fixed + depth of nesting. It is not possible to handle an arbitrary nesting depth. For some time, Perl has provided a facility that allows regular expres- - sions to recurse (amongst other things). It does this by interpolating - Perl code in the expression at run time, and the code can refer to the + sions to recurse (amongst other things). It does this by interpolating + Perl code in the expression at run time, and the code can refer to the expression itself. A Perl pattern using code interpolation to solve the parentheses problem can be created like this: @@ -8432,175 +8493,175 @@ RECURSIVE PATTERNS refers recursively to the pattern in which it appears. Obviously, PCRE2 cannot support the interpolation of Perl code. - Instead, it supports special syntax for recursion of the entire pat- + Instead, it supports special syntax for recursion of the entire pat- tern, and also for individual subpattern recursion. After its introduc- - tion in PCRE1 and Python, this kind of recursion was subsequently + tion in PCRE1 and Python, this kind of recursion was subsequently introduced into Perl at release 5.10. - A special item that consists of (? followed by a number greater than - zero and a closing parenthesis is a recursive subroutine call of the - subpattern of the given number, provided that it occurs inside that - subpattern. (If not, it is a non-recursive subroutine call, which is - described in the next section.) The special item (?R) or (?0) is a + A special item that consists of (? followed by a number greater than + zero and a closing parenthesis is a recursive subroutine call of the + subpattern of the given number, provided that it occurs inside that + subpattern. (If not, it is a non-recursive subroutine call, which is + described in the next section.) The special item (?R) or (?0) is a recursive call of the entire regular expression. - This PCRE2 pattern solves the nested parentheses problem (assume the + This PCRE2 pattern solves the nested parentheses problem (assume the PCRE2_EXTENDED option is set so that white space is ignored): \( ( [^()]++ | (?R) )* \) - First it matches an opening parenthesis. Then it matches any number of - substrings which can either be a sequence of non-parentheses, or a - recursive match of the pattern itself (that is, a correctly parenthe- + First it matches an opening parenthesis. Then it matches any number of + substrings which can either be a sequence of non-parentheses, or a + recursive match of the pattern itself (that is, a correctly parenthe- sized substring). Finally there is a closing parenthesis. Note the use of a possessive quantifier to avoid backtracking into sequences of non- parentheses. - If this were part of a larger pattern, you would not want to recurse + If this were part of a larger pattern, you would not want to recurse the entire pattern, so instead you could use this: ( \( ( [^()]++ | (?1) )* \) ) - We have put the pattern into parentheses, and caused the recursion to + We have put the pattern into parentheses, and caused the recursion to refer to them instead of the whole pattern. - In a larger pattern, keeping track of parenthesis numbers can be - tricky. This is made easier by the use of relative references. Instead + In a larger pattern, keeping track of parenthesis numbers can be + tricky. This is made easier by the use of relative references. Instead of (?1) in the pattern above you can write (?-2) to refer to the second - most recently opened parentheses preceding the recursion. In other - words, a negative number counts capturing parentheses leftwards from + most recently opened parentheses preceding the recursion. In other + words, a negative number counts capturing parentheses leftwards from the point at which it is encountered. Be aware however, that if duplicate subpattern numbers are in use, rel- - ative references refer to the earliest subpattern with the appropriate + ative references refer to the earliest subpattern with the appropriate number. Consider, for example: (?|(a)|(b)) (c) (?-2) - The first two capturing groups (a) and (b) are both numbered 1, and - group (c) is number 2. When the reference (?-2) is encountered, the + The first two capturing groups (a) and (b) are both numbered 1, and + group (c) is number 2. When the reference (?-2) is encountered, the second most recently opened parentheses has the number 1, but it is the - first such group (the (a) group) to which the recursion refers. This - would be the same if an absolute reference (?1) was used. In other - words, relative references are just a shorthand for computing a group + first such group (the (a) group) to which the recursion refers. This + would be the same if an absolute reference (?1) was used. In other + words, relative references are just a shorthand for computing a group number. - It is also possible to refer to subsequently opened parentheses, by - writing references such as (?+2). However, these cannot be recursive - because the reference is not inside the parentheses that are refer- - enced. They are always non-recursive subroutine calls, as described in + It is also possible to refer to subsequently opened parentheses, by + writing references such as (?+2). However, these cannot be recursive + because the reference is not inside the parentheses that are refer- + enced. They are always non-recursive subroutine calls, as described in the next section. - An alternative approach is to use named parentheses. The Perl syntax - for this is (?&name); PCRE1's earlier syntax (?P>name) is also sup- + An alternative approach is to use named parentheses. The Perl syntax + for this is (?&name); PCRE1's earlier syntax (?P>name) is also sup- ported. We could rewrite the above example as follows: (? \( ( [^()]++ | (?&pn) )* \) ) - If there is more than one subpattern with the same name, the earliest + If there is more than one subpattern with the same name, the earliest one is used. The example pattern that we have been looking at contains nested unlim- - ited repeats, and so the use of a possessive quantifier for matching - strings of non-parentheses is important when applying the pattern to + ited repeats, and so the use of a possessive quantifier for matching + strings of non-parentheses is important when applying the pattern to strings that do not match. For example, when this pattern is applied to (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa() - it yields "no match" quickly. However, if a possessive quantifier is - not used, the match runs for a very long time indeed because there are - so many different ways the + and * repeats can carve up the subject, + it yields "no match" quickly. However, if a possessive quantifier is + not used, the match runs for a very long time indeed because there are + so many different ways the + and * repeats can carve up the subject, and all have to be tested before failure can be reported. - At the end of a match, the values of capturing parentheses are those - from the outermost level. If you want to obtain intermediate values, a + At the end of a match, the values of capturing parentheses are those + from the outermost level. If you want to obtain intermediate values, a callout function can be used (see below and the pcre2callout documenta- tion). If the pattern above is matched against (ab(cd)ef) - the value for the inner capturing parentheses (numbered 2) is "ef", - which is the last value taken on at the top level. If a capturing sub- - pattern is not matched at the top level, its final captured value is - unset, even if it was (temporarily) set at a deeper level during the + the value for the inner capturing parentheses (numbered 2) is "ef", + which is the last value taken on at the top level. If a capturing sub- + pattern is not matched at the top level, its final captured value is + unset, even if it was (temporarily) set at a deeper level during the matching process. - Do not confuse the (?R) item with the condition (R), which tests for - recursion. Consider this pattern, which matches text in angle brack- - ets, allowing for arbitrary nesting. Only digits are allowed in nested - brackets (that is, when recursing), whereas any characters are permit- + Do not confuse the (?R) item with the condition (R), which tests for + recursion. Consider this pattern, which matches text in angle brack- + ets, allowing for arbitrary nesting. Only digits are allowed in nested + brackets (that is, when recursing), whereas any characters are permit- ted at the outer level. < (?: (?(R) \d++ | [^<>]*+) | (?R)) * > - In this pattern, (?(R) is the start of a conditional subpattern, with - two different alternatives for the recursive and non-recursive cases. + In this pattern, (?