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| author | ph10 <ph10@6239d852-aaf2-0410-a92c-79f79f948069> | 2020-01-27 15:36:29 +0000 |
|---|---|---|
| committer | ph10 <ph10@6239d852-aaf2-0410-a92c-79f79f948069> | 2020-01-27 15:36:29 +0000 |
| commit | 8d66348cbeb82ab7234aee227e483796a771ee19 (patch) | |
| tree | 3fa885f567a824d08a79f89ffa36468a85602964 /doc/pcre2.txt | |
| parent | b251f0bc17a4d5a3b3f7690432113c773bcbe13f (diff) | |
| download | pcre2-8d66348cbeb82ab7234aee227e483796a771ee19.tar.gz | |
Documentation update.
git-svn-id: svn://vcs.exim.org/pcre2/code/trunk@1214 6239d852-aaf2-0410-a92c-79f79f948069
Diffstat (limited to 'doc/pcre2.txt')
| -rw-r--r-- | doc/pcre2.txt | 869 |
1 files changed, 437 insertions, 432 deletions
diff --git a/doc/pcre2.txt b/doc/pcre2.txt index bbdb2e6..a05a57f 100644 --- a/doc/pcre2.txt +++ b/doc/pcre2.txt @@ -8396,7 +8396,12 @@ NON-ATOMIC ASSERTIONS pattern match fails exactly as before because nothing has changed, so using a non-atomic assertion just wastes resources. - Non-atomic assertions are not supported by the alternative matching + There is one exception to backtracking into a non-atomic assertion. If + an (*ACCEPT) control verb is triggered, the assertion succeeds atomi- + cally. That is, a subsequent match failure cannot backtrack into the + assertion. + + Non-atomic assertions are not supported by the alternative matching function pcre2_dfa_match(). They are supported by JIT, but only if they do not contain any control verbs such as (*ACCEPT). (This may change in future). Note that assertions that appear as conditions for conditional @@ -8405,42 +8410,42 @@ NON-ATOMIC ASSERTIONS SCRIPT RUNS - 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. + 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 oc- - curs. Script runs can be used to detect spoofing attacks using charac- - ters that look the same, but are from different scripts. The string - "paypal.com" is an infamous example, where the letters could be a mix- + 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 oc- + curs. Script runs can be used to detect spoofing attacks using charac- + ters that look the same, but are from different scripts. The string + "paypal.com" is an infamous example, where the letters could be a mix- ture of Latin and Cyrillic. This pattern ensures that the matched char- acters 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 + 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 + 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 + 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:(?>...)) @@ -8448,13 +8453,13 @@ SCRIPT RUNS 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 + 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- + 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 + Warning: The (*ACCEPT) control verb (see below) should not be used within a script run group, because it causes an immediate exit from the group, bypassing the script run checking. @@ -8463,116 +8468,116 @@ CONDITIONAL GROUPS It is possible to cause the matching process to obey a pattern fragment conditionally or to choose between two alternative fragments, depending - on the result of an assertion, or whether a specific capture group has + on the result of an assertion, or whether a specific capture group has already been matched. The two possible forms of conditional group 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- - natives in the group, a compile-time error occurs. Each of the two al- + 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 group, a compile-time error occurs. Each of the two al- ternatives may itself contain nested groups of any form, including con- - ditional groups; the restriction to two alternatives applies only at - the level of the condition itself. This pattern fragment is an example + ditional groups; the restriction to two alternatives applies only at + the level of the condition itself. This pattern fragment is an example where the alternatives are complex: (?(1) (A|B|C) | (D | (?(2)E|F) | E) ) There are five kinds of condition: references to capture groups, refer- - ences to recursion, two pseudo-conditions called DEFINE and VERSION, + ences to recursion, two pseudo-conditions called DEFINE and VERSION, and assertions. Checking for a used capture group by number - If the text between the parentheses consists of a sequence of digits, - the condition is true if a capture group of that number has previously - matched. If there is more than one capture group with the same number - (see the earlier section about duplicate group numbers), the condition + If the text between the parentheses consists of a sequence of digits, + the condition is true if a capture group of that number has previously + matched. If there is more than one capture group with the same number + (see the earlier section about duplicate group numbers), the condition is true if any of them have matched. An alternative notation is to pre- cede the digits with a plus or minus sign. In this case, the group num- - ber is relative rather than absolute. The most recently opened capture - group 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 capture group can be referenced as (?(+1), and so on. - (The value zero in any of these forms is not used; it provokes a com- + ber is relative rather than absolute. The most recently opened capture + group 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 capture group can be referenced as (?