| .TH PCRE2CALLOUT 3 "03 February 2019" "PCRE2 10.33" |
| .SH NAME |
| PCRE2 - Perl-compatible regular expressions (revised API) |
| .SH SYNOPSIS |
| .rs |
| .sp |
| .B #include <pcre2.h> |
| .PP |
| .SM |
| .nf |
| .B int (*pcre2_callout)(pcre2_callout_block *, void *); |
| .sp |
| .B int pcre2_callout_enumerate(const pcre2_code *\fIcode\fP, |
| .B " int (*\fIcallback\fP)(pcre2_callout_enumerate_block *, void *)," |
| .B " void *\fIuser_data\fP);" |
| .fi |
| . |
| .SH DESCRIPTION |
| .rs |
| .sp |
| PCRE2 provides a feature called "callout", which is a means of temporarily |
| passing control to the caller of PCRE2 in the middle of pattern matching. The |
| caller of PCRE2 provides an external function by putting its entry point in |
| a match context (see \fBpcre2_set_callout()\fP in the |
| .\" HREF |
| \fBpcre2api\fP |
| .\" |
| documentation). |
| .P |
| When using the \fBpcre2_substitute()\fP function, an additional callout feature |
| is available. This does a callout after each change to the subject string and |
| is described in the |
| .\" HREF |
| \fBpcre2api\fP |
| .\" |
| documentation; the rest of this document is concerned with callouts during |
| pattern matching. |
| .P |
| Within a regular expression, (?C<arg>) indicates a point at which the external |
| function is to be called. Different callout points can be identified by putting |
| a number less than 256 after the letter C. The default value is zero. |
| Alternatively, the argument may be a delimited string. The starting delimiter |
| must be one of ` ' " ^ % # $ { and the ending delimiter is the same as the |
| start, except for {, where the ending delimiter is }. If the ending delimiter |
| is needed within the string, it must be doubled. For example, this pattern has |
| two callout points: |
| .sp |
| (?C1)abc(?C"some ""arbitrary"" text")def |
| .sp |
| If the PCRE2_AUTO_CALLOUT option bit is set when a pattern is compiled, PCRE2 |
| automatically inserts callouts, all with number 255, before each item in the |
| pattern except for immediately before or after an explicit callout. For |
| example, if PCRE2_AUTO_CALLOUT is used with the pattern |
| .sp |
| A(?C3)B |
| .sp |
| it is processed as if it were |
| .sp |
| (?C255)A(?C3)B(?C255) |
| .sp |
| Here is a more complicated example: |
| .sp |
| A(\ed{2}|--) |
| .sp |
| With PCRE2_AUTO_CALLOUT, this pattern is processed as if it were |
| .sp |
| (?C255)A(?C255)((?C255)\ed{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255) |
| .sp |
| Notice that there is a callout before and after each parenthesis and |
| alternation bar. If the pattern contains a conditional group whose condition is |
| an assertion, an automatic callout is inserted immediately before the |
| condition. Such a callout may also be inserted explicitly, for example: |
| .sp |
| (?(?C9)(?=a)ab|de) (?(?C%text%)(?!=d)ab|de) |
| .sp |
| This applies only to assertion conditions (because they are themselves |
| independent groups). |
| .P |
| Callouts can be useful for tracking the progress of pattern matching. The |
| .\" HREF |
| \fBpcre2test\fP |
| .\" |
| program has a pattern qualifier (/auto_callout) that sets automatic callouts. |
| When any callouts are present, the output from \fBpcre2test\fP indicates how |
| the pattern is being matched. This is useful information when you are trying to |
| optimize the performance of a particular pattern. |
| . |
| . |
| .SH "MISSING CALLOUTS" |
| .rs |
| .sp |
| You should be aware that, because of optimizations in the way PCRE2 compiles |
| and matches patterns, callouts sometimes do not happen exactly as you might |
| expect. |
| . |
| . |
| .SS "Auto-possessification" |
| .rs |
| .sp |
| At compile time, PCRE2 "auto-possessifies" repeated items when it knows that |
| what follows cannot be part of the repeat. For example, a+[bc] is compiled as |
| if it were a++[bc]. The \fBpcre2test\fP output when this pattern is compiled |
| with PCRE2_ANCHORED and PCRE2_AUTO_CALLOUT and then applied to the string |
| "aaaa" is: |
| .sp |
| --->aaaa |
| +0 ^ a+ |