(R) is the start of a conditional subpattern, with + two different alternatives for the recursive and non-recursive cases. The (?R) item is the actual recursive call. Differences in recursion processing between PCRE2 and Perl Some former differences between PCRE2 and Perl no longer exist. - Before release 10.30, recursion processing in PCRE2 differed from Perl - in that a recursive subpattern call was always treated as an atomic - group. That is, once it had matched some of the subject string, it was - never re-entered, even if it contained untried alternatives and there - was a subsequent matching failure. (Historical note: PCRE implemented + Before release 10.30, recursion processing in PCRE2 differed from Perl + in that a recursive subpattern call was always treated as an atomic + group. That is, once it had matched some of the subject string, it was + never re-entered, even if it contained untried alternatives and there + was a subsequent matching failure. (Historical note: PCRE implemented recursion before Perl did.) - Starting with release 10.30, recursive subroutine calls are no longer + Starting with release 10.30, recursive subroutine calls are no longer treated as atomic. That is, they can be re-entered to try unused alter- - natives if there is a matching failure later in the pattern. This is - now compatible with the way Perl works. If you want a subroutine call + natives if there is a matching failure later in the pattern. This is + now compatible with the way Perl works. If you want a subroutine call to be atomic, you must explicitly enclose it in an atomic group. - Supporting backtracking into recursions simplifies certain types of + Supporting backtracking into recursions simplifies certain types of recursive pattern. For example, this pattern matches palindromic strings: ^((.)(?1)\2|.?)$ - The second branch in the group matches a single central character in - the palindrome when there are an odd number of characters, or nothing - when there are an even number of characters, but in order to work it - has to be able to try the second case when the rest of the pattern + The second branch in the group matches a single central character in + the palindrome when there are an odd number of characters, or nothing + when there are an even number of characters, but in order to work it + has to be able to try the second case when the rest of the pattern match fails. If you want to match typical palindromic phrases, the pat- - tern has to ignore all non-word characters, which can be done like + tern has to ignore all non-word characters, which can be done like this: ^\W*+((.)\W*+(?1)\W*+\2|\W*+.?)\W*+$ - If run with the PCRE2_CASELESS option, this pattern matches phrases - such as "A man, a plan, a canal: Panama!". Note the use of the posses- - sive quantifier *+ to avoid backtracking into sequences of non-word + If run with the PCRE2_CASELESS option, this pattern matches phrases + such as "A man, a plan, a canal: Panama!". Note the use of the posses- + sive quantifier *+ to avoid backtracking into sequences of non-word characters. Without this, PCRE2 takes a great deal longer (ten times or - more) to match typical phrases, and Perl takes so long that you think + more) to match typical phrases, and Perl takes so long that you think it has gone into a loop. - Another way in which PCRE2 and Perl used to differ in their recursion - processing is in the handling of captured values. Formerly in Perl, - when a subpattern was called recursively or as a subpattern (see the - next section), it had no access to any values that were captured out- - side the recursion, whereas in PCRE2 these values can be referenced. + Another way in which PCRE2 and Perl used to differ in their recursion + processing is in the handling of captured values. Formerly in Perl, + when a subpattern was called recursively or as a subpattern (see the + next section), it had no access to any values that were captured out- + side the recursion, whereas in PCRE2 these values can be referenced. Consider this pattern: ^(.)(\1|a(?2)) - This pattern matches "bab". The first capturing parentheses match "b", + This pattern matches "bab". The first capturing parentheses match "b", then in the second group, when the backreference \1 fails to match "b", the second alternative matches "a" and then recurses. In the recursion, - \1 does now match "b" and so the whole match succeeds. This match used + \1 does now match "b" and so the whole match succeeds. This match used to fail in Perl, but in later versions (I tried 5.024) it now works. SUBPATTERNS AS SUBROUTINES - If the syntax for a recursive subpattern call (either by number or by + 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 a bit like a subroutine in a programming language. More accurately, PCRE2 - treats the referenced subpattern as an independent subpattern which it - tries to match at the current matching position. The called subpattern - may be defined before or after the reference. A numbered reference can + treats the referenced subpattern as an independent subpattern which it + tries to match at the current matching position. The called subpattern + may be defined before or after the reference. A numbered reference can be absolute or relative, as in these examples: (...(absolute)...)...(?2)... @@ -8611,106 +8672,106 @@ SUBPATTERNS AS SUBROUTINES (sens|respons)e and \1ibility - matches "sense and sensibility" and "response and responsibility", but + matches "sense and sensibility" and "response and responsibility", but not "sense and responsibility". If instead the pattern (sens|respons)e and (?1)ibility - is used, it does match "sense and responsibility" as well as the other - two strings. Another example is given in the discussion of DEFINE + is used, it does match "sense and responsibility" as well as the other + two strings. Another example is given in the discussion of DEFINE above. - Like recursions, subroutine calls used to be treated as atomic, but - this changed at PCRE2 release 10.30, so backtracking into subroutine - calls can now occur. However, any capturing parentheses that are set + Like recursions, subroutine calls used to be treated as atomic, but + this changed at PCRE2 release 10.30, so backtracking into subroutine + calls can now occur. However, any capturing parentheses that are set during the subroutine call revert to their previous values afterwards. - Processing options such as case-independence are fixed when a subpat- - tern is defined, so if it is used as a subroutine, such options cannot + Processing options such as case-independence are fixed when a subpat- + tern is defined, so if it is used as a subroutine, such options cannot be changed for different calls. For example, consider this pattern: (abc)(?i:(?-1)) - It matches "abcabc". It does not match "abcABC" because the change of + It matches "abcabc". It does not match "abcABC" because the change of processing option does not affect the called subpattern. - The behaviour of backtracking control verbs in subpatterns when called + The behaviour of backtracking control verbs in subpatterns when called as subroutines is described in the section entitled "Backtracking verbs in subroutines" below. ONIGURUMA SUBROUTINE SYNTAX - For compatibility with Oniguruma, the non-Perl syntax \g followed by a + 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 syntax for referencing a subpattern as a subroutine, - possibly recursively. Here are two of the examples used above, rewrit- + an alternative syntax for referencing a subpattern as a subroutine, + possibly recursively. Here are two of the examples used above, rewrit- ten using this syntax: (? \( ( (?>[^()]+) | \g )* \) ) (sens|respons)e and \g'1'ibility - PCRE2 supports an extension to Oniguruma: if a number is preceded by a + PCRE2 supports an extension to Oniguruma: if a number is preceded by a plus or a minus sign it is taken as a relative reference. For example: (abc)(?i:\g<-1>) - Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are not - synonymous. The former is a backreference; the latter is a subroutine + Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are not + synonymous. The former is a backreference; the latter is a subroutine call. CALLOUTS Perl has a feature whereby using the sequence (?{...