(+1), and so on. + (The value zero in any of these forms is not used; it provokes a com- pile-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 group that tests whether or not the first - capture group matched. If it did, that is, if subject started with an - opening parenthesis, the condition is true, and so the yes-pattern is - executed and a closing parenthesis is required. Otherwise, since no- + ond part matches one or more characters that are not parentheses. The + third part is a conditional group that tests whether or not the first + capture group matched. If it did, that is, if subject started with an + opening parenthesis, the condition is true, and so the yes-pattern is + executed and a closing parenthesis is required. Otherwise, since no- pattern is not present, the conditional group matches nothing. In other - words, this pattern matches a sequence of non-parentheses, optionally + 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 capture group by name - Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a - used capture group 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 (?(<name>)...) or (?('name')...) to test for a + used capture group 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 group gives this: (?<OPEN> \( )? [^()]+ (?(<OPEN>) \) ) - If the name used in a condition of this kind is a duplicate, the test - is applied to all groups of the same name, and is true if any one of + If the name used in a condition of this kind is a duplicate, the test + is applied to all groups 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 "Groups as + "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 "Groups as subroutines" below for details of recursion and subroutine calls. - If a condition is the string (R), and there is no capture group with - the name R, the condition is true if matching is currently in a recur- - sion or subroutine call to the whole pattern or any capture group. If - digits follow the letter R, and there is no group with that name, the - condition is true if the most recent call is into a group with the - given number, which must exist somewhere in the overall pattern. This + If a condition is the string (R), and there is no capture group with + the name R, the condition is true if matching is currently in a recur- + sion or subroutine call to the whole pattern or any capture group. If + digits follow the letter R, and there is no group with that name, the + condition is true if the most recent call is into a group with the + 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 capture group with a matching - name, the condition tests for its being set, as described in the sec- - tion above, instead of testing for recursion. For example, creating a - group with the name R1 by adding (?<R1>) to the above pattern com- + However, in both cases, if there is a capture group with a matching + name, the condition tests for its being set, as described in the sec- + tion above, instead of testing for recursion. For example, creating a + group with the name R1 by adding (?<R1>) to the above pattern com- pletely changes its meaning. If a name preceded by ampersand follows the letter R, for example: (?(R&name)...) - the condition is true if the most recent recursion is into a group of + the condition is true if the most recent recursion is into a group 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 - duplicate, the test is applied to all groups of the same name, and is + the current level. If the name used in a condition of this kind is a + duplicate, the test is applied to all groups of the same name, and is true if any one of them is the most recent recursion. At "top level", all these recursion test conditions are false. @@ -8580,111 +8585,111 @@ CONDITIONAL GROUPS Defining capture groups 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 rest of the conditional group. It is al- - ways skipped if control reaches this point in the pattern; the idea of - DEFINE is that it can be used to define subroutines that can be refer- - enced from elsewhere. (The use of subroutines is described below.) For - example, a pattern to match an IPv4 address such as "192.168.23.245" + ways skipped if control reaches this point in the pattern; the idea of + DEFINE is that it can be used to define subroutines that can be refer- + enced from elsewhere. (The use of subroutines is described below.) For + example, a pattern to match an IPv4 address such as "192.168.23.245" could be written like this (ignore white space and line breaks): (?(DEFINE) (?<byte> 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 a - parenthesized assertion. This may be a positive or negative lookahead - or lookbehind assertion. However, it must be a traditional atomic as- + If the condition is not in any of the above formats, it must be a + parenthesized assertion. This may be a positive or negative lookahead + or lookbehind assertion. However, it must be a traditional atomic as- sertion, not one of the PCRE2-specific non-atomic assertions. - Consider this pattern, again containing non-significant white space, + 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 op- + The condition is a positive lookahead assertion that matches an op- tional 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 let- - ter 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 + ter 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 capture groups, any cap- - turing that occurs in a matching branch is retained afterwards, for - both positive and negative assertions, because matching always contin- - ues after the assertion, whether it succeeds or fails. (Compare non- - conditional assertions, for which captures are retained only for posi- + turing that occurs in a matching branch is retained afterwards, for + both positive and negative assertions, because matching always contin- + ues after the assertion, whether it succeeds or fails. (Compare non- + conditional assertions, for which captures are retained only for posi- tive 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 group name or number. The characters that + 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 group 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: @@ -8693,67 +8698,67 @@ RECURSIVE PATTERNS The (?p{...