| +2 ^ ^ [bc] |
| No match |
| .sp |
| This indicates that when matching [bc] fails, there is no backtracking into a+ |
| (because it is being treated as a++) and therefore the callouts that would be |
| taken for the backtracks do not occur. You can disable the auto-possessify |
| feature by passing PCRE2_NO_AUTO_POSSESS to \fBpcre2_compile()\fP, or starting |
| the pattern with (*NO_AUTO_POSSESS). In this case, the output changes to this: |
| .sp |
| --->aaaa |
| +0 ^ a+ |
| +2 ^ ^ [bc] |
| +2 ^ ^ [bc] |
| +2 ^ ^ [bc] |
| +2 ^^ [bc] |
| No match |
| .sp |
| This time, when matching [bc] fails, the matcher backtracks into a+ and tries |
| again, repeatedly, until a+ itself fails. |
| . |
| . |
| .SS "Automatic .* anchoring" |
| .rs |
| .sp |
| By default, an optimization is applied when .* is the first significant item in |
| a pattern. If PCRE2_DOTALL is set, so that the dot can match any character, the |
| pattern is automatically anchored. If PCRE2_DOTALL is not set, a match can |
| start only after an internal newline or at the beginning of the subject, and |
| \fBpcre2_compile()\fP remembers this. If a pattern has more than one top-level |
| branch, automatic anchoring occurs if all branches are anchorable. |
| .P |
| This optimization is disabled, however, if .* is in an atomic group or if there |
| is a backreference to the capture group in which it appears. It is also |
| disabled if the pattern contains (*PRUNE) or (*SKIP). However, the presence of |
| callouts does not affect it. |
| .P |
| For example, if the pattern .*\ed is compiled with PCRE2_AUTO_CALLOUT and |
| applied to the string "aa", the \fBpcre2test\fP output is: |
| .sp |
| --->aa |
| +0 ^ .* |
| +2 ^ ^ \ed |
| +2 ^^ \ed |
| +2 ^ \ed |
| No match |
| .sp |
| This shows that all match attempts start at the beginning of the subject. In |
| other words, the pattern is anchored. You can disable this optimization by |
| passing PCRE2_NO_DOTSTAR_ANCHOR to \fBpcre2_compile()\fP, or starting the |
| pattern with (*NO_DOTSTAR_ANCHOR). In this case, the output changes to: |
| .sp |
| --->aa |
| +0 ^ .* |
| +2 ^ ^ \ed |
| +2 ^^ \ed |
| +2 ^ \ed |
| +0 ^ .* |
| +2 ^^ \ed |
| +2 ^ \ed |
| No match |
| .sp |
| This shows more match attempts, starting at the second subject character. |
| Another optimization, described in the next section, means that there is no |
| subsequent attempt to match with an empty subject. |
| . |
| . |
| .SS "Other optimizations" |
| .rs |
| .sp |
| Other optimizations that provide fast "no match" results also affect callouts. |
| For example, if the pattern is |
| .sp |
| ab(?C4)cd |
| .sp |
| PCRE2 knows that any matching string must contain the letter "d". If the |
| subject string is "abyz", the lack of "d" means that matching doesn't ever |
| start, and the callout is never reached. However, with "abyd", though the |
| result is still no match, the callout is obeyed. |
| .P |
| For most patterns PCRE2 also knows the minimum length of a matching string, and |
| will immediately give a "no match" return without actually running a match if |
| the subject is not long enough, or, for unanchored patterns, if it has been |
| scanned far enough. |
| .P |
| You can disable these optimizations by passing the PCRE2_NO_START_OPTIMIZE |
| option to \fBpcre2_compile()\fP, or by starting the pattern with |
| (*NO_START_OPT). This slows down the matching process, but does ensure that |
| callouts such as the example above are obeyed. |
| . |
| . |
| .\" HTML <a name="calloutinterface"></a> |
| .SH "THE CALLOUT INTERFACE" |
| .rs |
| .sp |
| During matching, when PCRE2 reaches a callout point, if an external function is |
| provided in the match context, it is called. This applies to both normal, |
| DFA, and JIT matching. The first argument to the callout function is a pointer |
| to a \fBpcre2_callout\fP block. The second argument is the void * callout data |
| that was supplied when the callout was set up by calling |
| \fBpcre2_set_callout()\fP (see the |
| .\" HREF |
| \fBpcre2api\fP |
| .\" |