}) causes arbitrary - Perl code to be obeyed in the middle of matching a regular expression. + Perl code to be obeyed in the middle of matching a regular expression. This makes it possible, amongst other things, to extract different sub- strings that match the same pair of parentheses when there is a repeti- tion. - PCRE2 provides a similar feature, but of course it cannot obey arbi- - trary Perl code. The feature is called "callout". The caller of PCRE2 - provides an external function by putting its entry point in a match - context using the function pcre2_set_callout(), and then passing that - context to pcre2_match() or pcre2_dfa_match(). If no match context is + PCRE2 provides a similar feature, but of course it cannot obey arbi- + trary Perl code. The feature is called "callout". The caller of PCRE2 + provides an external function by putting its entry point in a match + context using the function pcre2_set_callout(), and then passing that + context to pcre2_match() or pcre2_dfa_match(). If no match context is passed, or if the callout entry point is set to NULL, callouts are dis- abled. - Within a regular expression, (?C ) indicates a point at which the - external function is to be called. There are two kinds of callout: - those with a numerical argument and those with a string argument. (?C) - on its own with no argument is treated as (?C0). A numerical argument - allows the application to distinguish between different callouts. - String arguments were added for release 10.20 to make it possible for - script languages that use PCRE2 to embed short scripts within patterns + Within a regular expression, (?C ) indicates a point at which the + external function is to be called. There are two kinds of callout: + those with a numerical argument and those with a string argument. (?C) + on its own with no argument is treated as (?C0). A numerical argument + allows the application to distinguish between different callouts. + String arguments were added for release 10.20 to make it possible for + script languages that use PCRE2 to embed short scripts within patterns in a similar way to Perl. During matching, when PCRE2 reaches a callout point, the external func- - tion is called. It is provided with the number or string argument of - the callout, the position in the pattern, and one item of data that is + tion is called. It is provided with the number or string argument of + the callout, the position in the pattern, and one item of data that is also set in the match block. The callout function may cause matching to proceed, to backtrack, or to fail. - By default, PCRE2 implements a number of optimizations at 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, including a complete - description of the programming interface to the callout function, are + By default, PCRE2 implements a number of optimizations at 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, including a complete + description of the programming interface to the callout function, are given in the pcre2callout documentation. Callouts with numerical arguments - If you just want to have a means of identifying different callout - points, put a number less than 256 after the letter C. For example, + If you just want to have a means of identifying different callout + points, put a number less than 256 after the letter C. For example, this pattern has two callout points: (?C1)abc(?C2)def - If the PCRE2_AUTO_CALLOUT flag is passed to pcre2_compile(), numerical - callouts are automatically installed before each item in the pattern. - They are all numbered 255. If there is a conditional group in the pat- + If the PCRE2_AUTO_CALLOUT flag is passed to pcre2_compile(), numerical + callouts are automatically installed before each item in the pattern. + They are all numbered 255. If there is a conditional group in the pat- tern whose condition is an assertion, an additional callout is inserted - just before the condition. An explicit callout may also be set at this + just before the condition. An explicit callout may also be set at this position, as in this example: (?(?C9)(?=a)abc|def) @@ -8720,60 +8781,60 @@ CALLOUTS Callouts with string arguments - A delimited string may be used instead of a number as a callout argu- - ment. The starting delimiter must be one of ` ' " ^ % # $ { and the + A delimited string may be used instead of a number as a callout argu- + ment. The starting delimiter must be one of ` ' " ^ % # $ { and the ending delimiter is the same as the start, except for {, where the end- - ing delimiter is }. If the ending delimiter is needed within the + ing delimiter is }. If the ending delimiter is needed within the string, it must be doubled. For example: (?C'ab ''c'' d')xyz(?C{any text})pqr - The doubling is removed before the string is passed to the callout + The doubling is removed before the string is passed to the callout function. BACKTRACKING CONTROL - There are a number of special "Backtracking Control Verbs" (to use - Perl's terminology) that modify the behaviour of backtracking during - matching. They are generally of the form (*VERB) or (*VERB:NAME). Some - verbs take either form, possibly behaving differently depending on + There are a number of special "Backtracking Control Verbs" (to use + Perl's terminology) that modify the behaviour of backtracking during + matching. They are generally of the form (*VERB) or (*VERB:NAME). Some + verbs take either form, possibly behaving differently depending on whether or not a name is present. - By default, for compatibility with Perl, a name is any sequence of + By default, for compatibility with Perl, a name is any sequence of characters that does not include a closing parenthesis. The name is not - processed in any way, and it is not possible to include a closing - parenthesis in the name. This can be changed by setting the - PCRE2_ALT_VERBNAMES option, but the result is no longer Perl-compati- + processed in any way, and it is not possible to include a closing + parenthesis in the name. This can be changed by setting the + PCRE2_ALT_VERBNAMES option, but the result is no longer Perl-compati- ble. - When PCRE2_ALT_VERBNAMES is set, backslash processing is applied to - verb names and only an unescaped closing parenthesis terminates the - name. However, the only backslash items that are permitted are \Q, \E, - and sequences such as \x{100} that define character code points. Char- + When PCRE2_ALT_VERBNAMES is set, backslash processing is applied to + verb names and only an unescaped closing parenthesis terminates the + name. However, the only backslash items that are permitted are \Q, \E, + and sequences such as \x{100} that define character code points. Char- acter type escapes such as \d are faulted. A closing parenthesis can be included in a name either as \) or between - \Q and \E. In addition to backslash processing, if the PCRE2_EXTENDED + \Q and \E. In addition to backslash processing, if the PCRE2_EXTENDED or PCRE2_EXTENDED_MORE option is also set, unescaped whitespace in verb names is skipped, and #-comments are recognized, exactly as in the rest - of the pattern. PCRE2_EXTENDED and PCRE2_EXTENDED_MORE do not affect + of the pattern. PCRE2_EXTENDED and PCRE2_EXTENDED_MORE do not affect verb names unless PCRE2_ALT_VERBNAMES is also set. - The maximum length of a 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 maximum length of a 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 pat- tern. - Since these verbs are specifically related to backtracking, most of - them can be used only when the pattern is to be matched using the tra- + Since these verbs are specifically related to backtracking, most of + them can be used only when the pattern is to be matched using the tra- ditional matching function, because that uses a backtracking algorithm. - With the exception of (*FAIL), which behaves like a failing negative + With the exception of (*FAIL), which behaves like a failing negative assertion, the backtracking control verbs cause an error if encountered by the DFA matching function. - The behaviour of these verbs in repeated groups, assertions, and in + The behaviour of these verbs in repeated groups, assertions, and in subpatterns called as subroutines (whether or not recursively) is docu- mented below. @@ -8781,16 +8842,16 @@ BACKTRACKING CONTROL PCRE2 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 + may know the minimum length of matching subject, or that a particular character must be present. When one of these optimizations bypasses the - running of a match, any included backtracking verbs will not, of + 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 PCRE2_NO_START_OPTIMIZE option when calling pcre2_com- - pile(), or by starting the pattern with (*NO_START_OPT). There is more + by setting the PCRE2_NO_START_OPTIMIZE option when calling pcre2_com- + pile(), or by starting the pattern with (*NO_START_OPT). There is more discussion of this option in the section entitled "Compiling a pattern" in the pcre2api documentation. - Experiments with Perl suggest that it too has similar optimizations, + Experiments with Perl suggest that it too has similar optimizations, and like PCRE2, turning them off can change the result of a match. Verbs that act immediately @@ -8799,63 +8860,67 @@ BACKTRACKING CONTROL (*ACCEPT) or (*ACCEPT: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 + 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) in triggered in a posi- - tive assertion, the assertion succeeds; in a negative assertion, the + tive assertion, the assertion succeeds; in a negative assertion, the assertion fails. - If (*ACCEPT) is inside capturing parentheses, the data so far is cap- + If (*ACCEPT) is inside capturing parentheses, the data so far is cap- tured. For example: A((?:A|B(*ACCEPT)|C)D) - This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is cap- + This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is cap- tured by the outer parentheses. + Warning: (*ACCEPT) should not be used within a script run subpattern, + because it causes an immediate exit from the subpattern, bypassing the + script run checking. + (*FAIL) or (*FAIL:NAME) - This verb causes a matching failure, forcing backtracking to occur. It - may be abbreviated to (*F). It is equivalent to (?!) but easier to + This verb causes a matching failure, forcing backtracking to occur. It + may be abbreviated to (*F). It is equivalent to (?!) but easier to read. The Perl documentation notes that it is probably useful only when combined with (?