}) item interpolates Perl code at run time, and in this case refers recursively to the pattern in which it appears. - Obviously, PCRE2 cannot support the interpolation of Perl code. In- - stead, it supports special syntax for recursion of the entire pattern, + Obviously, PCRE2 cannot support the interpolation of Perl code. In- + stead, it supports special syntax for recursion of the entire pattern, and also for individual capture group recursion. After its introduction 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 - capture group of the given number, provided that it occurs inside that - group. (If not, it is a non-recursive subroutine call, which is de- + A special item that consists of (? followed by a number greater than + zero and a closing parenthesis is a recursive subroutine call of the + capture group of the given number, provided that it occurs inside that + group. (If not, it is a non-recursive subroutine call, which is de- scribed in the next section.) The special item (?R) or (?0) is a recur- sive 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 re- + First it matches an opening parenthesis. Then it matches any number of + substrings which can either be a sequence of non-parentheses, or a re- cursive match of the pattern itself (that is, a correctly parenthesized - substring). Finally there is a closing parenthesis. Note the use of a - possessive quantifier to avoid backtracking into sequences of non- + 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 capture group numbers are in use, - relative references refer to the earliest group with the appropriate + Be aware however, that if duplicate capture group numbers are in use, + relative references refer to the earliest group with the appropriate number. Consider, for example: (?|(a)|(b)) (c) (?-2) The first two capture 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 + (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, rela- + the same if an absolute reference (?1) was used. In other words, rela- tive references are just a shorthand for computing a group number. - It is also possible to refer to subsequent capture groups, by writing - references such as (?+2). However, these cannot be recursive because - the reference is not inside the parentheses that are referenced. They - are always non-recursive subroutine calls, as described in the next + It is also possible to refer to subsequent capture groups, by writing + references such as (?+2). However, these cannot be recursive because + the reference is not inside the parentheses that are referenced. 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> \( ( [^()]++ | (?&pn) )* \) ) @@ -8762,57 +8767,57 @@ RECURSIVE PATTERNS 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 capture group - 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 + the value for the inner capturing parentheses (numbered 2) is "ef", + which is the last value taken on at the top level. If a capture group + 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 group, with two - different alternatives for the recursive and non-recursive cases. The + In this pattern, (?(R) is the start of a conditional group, 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 subroutine 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 subroutine 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 re- @@ -8820,47 +8825,47 @@ RECURSIVE PATTERNS ^((.)(?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 group was called recursively or as a subroutine (see the next + 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 group was called recursively or as a subroutine (see the next section), it had no access to any values that were captured outside the - recursion, whereas in PCRE2 these values can be referenced. Consider + 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. GROUPS AS SUBROUTINES - If the syntax for a recursive group 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 + If the syntax for a recursive group 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 group as an independent subpattern which it tries - to match at the current matching position. The called group may be de- - fined before or after the reference. A numbered reference can be abso- + to match at the current matching position. The called group may be de- + fined before or after the reference. A numbered reference can be abso- lute or relative, as in these examples: (...(absolute)...)...(?2)... @@ -8871,106 +8876,106 @@ GROUPS 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 group is - defined, so if it is used as a subroutine, such options cannot be + Processing options such as case-independence are fixed when a group 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 group. - The behaviour of backtracking control verbs in groups when called as + The behaviour of backtracking control verbs in groups 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 calling a group as a subroutine, possibly re- - cursively. Here are two of the examples used above, rewritten using + cursively. Here are two of the examples used above, rewritten using this syntax: (?<pn> \( ( (?