| documentation). The callout block structure contains the following fields, not |
| necessarily in this order: |
| .sp |
| uint32_t \fIversion\fP; |
| uint32_t \fIcallout_number\fP; |
| uint32_t \fIcapture_top\fP; |
| uint32_t \fIcapture_last\fP; |
| uint32_t \fIcallout_flags\fP; |
| PCRE2_SIZE *\fIoffset_vector\fP; |
| PCRE2_SPTR \fImark\fP; |
| PCRE2_SPTR \fIsubject\fP; |
| PCRE2_SIZE \fIsubject_length\fP; |
| PCRE2_SIZE \fIstart_match\fP; |
| PCRE2_SIZE \fIcurrent_position\fP; |
| PCRE2_SIZE \fIpattern_position\fP; |
| PCRE2_SIZE \fInext_item_length\fP; |
| PCRE2_SIZE \fIcallout_string_offset\fP; |
| PCRE2_SIZE \fIcallout_string_length\fP; |
| PCRE2_SPTR \fIcallout_string\fP; |
| .sp |
| The \fIversion\fP field contains the version number of the block format. The |
| current version is 2; the three callout string fields were added for version 1, |
| and the \fIcallout_flags\fP field for version 2. If you are writing an |
| application that might use an earlier release of PCRE2, you should check the |
| version number before accessing any of these fields. The version number will |
| increase in future if more fields are added, but the intention is never to |
| remove any of the existing fields. |
| . |
| . |
| .SS "Fields for numerical callouts" |
| .rs |
| .sp |
| For a numerical callout, \fIcallout_string\fP is NULL, and \fIcallout_number\fP |
| contains the number of the callout, in the range 0-255. This is the number |
| that follows (?C for callouts that part of the pattern; it is 255 for |
| automatically generated callouts. |
| . |
| . |
| .SS "Fields for string callouts" |
| .rs |
| .sp |
| For callouts with string arguments, \fIcallout_number\fP is always zero, and |
| \fIcallout_string\fP points to the string that is contained within the compiled |
| pattern. Its length is given by \fIcallout_string_length\fP. Duplicated ending |
| delimiters that were present in the original pattern string have been turned |
| into single characters, but there is no other processing of the callout string |
| argument. An additional code unit containing binary zero is present after the |
| string, but is not included in the length. The delimiter that was used to start |
| the string is also stored within the pattern, immediately before the string |
| itself. You can access this delimiter as \fIcallout_string\fP[-1] if you need |
| it. |
| .P |
| The \fIcallout_string_offset\fP field is the code unit offset to the start of |
| the callout argument string within the original pattern string. This is |
| provided for the benefit of applications such as script languages that might |
| need to report errors in the callout string within the pattern. |
| . |
| . |
| .SS "Fields for all callouts" |
| .rs |
| .sp |
| The remaining fields in the callout block are the same for both kinds of |
| callout. |
| .P |
| The \fIoffset_vector\fP field is a pointer to a vector of capturing offsets |
| (the "ovector"). You may read the elements in this vector, but you must not |
| change any of them. |
| .P |
| For calls to \fBpcre2_match()\fP, the \fIoffset_vector\fP field is not (since |
| release 10.30) a pointer to the actual ovector that was passed to the matching |
| function in the match data block. Instead it points to an internal ovector of a |
| size large enough to hold all possible captured substrings in the pattern. Note |
| that whenever a recursion or subroutine call within a pattern completes, the |
| capturing state is reset to what it was before. |
| .P |
| The \fIcapture_last\fP field contains the number of the most recently captured |
| substring, and the \fIcapture_top\fP field contains one more than the number of |
| the highest numbered captured substring so far. If no substrings have yet been |
| captured, the value of \fIcapture_last\fP is 0 and the value of |
| \fIcapture_top\fP is 1. The values of these fields do not always differ by one; |
| for example, when the callout in the pattern ((a)(b))(?C2) is taken, |
| \fIcapture_last\fP is 1 but \fIcapture_top\fP is 4. |
| .P |