{}) or (??{}). Those are, of course, Perl features that - are not present in PCRE2. The nearest equivalent is the callout fea- + are not present in PCRE2. The nearest equivalent is the callout fea- ture, as for example in this pattern: a+(?C)(*FAIL) - A match with the string "aaaa" always fails, but the callout is taken + A match with the string "aaaa" always fails, but the callout is taken before each backtrack happens (in this example, 10 times). - (*ACCEPT:NAME) and (*FAIL:NAME) behave exactly the same as + (*ACCEPT:NAME) and (*FAIL:NAME) behave exactly the same as (*MARK:NAME)(*ACCEPT) and (*MARK:NAME)(*FAIL), respectively. Recording which path was taken - There is one verb whose main purpose is to track how a match was - arrived at, though it also has a secondary use in conjunction with + There is one verb whose main purpose is to track how a match was + arrived at, though it also has a secondary use in conjunction with advancing the match starting point (see (*SKIP) below). (*MARK:NAME) or (*:NAME) - A name is always required with this verb. There may be as many - instances of (*MARK) as you like in a pattern, and their names do not + A name is always required with this verb. There may be as many + instances of (*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:NAME) on the matching path is passed back to the caller as described in the sec- tion entitled "Other information about the match" in the pcre2api docu- - mentation. This applies to all instances of (*MARK), including those - inside assertions and atomic groups. (There are differences in those - cases when (*MARK) is used in conjunction with (*SKIP) as described + mentation. This applies to all instances of (*MARK), including those + inside assertions and atomic groups. (There are differences in those + cases when (*MARK) is used in conjunction with (*SKIP) as described below.) - As well as (*MARK), the (*COMMIT), (*PRUNE) and (*THEN) verbs may have - associated NAME arguments. Whichever is last on the matching path is + As well as (*MARK), the (*COMMIT), (*PRUNE) and (*THEN) verbs may have + associated NAME arguments. Whichever is last on the matching path is passed back. See below for more details of these other verbs. - Here is an example of pcre2test output, where the "mark" modifier + Here is an example of pcre2test output, where the "mark" modifier requests the retrieval and outputting of (*MARK) data: re> /X(*MARK:A)Y|X(*MARK:B)Z/mark @@ -8867,76 +8932,76 @@ BACKTRACKING CONTROL MK: B The (*MARK) name is tagged with "MK:" in this output, and in this exam- - ple it indicates which of the two alternatives matched. This is a more - efficient way of obtaining this information than putting each alterna- + ple it indicates which of the two alternatives matched. This is a more + efficient way of obtaining this information than putting each alterna- tive in its own capturing parentheses. - 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-encoun- + 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-encoun- tered. This does not happen for negative assertions or failing positive assertions. - After a partial match or a failed match, the last encountered name in + 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/mark data> XP No match, mark = B - Note that in this unanchored example the mark is retained from the + Note that in this unanchored example the mark is retained from the match 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 PCRE2_NO_START_OPTIMIZE option (see above) to + If you are interested in (*MARK) values after failed matches, you + should probably set the PCRE2_NO_START_OPTIMIZE option (see above) to ensure that the match is always attempted. Verbs that act after backtracking The following verbs do nothing when they are encountered. Matching con- - tinues with what follows, but if there is a subsequent match failure, - causing a backtrack to the verb, a failure is forced. That is, back- - tracking cannot pass to the left of the verb. However, when one of + tinues with what follows, but if there is a subsequent match failure, + causing a backtrack to the verb, a failure is forced. That is, back- + tracking cannot pass to the left of the verb. However, when one of these verbs appears inside an atomic group or in a lookaround 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. Back- + that is true, its effect is confined to that group, because once the + group has been matched, there is never any backtracking into it. Back- tracking from beyond an assertion or an atomic group ignores the entire group, and seeks a preceeding backtracking point. - These verbs differ in exactly what kind of failure occurs when back- - tracking reaches them. The behaviour described below is what happens - when the verb is not in a subroutine or an assertion. Subsequent sec- + These verbs differ in exactly what kind of failure occurs when back- + tracking reaches them. The behaviour described below is what happens + when the verb is not in a subroutine or an assertion. Subsequent sec- tions cover these special cases. (*COMMIT) or (*COMMIT:NAME) - This verb causes the whole match to fail outright if there is a later + This verb causes the whole match to fail outright if there is a later matching failure that causes backtracking to reach it. Even if the pat- - tern is unanchored, no further attempts to find a match by advancing - the starting point take place. If (*COMMIT) is the only backtracking + tern 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 pcre2_match() is com- mitted to finding a match at the current starting point, or not at all. For example: a+(*COMMIT)b - This matches "xxaab" but not "aacaab". It can be thought of as a kind + This matches "xxaab" but not "aacaab". It can be thought of as a kind of dynamic anchor, or "I've started, so I must finish." - The behaviour of (*COMMIT:NAME) is not the same as (*MARK:NAME)(*COM- - MIT). It is like (*MARK:NAME) in that the name is remembered for pass- - ing back to the caller. However, (*SKIP:NAME) searches only for names - set with (*MARK), ignoring those set by (*COMMIT), (*PRUNE) and + The behaviour of (*COMMIT:NAME) is not the same as (*MARK:NAME)(*COM- + MIT). It is like (*MARK:NAME) in that the name is remembered for pass- + ing back to the caller. However, (*SKIP:NAME) searches only for names + set with (*MARK), ignoring those set by (*COMMIT), (*PRUNE) and (*THEN). - If there is more than one backtracking verb in a pattern, a different - one that follows (*COMMIT) may be triggered first, so merely passing + 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 PCRE2's start-of-match optimizations are turned off, as + Note that (*COMMIT) at the start of a pattern is not the same as an + anchor, unless PCRE2's start-of-match optimizations are turned off, as shown in this output from pcre2test: re> /(*COMMIT)abc/ @@ -8947,63 +9012,63 @@ BACKTRACKING CONTROL data> xyzabc No match - For the first pattern, PCRE2 knows that any match must start with "a", - so the optimization skips along the subject to "a" before applying the - pattern to the first set of data. The match attempt then succeeds. The - second pattern disables the optimization that skips along to the first - character. The pattern is now applied starting at "x", and so the - (*COMMIT) causes the match to fail without trying any other starting + For the first pattern, PCRE2 knows that any match must start with "a", + so the optimization skips along the subject to "a" before applying the + pattern to the first set of data. The match attempt then succeeds. The + second pattern disables the optimization that skips along to the first + character. The pattern is now applied starting at "x", and so the + (*COMMIT) causes the match to fail without trying any other starting points. (*PRUNE) or (*PRUNE:NAME) - This verb causes the match to fail at the current starting position in + This verb causes the match to fail at the current starting position in the subject if there is a later matching failure that causes backtrack- - ing 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 quan- + ing 