>[^()]+) | \g<pn> )* \) ) (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<arg>) 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<arg>) 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) @@ -8980,78 +8985,78 @@ 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 + 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, and may behave differently depending on whether - or not a name argument is present. The names are not required to be + or not a name argument is present. The names are not required to be unique within the pattern. - 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. Except for (*ACCEPT), they may not be quantified. - 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 - capture groups called as subroutines (whether or not recursively) is + The behaviour of these verbs in repeated groups, assertions, and in + capture groups called as subroutines (whether or not recursively) is documented below. Optimizations that affect backtracking verbs 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 @@ -9060,77 +9065,77 @@ 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 capture group that is + This verb causes the match to end successfully, skipping the remainder + of the pattern. However, when it is inside a capture group that is called as a subroutine, only that group 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. - (*ACCEPT) is the only backtracking verb that is allowed to be quanti- - fied because an ungreedy quantification with a minimum of zero acts + (*ACCEPT) is the only backtracking verb that is allowed to be quanti- + fied because an ungreedy quantification with a minimum of zero acts only when a backtrack happens. Consider, for example, (A(*ACCEPT)??B)C - where A, B, and C may be complex expressions. After matching "A", the - matcher processes "BC"; if that fails, causing a backtrack, (*ACCEPT) - is triggered and the match succeeds. In both cases, all but C is cap- - tured. Whereas (*COMMIT) (see below) means "fail on backtrack", a re- + where A, B, and C may be complex expressions. After matching "A", the + matcher processes "BC"; if that fails, causing a backtrack, (*ACCEPT) + is triggered and the match succeeds. In both cases, all but C is cap- + tured. Whereas (*COMMIT) (see below) means "fail on backtrack", a re- peated (*ACCEPT) of this type means "succeed on backtrack". - Warning: (*ACCEPT) should not be used within a script run group, be- - cause it causes an immediate exit from the group, bypassing the script + Warning: (*ACCEPT) should not be used within a script run group, be- + cause it causes an immediate exit from the group, 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 the same as (*MARK:NAME)(*AC- - CEPT) and (*MARK:NAME)(*FAIL), respectively, that is, a (*MARK) is + (*ACCEPT:NAME) and (*FAIL:NAME) behave the same as (*MARK:NAME)(*AC- + CEPT) and (*MARK:NAME)(*FAIL), respectively, that is, a (*MARK) is recorded just before the verb acts. Recording which path was taken - There is one verb whose main purpose is to track how a match was ar- - rived at, though it also has a secondary use in conjunction with ad- + There is one verb whose main purpose is to track how a match was ar- + rived at, though it also has a secondary use in conjunction with ad- vancing the match starting point (see (*SKIP) below). (*MARK:NAME) or (*:NAME) - A name is always required with this verb. For all the other backtrack- + A name is always required with this verb. For all the other backtrack- ing control verbs, a NAME argument is optional. - When a match succeeds, the name of the last-encountered mark name on + 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) and other verbs, + mentation. This applies to all instances of (*MARK) and other verbs, including those inside assertions and atomic groups. However, there are - differences in those cases when (*MARK) is used in conjunction with + differences in those cases when (*MARK) is used in conjunction with (*SKIP) as described below. - The mark name that was last encountered on the matching path is passed - back. A verb without a NAME argument is ignored for this purpose. Here - is an example of pcre2test output, where the "mark" modifier requests + The mark name that was last encountered on the matching path is passed + back. A verb without a NAME argument is ignored for this purpose. 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 @@ -9142,76 +9147,76 @@ 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 preceding 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 + 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 that are set with (*MARK), ignoring those set by any of the other back- tracking verbs. - 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 an- - chor, unless PCRE2's start-of-match optimizations are turned off, as + chor, unless PCRE2's start-of-match optimizations are turned off, as shown in this output from pcre2test: re> /(*COMMIT)abc/ @@ -9222,68 +9227,68 @@ 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 other backtracking verbs. (*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 at- - tempt 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 at- + tempt would start at the second character instead of skipping on to "c". - If (*SKIP) is used to specify a new starting position that is the same - as the starting position of the current match, or (by being inside a - lookbehind) earlier, the position specified by (*SKIP) is ignored, and + If (*SKIP) is used to specify a new starting position that is the same + as the starting position of the current match, or (by being inside a + lookbehind) earlier, the