| The contents of ovector[2] to ovector[<capture_top>*2-1] can be inspected in |
| order to extract substrings that have been matched so far, in the same way as |
| extracting substrings after a match has completed. The values in ovector[0] and |
| ovector[1] are always PCRE2_UNSET because the match is by definition not |
| complete. Substrings that have not been captured but whose numbers are less |
| than \fIcapture_top\fP also have both of their ovector slots set to |
| PCRE2_UNSET. |
| .P |
| For DFA matching, the \fIoffset_vector\fP field points to the ovector that was |
| passed to the matching function in the match data block for callouts at the top |
| level, but to an internal ovector during the processing of pattern recursions, |
| lookarounds, and atomic groups. However, these ovectors hold no useful |
| information because \fBpcre2_dfa_match()\fP does not support substring |
| capturing. The value of \fIcapture_top\fP is always 1 and the value of |
| \fIcapture_last\fP is always 0 for DFA matching. |
| .P |
| The \fIsubject\fP and \fIsubject_length\fP fields contain copies of the values |
| that were passed to the matching function. |
| .P |
| The \fIstart_match\fP field normally contains the offset within the subject at |
| which the current match attempt started. However, if the escape sequence \eK |
| has been encountered, this value is changed to reflect the modified starting |
| point. If the pattern is not anchored, the callout function may be called |
| several times from the same point in the pattern for different starting points |
| in the subject. |
| .P |
| The \fIcurrent_position\fP field contains the offset within the subject of the |
| current match pointer. |
| .P |
| The \fIpattern_position\fP field contains the offset in the pattern string to |
| the next item to be matched. |
| .P |
| The \fInext_item_length\fP field contains the length of the next item to be |
| processed in the pattern string. When the callout is at the end of the pattern, |
| the length is zero. When the callout precedes an opening parenthesis, the |
| length includes meta characters that follow the parenthesis. For example, in a |
| callout before an assertion such as (?=ab) the length is 3. For an an |
| alternation bar or a closing parenthesis, the length is one, unless a closing |
| parenthesis is followed by a quantifier, in which case its length is included. |
| (This changed in release 10.23. In earlier releases, before an opening |
| parenthesis the length was that of the entire group, and before an alternation |
| bar or a closing parenthesis the length was zero.) |
| .P |
| The \fIpattern_position\fP and \fInext_item_length\fP fields are intended to |
| help in distinguishing between different automatic callouts, which all have the |
| same callout number. However, they are set for all callouts, and are used by |
| \fBpcre2test\fP to show the next item to be matched when displaying callout |
| information. |
| .P |
| In callouts from \fBpcre2_match()\fP the \fImark\fP field contains a pointer to |
| the zero-terminated name of the most recently passed (*MARK), (*PRUNE), or |
| (*THEN) item in the match, or NULL if no such items have been passed. Instances |
| of (*PRUNE) or (*THEN) without a name do not obliterate a previous (*MARK). In |
| callouts from the DFA matching function this field always contains NULL. |
| .P |
| The \fIcallout_flags\fP field is always zero in callouts from |
| \fBpcre2_dfa_match()\fP or when JIT is being used. When \fBpcre2_match()\fP |
| without JIT is used, the following bits may be set: |
| .sp |
| PCRE2_CALLOUT_STARTMATCH |
| .sp |
| This is set for the first callout after the start of matching for each new |
| starting position in the subject. |
| .sp |
| PCRE2_CALLOUT_BACKTRACK |
| .sp |
| This is set if there has been a matching backtrack since the previous callout, |
| or since the start of matching if this is the first callout from a |
| \fBpcre2_match()\fP run. |
| .P |
| Both bits are set when a backtrack has caused a "bumpalong" to a new starting |