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 quan- tifier, 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 + any other way. In an anchored pattern (*PRUNE) has the same effect as (*COMMIT). The behaviour of (*PRUNE:NAME) is 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 + to the caller. However, (*SKIP:NAME) searches only for names set with (*MARK), ignoring those set by (*COMMIT), (*PRUNE) or (*THEN). (*SKIP) - This verb, when given without a name, is like (*PRUNE), except that if - the pattern is unanchored, the "bumpalong" advance is not to the next + This verb, when given without a name, is like (*PRUNE), except that if + the pattern is unanchored, the "bumpalong" advance is not to the next character, but to the position in the subject where (*SKIP) was encoun- - tered. (*SKIP) signifies that whatever text was matched leading up to - it cannot be part of a successful match if there is a later mismatch. + tered. (*SKIP) signifies that whatever text was matched leading up to + it cannot be part of a successful match if there is a later mismatch. Consider: a+(*SKIP)b - If the subject is "aaaac...", after the first match attempt fails - (starting at the first character in the string), the starting point + If the subject is "aaaac...", after the first match attempt fails + (starting at the first character in the string), the starting point skips on to start the next attempt at "c". Note that a possessive quan- - tifer does not have the same effect as this example; although it would - suppress backtracking during the first match attempt, the second - attempt would start at the second character instead of skipping on to + tifer does not have the same effect as this example; although it would + suppress backtracking during the first match attempt, the second + attempt would start at the second character instead of skipping on to "c". (*SKIP:NAME) - When (*SKIP) has an associated name, its behaviour is modified. When - such a (*SKIP) 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 corre- - sponds to that (*MARK) instead of to where (*SKIP) was encountered. If + When (*SKIP) has an associated name, its behaviour is modified. When + such a (*SKIP) 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 corre- + sponds to that (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a matching name is found, the (*SKIP) is ignored. - The search for a (*MARK) name uses the normal backtracking mechanism, - which means that it does not see (*MARK) settings that are inside + The search for a (*MARK) name uses the normal backtracking mechanism, + which means that it does not see (*MARK) settings that are inside atomic groups or assertions, because they are never re-entered by back- tracking. Compare the following pcre2test examples: @@ -9017,106 +9082,106 @@ BACKTRACKING CONTROL 0: b 1: b - In the first example, the (*MARK) setting is in an atomic group, so it + In the first example, the (*MARK) setting is in an atomic group, so it is not seen when (*SKIP:X) triggers, causing the (*SKIP) to be ignored. - This allows the second branch of the pattern to be tried at the first - character position. In the second example, the (*MARK) setting is not - in an atomic group. This allows (*SKIP:X) to find the (*MARK) when it + This allows the second branch of the pattern to be tried at the first + character position. In the second example, the (*MARK) setting is not + in an atomic group. This allows (*SKIP:X) to find the (*MARK) when it backtracks, and this causes a new matching attempt to start at the sec- - ond character. This time, the (*MARK) is never seen because "a" does + ond character. This time, the (*MARK) is never seen because "a" does not match "b", so the matcher immediately jumps to the second branch of the pattern. - Note that (*SKIP:NAME) searches only for names set by (*MARK:NAME). It - ignores names that are set by (*COMMIT:NAME), (*PRUNE:NAME) or + Note that (*SKIP:NAME) searches only for names set by (*MARK:NAME). It + ignores names that are set by (*COMMIT:NAME), (*PRUNE:NAME) or (*THEN:NAME). (*THEN) or (*THEN:NAME) - This verb causes a skip to the next innermost alternative when back- - tracking reaches it. That is, it cancels any further backtracking - within the current alternative. Its name comes from the observation + This verb causes a skip to the next innermost alternative when back- + tracking 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. If that succeeds and BAR fails, COND3 is tried. If subse- - quently BAZ fails, there are no more alternatives, so there is a back- - track to whatever came before the entire group. If (*THEN) is not + 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. If that succeeds and BAR fails, COND3 is tried. If subse- + quently BAZ fails, there are no more alternatives, so there is a back- + track to whatever came before the entire group. If (*THEN) is not inside an alternation, it acts like (*PRUNE). - The behaviour of (*THEN:NAME) is not the same as (*MARK:NAME)(*THEN). + The behaviour of (*THEN:NAME) is 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 + to the caller. However, (*SKIP:NAME) searches only for names set with (*MARK), ignoring those set by (*COMMIT), (*PRUNE) and (*THEN). - 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 + 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: A (B(*THEN)C) | D - If A and B are matched, but there is a failure in C, matching does not + If A and B are matched, but there is a failure in C, matching does not backtrack into A; instead it moves to the next alternative, that is, D. - However, if the subpattern containing (*THEN) is given an alternative, + However, if the subpattern containing (*THEN) is given an alternative, it behaves differently: A (B(*THEN)C | (*FAIL)) | D - The effect of (*THEN) is now confined to the inner subpattern. After a + The effect of (*THEN) is now confined to the inner subpattern. After a failure in C, matching moves to (*FAIL), which causes the whole subpat- - tern to fail because there are no more alternatives to try. In this + tern to fail because there are no more alternatives to try. In this case, matching does now backtrack into A. - Note that a conditional subpattern is not considered as having two - alternatives, because only one is ever used. In other words, the | + 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: ^.*? (?(?=a) a | b(*THEN)c ) - If the subject is "ba", this pattern does not match. Because .*? is - ungreedy, it initially matches zero characters. The condition (?=a) - then fails, the character "b" is matched, but "c" is not. At this - point, matching does not backtrack to .*? as might perhaps be expected - from the presence of the | character. The conditional subpattern is + If the subject is "ba", this pattern does not match. Because .*? is + ungreedy, it initially matches zero characters. The condition (?=a) + then fails, the character "b" is matched, but "c" is not. At this + point, matching does not backtrack to .*? as might perhaps be expected + from the presence of the | character. The conditional subpattern is part of the single alternative that comprises the whole pattern, and so - the match fails. (If there was a backtrack into .*?, allowing it to + the match fails. (If there was a backtrack into .*?, allowing it to match "b", the match would succeed.) - The verbs just described provide four different "strengths" of control + The verbs just described provide four different "strengths" of control when subsequent matching fails. (*THEN) is the weakest, carrying on the - match at the next alternative. (*PRUNE) comes next, failing the match - at the current starting position, but allowing an advance to the next - character (for an unanchored pattern). (*SKIP) is similar, except that + match at the next alternative. (*PRUNE) comes next, failing the match + at the current starting position, but allowing an advance to the next + character (for an unanchored pattern). (*SKIP) is similar, except that the advance may be more than one character. (*COMMIT) is the strongest, causing the entire match to fail. More than one backtracking verb - If more than one backtracking verb is present in a pattern, the one - that is backtracked onto first acts. For example, consider this pat- + If more than one backtracking verb is present in a pattern, the one + that is backtracked onto first acts. For example, consider this pat- tern, where A, B, etc. are complex pattern fragments: (A(*COMMIT)B(*THEN)C|ABD) - If A matches but B fails, the backtrack to (*COMMIT) causes the entire + 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 + (*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 + causes it to be triggered, and its action is taken. There can never be a backtrack onto (*COMMIT). Backtracking verbs in repeated groups @@ -9126,73 +9191,73 @@ BACKTRACKING CONTROL /(a(*COMMIT)b)+ac/ - If the subject is "abac", Perl matches unless its optimizations are - disabled, but PCRE2 always fails because the (*COMMIT) in the second + If the subject is "abac", Perl matches unless its optimizations are + disabled, but PCRE2 always fails because the (*COMMIT) in the second repeat of the group acts. Backtracking verbs in assertions - (*FAIL) in any assertion has its normal effect: it forces an immediate - backtrack. The behaviour of the other backtracking verbs depends on - whether or not the assertion is standalone or acting as the condition + (*FAIL) in any assertion has its normal effect: it forces an immediate + backtrack. The behaviour of the other backtracking verbs depends on + whether or not the assertion is standalone or acting as the condition in a conditional subpattern. - (*ACCEPT) in a standalone positive assertion causes the assertion to - succeed without any further processing; captured strings and a (*MARK) - name (if set) are retained. In a standalone negative assertion, - (*ACCEPT) causes the assertion to fail without any further processing; + (*ACCEPT) in a standalone positive assertion causes the assertion to + succeed without any further processing; captured strings and a (*MARK) + name (if set) are retained. In a standalone negative assertion, + (*ACCEPT) causes the assertion to fail without any further processing; captured substrings and any (*MARK) name are discarded. - If the assertion is a condition, (*ACCEPT) causes the condition to be - true for a positive assertion and false for a negative one; captured + If the assertion is a condition, (*ACCEPT) causes the condition to be + true for a positive assertion and false for a negative one; captured substrings are retained in both cases. The remaining verbs act only when a later failure causes a backtrack to - reach them. This means that their effect is confined to the assertion, + reach them. This means that their effect is confined to the assertion, because lookaround assertions are atomic. A backtrack that occurs after an assertion is complete does not jump back into the assertion. Note in - particular that a (*MARK) name that is set in an assertion is not + particular that a (*MARK) name that is set in an assertion is not "seen" by an instance of (*SKIP:NAME) latter in the pattern. - The effect of (*THEN) is not allowed to escape beyond an assertion. If - there are no more branches to try, (*THEN) causes a positive assertion + The effect of (*THEN) is not allowed to escape beyond an assertion. If + there are no more branches to try, (*THEN) causes a positive assertion to be false, and a negative assertion to be true. - The other backtracking verbs are not treated specially if they appear - in a standalone positive assertion. In a conditional positive asser- + The other backtracking verbs are not treated specially if they appear + in a standalone positive assertion. In a conditional positive asser- tion, backtracking (from within the assertion) into (*COMMIT), (*SKIP), - or (*PRUNE) causes the condition to be false. However, for both stand- + or (*PRUNE) causes the condition to be false. However, for both stand- alone and conditional negative assertions, backtracking into (*COMMIT), (*SKIP), or (*PRUNE) causes the assertion to be true, without consider- ing any further alternative branches. Backtracking verbs in subroutines - These behaviours occur whether or not the subpattern is called recur- + These behaviours occur whether or not the subpattern is called recur- sively. - (*ACCEPT) in a subpattern called as a subroutine causes the subroutine - match to succeed without any further processing. Matching then contin- - ues after the subroutine call. Perl documents this behaviour. Perl's + (*ACCEPT) in a subpattern called as a subroutine causes the subroutine + match to succeed without any further processing. Matching then contin- + ues after the subroutine call. Perl documents this behaviour. Perl's treatment of the other verbs in subroutines is different in some cases. - (*FAIL) in a subpattern called as a subroutine has its normal effect: + (*FAIL) in a subpattern called as a subroutine has its normal effect: it forces an immediate backtrack. - (*COMMIT), (*SKIP), and (*PRUNE) cause the subroutine match to fail + (*COMMIT), (*SKIP), and (*PRUNE) cause the subroutine match to fail when triggered by being backtracked to in a subpattern called as a sub- routine. There is then a backtrack at the outer level. (*THEN), when triggered, skips to the next alternative in the innermost enclosing group within the subpattern that has alternatives (its normal - behaviour). However, if there is no such group within the subroutine - subpattern, the subroutine match fails and there is a backtrack at the + behaviour). However, if there is no such group within the subroutine + subpattern, the subroutine match fails and there is a backtrack at the outer level. SEE ALSO - pcre2api(3), pcre2callout(3), pcre2matching(3), pcre2syntax(3), + pcre2api(3), pcre2callout(3), pcre2matching(3), pcre2syntax(3), pcre2(3). @@ -9205,7 +9270,7 @@ AUTHOR REVISION - Last updated: 24 September 2018 + Last updated: 12 October 2018 Copyright (c) 1997-2018 University of Cambridge. ------------------------------------------------------------------------------ @@ -10413,6 +10478,15 @@ LOOKAHEAD AND LOOKBEHIND ASSERTIONS Each top-level branch of a lookbehind must be of a fixed length. +SCRIPT RUNS + + (*script_run:...) ) script run, can be backtracked into + (*sr:...) ) + + (*atomic_script_run:...) ) atomic script run + (*asr:...) ) + + BACKREFERENCES \n reference by number (can be ambiguous) @@ -10525,7 +10599,7 @@ AUTHOR REVISION - Last updated: 24 September 2018 + Last updated: 10 October 2018 Copyright (c) 1997-2018 University of Cambridge. ------------------------------------------------------------------------------ @@ -10633,6 +10707,108 @@ CASE-EQUIVALENCE IN UTF MODES such. +SCRIPT RUNS + + The pattern constructs (*script_run:...) and (*atomic_script_run:...), + with synonyms (*sr:...) and (*asr:...), verify that the string matched + within the parentheses is a script run. In concept, a script run is a + sequence of characters that are all from the same Unicode script. How- + ever, because some scripts are commonly used together, and because some + diacritical and other marks are used with multiple scripts, it is not + that simple. + + Every Unicode character has a Script property, mostly with a value cor- + responding to the name of a script, such as Latin, Greek, or Cyrillic. + There are also three special values: + + "Unknown" is used for code points that have not been assigned, and also + for the surrogate code points. In the PCRE2 32-bit library, characters + whose code points are greater than the Unicode maximum (U+10FFFF), + which are accessible only in non-UTF mode, are assigned the Unknown + script. + + "Common" is used for characters that are used with many scripts. These + include punctuation, emoji, mathematical, musical, and currency sym- + bols, and the ASCII digits 0 to 9. + + "Inherited" is used for characters such as diacritical marks that mod- + ify a previous character. These are considered to take on the script of + the character that they modify. + + Some Inherited characters are used with many scripts, but many of them + are only normally used with a small number of scripts. For example, + U+102E0 (Coptic Epact thousands mark) is used only with Arabic and Cop- + tic. In order to make it possible to check this, a Unicode property + called Script Extension exists. Its value is a list of scripts that + apply to the character. For the majority of characters, the list con- + tains just one script, the same one as the Script property. However, + for characters such as U+102E0 more than one Script is listed. There + are also some Common characters that have a single, non-Common script + in their Script Extension list. + + The next section describes the basic rules for deciding whether a given + string of characters is a script run. Note, however, that there are + some special cases involving the Chinese Han script, and an additional + constraint for decimal digits. These are covered in subsequent sec- + tions. + + Basic script run rules + + A string that is less than two characters long is a script run. This is + the only case in which an Unknown character can be part of a script + run. Longer strings are checked using only the Script Extensions prop- + erty, not the basic Script property. + + If a character's Script Extension property is the single value "Inher- + ited", it is always accepted as part of a script run. This is also true + for the property "Common", subject to the checking of decimal digits + described below. All the remaining characters in a script run must have + at least one script in common in their Script Extension lists. In set- + theoretic terminology, the intersection of all the sets of scripts must + not be empty. + + A simple example is an Internet name such as "google.com". The letters + are all in the Latin script, and the dot is Common, so this string is a + script run. However, the Cyrillic letter "o" looks exactly the same as + the Latin "o"; a string that looks the same, but with Cyrillic "o"s is + not a script run. + + More interesting examples involve characters with more than one script + in their Script Extension. Consider the following characters: + + U+060C Arabic comma + U+06D4 Arabic full stop + + The first has the Script Extension list Arabic, Hanifi Rohingya, Syr- + iac, and Thaana; the second has just Arabic and Hanifi Rohingya. Both + of them could appear in script runs of either Arabic or Hanifi + Rohingya. The first could also appear in Syriac or Thaana script runs, + but the second could not. + + The Chinese Han script + + The Chinese Han script is commonly used in conjunction with other + scripts for writing certain languages. Japanese uses the Hiragana and + Katakana scripts together with Han; Korean uses Hangul and Han; Tai- + wanese Mandarin uses Bopomofo and Han. These three combinations are + treated as special cases when checking script runs and are, in effect, + "virtual scripts". Thus, a script run may contain a mixture of Hira- + gana, Katakana, and Han, or a mixture of Hangul and Han, or a mixture + of Bopomofo and Han, but not, for example, a mixture of Hangul and + Bopomofo and Han. PCRE2 (like Perl) follows Unicode's Technical Stan- + dard 39 ("Unicode Security Mechanisms", http://uni- + code.org/reports/tr39/) in allowing such mixtures. + + Decimal digits + + Unicode contains many sets of 10 decimal digits in different scripts, + and some scripts (including the Common script) contain more than one + set. Some of these decimal digits them are visually indistinguishable + from the common ASCII digits. In addition to the script checking + described above, if a script run contains any decimal digits, they must + all come from the same set of 10 adjacent characters. + + VALIDITY OF UTF STRINGS When the PCRE2_UTF option is set, the strings passed as patterns and @@ -10788,7 +10964,7 @@ AUTHOR REVISION - Last updated: 02 September 2018 + Last updated: 12 October 2018 Copyright (c) 1997-2018 University of Cambridge. ------------------------------------------------------------------------------ diff --git a/doc/pcre2matching.3 b/doc/pcre2matching.3 index 81ce968..20ffac5 100644 --- a/doc/pcre2matching.3 +++ b/doc/pcre2matching.3 @@ -1,4 +1,4 @@ -.TH PCRE2MATCHING 3 "29 September 2014" "PCRE2 10.00" +.TH PCRE2MATCHING 3 "10 October 2018" "PCRE2 10.33" .SH NAME PCRE2 - Perl-compatible regular expressions (revised API) .SH "PCRE2 MATCHING ALGORITHMS" @@ -113,7 +113,8 @@ do want multiple matches in such cases, either use an ungreedy repeat ("a\ed+?") or set the PCRE2_NO_AUTO_POSSESS option when compiling. .P There are a number of features of PCRE2 regular expressions that are not -supported by the alternative matching algorithm. They are as follows: +supported or behave differently in the alternative matching function. Those +that are not supported cause an error if encountered. .P 1. Because the algorithm finds all possible matches, the greedy or ungreedy nature of repetition quantifiers is not relevant (though it may affect @@ -135,24 +136,26 @@ possibilities, and PCRE2's implementation of this algorithm does not attempt to do this. This means that no captured substrings are available. .P 3. Because no substrings are captured, backreferences within the pattern are -not supported, and cause errors if encountered. +not supported. .P 4. For the same reason, conditional expressions that use a backreference as the condition or test for a specific group recursion are not supported. .P -5. Because many paths through the tree may be active, the \eK escape sequence, -which resets the start of the match when encountered (but may be on some paths -and not on others), is not supported. It causes an error if encountered. +5. Again for the same reason, script runs are not supported. .P -6. Callouts are supported, but the value of the \fIcapture_top\fP field is +6. Because many paths through the tree may be active, the \eK escape sequence, +which resets the start of the match when encountered (but may be on some paths +and not on others), is not supported. +.P +7. Callouts are supported, but the value of the \fIcapture_top\fP field is always 1, and the value of the \fIcapture_last\fP field is always 0. .P -7. The \eC escape sequence, which (in the standard algorithm) always matches a +8. The \eC escape sequence, which (in the standard algorithm) always matches a single code unit, even in a UTF mode, is not supported in these modes, because the alternative algorithm moves through the subject string one character (not code unit) at a time, for all active paths through the tree. .P -8. Except for (*FAIL), the backtracking control verbs such as (*PRUNE) are not +9. Except for (*FAIL), the backtracking control verbs such as (*PRUNE) are not supported. (*FAIL) is supported, and behaves like a failing negative assertion. . . @@ -188,7 +191,7 @@ The alternative algorithm suffers from a number of disadvantages: because it has to search for all possible matches, but is also because it is less susceptible to optimization. .P -2. Capturing parentheses and backreferences are not supported. +2. Capturing parentheses, backreferences, and script runs are not supported. .P 3. Although atomic groups are supported, their use does not provide the performance advantage that it does for the standard algorithm. @@ -208,6 +211,6 @@ Cambridge, England. .rs .sp .nf -Last updated: 29 September 2014 -Copyright (c) 1997-2014 University of Cambridge. +Last updated: 10 October 2018 +Copyright (c) 1997-2018 University of Cambridge. .fi diff --git a/doc/pcre2pattern.3 b/doc/pcre2pattern.3 index 5e6baef..fa84844 100644 --- a/doc/pcre2pattern.3 +++ b/doc/pcre2pattern.3 @@ -1,4 +1,4 @@ -.TH PCRE2PATTERN 3 "24 September 2018" "PCRE2 10.33" +.TH PCRE2PATTERN 3 "12 October 2018" "PCRE2 10.33" .SH NAME PCRE2 - Perl-compatible regular expressions (revised API) .SH "PCRE2 REGULAR EXPRESSION DETAILS" @@ -755,7 +755,7 @@ sequences that match characters with specific properties are available. In 8-bit non-UTF-8 mode, these sequences are of course limited to testing characters whose code points are less than 256, but they do work in this mode. In 32-bit non-UTF mode, code points greater than 0x10ffff (the Unicode limit) -may be encountered. These are all treated as being in the Common script and +may be encountered. These are all treated as being in the Unknown script and with an unassigned type. The extra escape sequences are: .sp \ep{\fIxx\fP} a character with the \fIxx\fP property @@ -781,8 +781,10 @@ example: \ep{Greek} \eP{Han} .sp -Those that are not part of an identified script are lumped together as -"Common". The current list of scripts is: +Unassigned characters (and in non-UTF 32-bit mode, characters with code points +greater than 0x10FFFF) are assigned the "Unknown" script. Others that are not +part of an identified script are lumped together as "Common". The current list +of scripts is: .P Adlam, Ahom, @@ -928,6 +930,7 @@ Tibetan, Tifinagh, Tirhuta, Ugaritic, +Unknown, Vai, Warang_Citi, Yi, @@ -2603,6 +2606,73 @@ is another pattern that matches "foo" preceded by three digits and any three characters that are not "999". . . +.SH "SCRIPT RUNS" +.rs +.sp +In concept, a script run is a sequence of characters that are all from the same +Unicode script such as Latin or Greek. However, because some scripts are +commonly used together, and because some diacritical and other marks are used +with multiple scripts, it is not that simple. There is a full description of +the rules that PCRE2 uses in the section entitled +.\" HTML +.\" +"Script Runs" +.\" +in the +.\" HREF +\fBpcre2unicode\fP +.\" +documentation. +.P +If part of a pattern is enclosed between (*script_run: or (*sr: and a closing +parenthesis, it fails if the sequence of characters that it matches are not a +script run. After a failure, normal backtracking occurs. Script runs can be +used to detect spoofing attacks using characters that look the same, but are +from different scripts. The string "paypal.com" is an infamous example, where +the letters could be a mixture of Latin and Cyrillic. This pattern ensures that +the matched characters in a sequence of non-spaces that follow white space are +a script run: +.sp + \es+(*sr:\eS+) +.sp +To be sure that they are all from the Latin script (for example), a lookahead +can be used: +.sp + \es+(?=\ep{Latin})(*sr:\eS+) +.sp +This works as long as the first character is expected to be a character in that +script, and not (for example) punctuation, which is allowed with any script. If +this is not the case, a more creative lookahead is needed. For example, if +digits, underscore, and dots are permitted at the start: +.sp + \es+(?