position specified by (*SKIP) is ignored, and instead the normal "bumpalong" occurs. (*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: @@ -9297,105 +9302,105 @@ 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 + Note that (*SKIP:NAME) searches only for names set by (*MARK:NAME). It ignores names that are set by other backtracking verbs. (*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 in- + 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 in- side 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 other backtracking verbs. - A group that does not contain a | character is just a part of the en- - closing alternative; it is not a nested alternation with only one al- + A group that does not contain a | character is just a part of the en- + closing alternative; it is not a nested alternation with only one al- ternative. The effect of (*THEN) extends beyond such a group to the en- - closing alternative. Consider this pattern, where A, B, etc. are com- - plex pattern fragments that do not contain any | characters at this + closing alternative. Consider this pattern, where A, B, etc. are com- + plex 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 group containing (*THEN) is given an alternative, it + However, if the group 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 group. After a fail- - ure in C, matching moves to (*FAIL), which causes the whole group to - fail because there are no more alternatives to try. In this case, + ure in C, matching moves to (*FAIL), which causes the whole group to + fail because there are no more alternatives to try. In this case, matching does backtrack into A. - Note that a conditional group is not considered as having two alterna- - tives, because only one is ever used. In other words, the | character - in a conditional group has a different meaning. Ignoring white space, + Note that a conditional group is not considered as having two alterna- + tives, because only one is ever used. In other words, the | character + in a conditional group 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 un- - greedy, 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 group 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 match "b", + greedy, 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 group 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 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 @@ -9405,50 +9410,50 @@ 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 group. - (*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, (*AC- + (*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, (*AC- CEPT) causes the assertion to fail without any further processing; cap- tured 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, for the Perl-compatible assertions, their + reach them. This means that, for the Perl-compatible assertions, their effect is confined to the assertion, because Perl lookaround assertions are atomic. A backtrack that occurs after such 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 "seen" by an instance + does not jump back into the assertion. Note in particular that a + (*MARK) name that is set in an assertion is not "seen" by an instance of (*SKIP:NAME) later in the pattern. - PCRE2 now supports non-atomic positive assertions, as described in the - section entitled "Non-atomic assertions" above. These assertions must - be standalone (not used as conditions). They are not Perl-compatible. - For these assertions, a later backtrack does jump back into the asser- - tion, and therefore verbs such as (*COMMIT) can be triggered by back- + PCRE2 now supports non-atomic positive assertions, as described in the + section entitled "Non-atomic assertions" above. These assertions must + be standalone (not used as conditions). They are not Perl-compatible. + For these assertions, a later backtrack does jump back into the asser- + tion, and therefore verbs such as (*COMMIT) can be triggered by back- tracks from later 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. @@ -9458,26 +9463,26 @@ BACKTRACKING CONTROL These behaviours occur whether or not the group is called recursively. (*ACCEPT) in a group called as a subroutine causes the subroutine match - to succeed without any further processing. Matching then continues af- - ter the subroutine call. Perl documents this behaviour. Perl's treat- + to succeed without any further processing. Matching then continues af- + ter the subroutine call. Perl documents this behaviour. Perl's treat- ment of the other verbs in subroutines is different in some cases. - (*FAIL) in a group called as a subroutine has its normal effect: it + (*FAIL) in a group called as a subroutine has its normal effect: it forces an immediate backtrack. - (*COMMIT), (*SKIP), and (*PRUNE) cause the subroutine match to fail - when triggered by being backtracked to in a group called as a subrou- + (*COMMIT), (*SKIP), and (*PRUNE) cause the subroutine match to fail + when triggered by being backtracked to in a group called as a subrou- tine. There is then a backtrack at the outer level. (*THEN), when triggered, skips to the next alternative in the innermost - enclosing group that has alternatives (its normal behaviour). However, + enclosing group that has alternatives (its normal behaviour). However, if there is no such group within the subroutine's group, 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). @@ -9490,7 +9495,7 @@ AUTHOR REVISION - Last updated: 24 January 2020 + Last updated: 27 January 2020 Copyright (c) 1997-2020 University of Cambridge. ------------------------------------------------------------------------------ |