| position in the subject. Output from \fBpcre2test\fP does not indicate the |
| presence of these bits unless the \fBcallout_extra\fP modifier is set. |
| .P |
| The information in the \fBcallout_flags\fP field is provided so that |
| applications can track and tell their users how matching with backtracking is |
| done. This can be useful when trying to optimize patterns, or just to |
| understand how PCRE2 works. There is no support in \fBpcre2_dfa_match()\fP |
| because there is no backtracking in DFA matching, and there is no support in |
| JIT because JIT is all about maximimizing matching performance. In both these |
| cases the \fBcallout_flags\fP field is always zero. |
| . |
| . |
| .SH "RETURN VALUES FROM CALLOUTS" |
| .rs |
| .sp |
| The external callout function returns an integer to PCRE2. If the value is |
| zero, matching proceeds as normal. If the value is greater than zero, matching |
| fails at the current point, but the testing of other matching possibilities |
| goes ahead, just as if a lookahead assertion had failed. If the value is less |
| than zero, the match is abandoned, and the matching function returns the |
| negative value. |
| .P |
| Negative values should normally be chosen from the set of PCRE2_ERROR_xxx |
| values. In particular, PCRE2_ERROR_NOMATCH forces a standard "no match" |
| failure. The error number PCRE2_ERROR_CALLOUT is reserved for use by callout |
| functions; it will never be used by PCRE2 itself. |
| . |
| . |
| .SH "CALLOUT ENUMERATION" |
| .rs |
| .sp |
| .nf |
| .B int pcre2_callout_enumerate(const pcre2_code *\fIcode\fP, |
| .B " int (*\fIcallback\fP)(pcre2_callout_enumerate_block *, void *)," |
| .B " void *\fIuser_data\fP);" |
| .fi |
| .sp |
| A script language that supports the use of string arguments in callouts might |
| like to scan all the callouts in a pattern before running the match. This can |
| be done by calling \fBpcre2_callout_enumerate()\fP. The first argument is a |
| pointer to a compiled pattern, the second points to a callback function, and |
| the third is arbitrary user data. The callback function is called for every |
| callout in the pattern in the order in which they appear. Its first argument is |
| a pointer to a callout enumeration block, and its second argument is the |
| \fIuser_data\fP value that was passed to \fBpcre2_callout_enumerate()\fP. The |
| data block contains the following fields: |
| .sp |
| \fIversion\fP Block version number |
| \fIpattern_position\fP Offset to next item in pattern |
| \fInext_item_length\fP Length of next item in pattern |
| \fIcallout_number\fP Number for numbered callouts |
| \fIcallout_string_offset\fP Offset to string within pattern |
| \fIcallout_string_length\fP Length of callout string |
| \fIcallout_string\fP Points to callout string or is NULL |
| .sp |
| The version number is currently 0. It will increase if new fields are ever |
| added to the block. The remaining fields are the same as their namesakes in the |
| \fBpcre2_callout\fP block that is used for callouts during matching, as |
| described |
| .\" HTML <a href="#calloutinterface"> |
| .\" </a> |
| above. |
| .\" |
| .P |
| Note that the value of \fIpattern_position\fP is unique for each callout. |
| However, if a callout occurs inside a group that is quantified with a non-zero |
| minimum or a fixed maximum, the group is replicated inside the compiled |
| pattern. For example, a pattern such as /(a){2}/ is compiled as if it were |
| /(a)(a)/. This means that the callout will be enumerated more than once, but |
| with the same value for \fIpattern_position\fP in each case. |
| .P |
| The callback function should normally return zero. If it returns a non-zero |
| value, scanning the pattern stops, and that value is returned from |
| \fBpcre2_callout_enumerate()\fP. |
| . |
| . |
| .SH AUTHOR |
| .rs |
| .sp |
| .nf |
| Philip Hazel |
| University Computing Service |
| Cambridge, England. |
| .fi |
| . |
| . |
| .SH REVISION |
| .rs |
| .sp |
| .nf |
| Last updated: 03 February 2019 |
| Copyright (c) 1997-2019 University of Cambridge. |
| .fi |