=[0-9_.]*\ep{Latin})(*sr:\eS+) +.sp +.P +In many cases, backtracking into a script run pattern fragment is not +desirable. The script run can employ an atomic group to prevent this. Because +this is a common requirement, a shorthand notation is provided by +(*atomic_script_run: or (*asr: +.sp + (*asr:...) is the same as (*sr:(?>...)) +.sp +Note that the atomic group is inside the script run. Putting it outside would +not prevent backtracking into the script run pattern. +.P +Support for script runs is not available if PCRE2 is compiled without Unicode +support. A compile-time error is given if any of the above constructs is +encountered. Script runs are not supported by the alternate matching function, +\fBpcre2_dfa_match()\fP because they use the same mechanism as capturing +parentheses. +.P +\fBWarning:\fP The (*ACCEPT) control verb +.\" HTML +.\" +(see below) +.\" +should not be used within a script run subpattern, because it causes an +immediate exit from the subpattern, bypassing the script run checking. +. +. .\" HTML .SH "CONDITIONAL SUBPATTERNS" .rs @@ -3267,6 +3337,7 @@ Experiments with Perl suggest that it too has similar optimizations, and like PCRE2, turning them off can change the result of a match. . . +.\" HTML .SS "Verbs that act immediately" .rs .sp @@ -3287,6 +3358,10 @@ example: .sp This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by the outer parentheses. +.P +\fBWarning:\fP (*ACCEPT) should not be used within a script run subpattern, +because it causes an immediate exit from the subpattern, bypassing the script +run checking. .sp (*FAIL) or (*FAIL:NAME) .sp @@ -3692,6 +3767,6 @@ Cambridge, England. .rs .sp .nf -Last updated: 24 September 2018 +Last updated: 12 October 2018 Copyright (c) 1997-2018 University of Cambridge. .fi diff --git a/doc/pcre2syntax.3 b/doc/pcre2syntax.3 index 2daf9a9..07b7d80 100644 --- a/doc/pcre2syntax.3 +++ b/doc/pcre2syntax.3 @@ -1,4 +1,4 @@ -.TH PCRE2SYNTAX 3 "24 September 2018" "PCRE2 10.33" +.TH PCRE2SYNTAX 3 "10 October 2018" "PCRE2 10.33" .SH NAME PCRE2 - Perl-compatible regular expressions (revised API) .SH "PCRE2 REGULAR EXPRESSION SYNTAX SUMMARY" @@ -511,6 +511,16 @@ setting with a similar syntax. Each top-level branch of a lookbehind must be of a fixed length. . . +.SH "SCRIPT RUNS" +.rs +.sp + (*script_run:...) ) script run, can be backtracked into + (*sr:...) ) +.sp + (*atomic_script_run:...) ) atomic script run + (*asr:...) ) +. +. .SH "BACKREFERENCES" .rs .sp @@ -633,6 +643,6 @@ Cambridge, England. .rs .sp .nf -Last updated: 24 September 2018 +Last updated: 10 October 2018 Copyright (c) 1997-2018 University of Cambridge. .fi diff --git a/doc/pcre2unicode.3 b/doc/pcre2unicode.3 index 877d887..f1f5810 100644 --- a/doc/pcre2unicode.3 +++ b/doc/pcre2unicode.3 @@ -1,4 +1,4 @@ -.TH PCRE2UNICODE 3 "02 September 2018" "PCRE2 10.32" +.TH PCRE2UNICODE 3 "12 October 2018" "PCRE2 10.33" .SH NAME PCRE - Perl-compatible regular expressions (revised API) .SH "UNICODE AND UTF SUPPORT" @@ -118,6 +118,108 @@ few Unicode characters such as Greek sigma have more than two code points that are case-equivalent, and these are treated as such. . . +.\" HTML +.SH "SCRIPT RUNS" +.rs +.sp +The pattern constructs (*script_run:...) and (*atomic_script_run:...), with +synonyms (*sr:...) and (*asr:...), verify that the string matched within the +parentheses is a script run. In concept, a script run is a sequence of +characters that are all from the same Unicode script. However, because some +scripts are commonly used together, and because some diacritical and other +marks are used with multiple scripts, it is not that simple. +.P +Every Unicode character has a Script property, mostly with a value +corresponding to the name of a script, such as Latin, Greek, or Cyrillic. There +are also three special values: +.P +"Unknown" is used for code points that have not been assigned, and also for the +surrogate code points. In the PCRE2 32-bit library, characters whose code +points are greater than the Unicode maximum (U+10FFFF), which are accessible +only in non-UTF mode, are assigned the Unknown script. +.P +"Common" is used for characters that are used with many scripts. These include +punctuation, emoji, mathematical, musical, and currency symbols, and the ASCII +digits 0 to 9. +.P +"Inherited" is used for characters such as diacritical marks that modify a +previous character. These are considered to take on the script of the character +that they modify. +.P +Some Inherited characters are used with many scripts, but many of them are only +normally used with a small number of scripts. For example, U+102E0 (Coptic +Epact thousands mark) is used only with Arabic and Coptic. In order to make it +possible to check this, a Unicode property called Script Extension exists. Its +value is a list of scripts that apply to the character. For the majority of +characters, the list contains just one script, the same one as the Script +property. However, for characters such as U+102E0 more than one Script is +listed. There are also some Common characters that have a single, non-Common +script in their Script Extension list. +.P +The next section describes the basic rules for deciding whether a given string +of characters is a script run. Note, however, that there are some special cases +involving the Chinese Han script, and an additional constraint for decimal +digits. These are covered in subsequent sections. +. +. +.SS "Basic script run rules" +.rs +.sp +A string that is less than two characters long is a script run. This is the +only case in which an Unknown character can be part of a script run. Longer +strings are checked using only the Script Extensions property, not the basic +Script property. +.P +If a character's Script Extension property is the single value "Inherited", it +is always accepted as part of a script run. This is also true for the property +"Common", subject to the checking of decimal digits described below. All the +remaining characters in a script run must have at least one script in common in +their Script Extension lists. In set-theoretic terminology, the intersection of +all the sets of scripts must not be empty. +.P +A simple example is an Internet name such as "google.com". The letters are all +in the Latin script, and the dot is Common, so this string is a script run. +However, the Cyrillic letter "o" looks exactly the same as the Latin "o"; a +string that looks the same, but with Cyrillic "o"s is not a script run. +.P +More interesting examples involve characters with more than one script in their +Script Extension. Consider the following characters: +.sp + U+060C Arabic comma + U+06D4 Arabic full stop +.sp +The first has the Script Extension list Arabic, Hanifi Rohingya, Syriac, and +Thaana; the second has just Arabic and Hanifi Rohingya. Both of them could +appear in script runs of either Arabic or Hanifi Rohingya. The first could also +appear in Syriac or Thaana script runs, but the second could not. +. +. +.SS "The Chinese Han script" +.rs +.sp +The Chinese Han script is commonly used in conjunction with other scripts for +writing certain languages. Japanese uses the Hiragana and Katakana scripts +together with Han; Korean uses Hangul and Han; Taiwanese Mandarin uses Bopomofo +and Han. These three combinations are treated as special cases when checking +script runs and are, in effect, "virtual scripts". Thus, a script run may +contain a mixture of Hiragana, Katakana, and Han, or a mixture of Hangul and +Han, or a mixture of Bopomofo and Han, but not, for example, a mixture of +Hangul and Bopomofo and Han. PCRE2 (like Perl) follows Unicode's Technical +Standard 39 ("Unicode Security Mechanisms", http://unicode.org/reports/tr39/) +in allowing such mixtures. +. +. +.SS "Decimal digits" +.rs +.sp +Unicode contains many sets of 10 decimal digits in different scripts, and some +scripts (including the Common script) contain more than one set. Some of these +decimal digits them are visually indistinguishable from the common ASCII +digits. In addition to the script checking described above, if a script run +contains any decimal digits, they must all come from the same set of 10 +adjacent characters. +. +. .SH "VALIDITY OF UTF STRINGS" .rs .sp @@ -285,6 +387,6 @@ Cambridge, England. .rs .sp .nf -Last updated: 02 September 2018 +Last updated: 12 October 2018 Copyright (c) 1997-2018 University of Cambridge. .fi diff --git a/testdata/testinput4 b/testdata/testinput4 index 7f2894b..9ab4191 100644 --- a/testdata/testinput4 +++ b/testdata/testinput4 @@ -2410,6 +2410,7 @@ \x{3031}\x{3041}\x{30a1}\x{2e80} [Hira Kata] Hira Kata Han \x{060c}\x{06d4}\x{0600}\x{10d00}\x{0700} [Arab Rohg Syrc Thaa] [Arab Rohg] Arab Rohg Syrc \x{060c}\x{06d4}\x{0700}\x{0600}\x{10d00} [Arab Rohg Syrc Thaa] [Arab Rohg] Syrc Arab Rohg + \x{2e80}\x{3041}\x{3001}\x{3031}\x{2e80} Han Hira [Bopo, Han, etc] [Hira Kata] Han /(?