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#include <pcre2.h>
int (*pcre2_callout)(pcre2_callout_block *);
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 pcre2_set_callout()) in the pcre2api documentation).
Within a regular expression, (?C) indicates the points 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. For example, this pattern has two callout points:
(?C1)abc(?C2)defIf 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. For example, if PCRE2_AUTO_CALLOUT is used with the pattern
A(\d{2}|--)
it is processed as if it were
(?(?C9)(?=a)ab|de)This applies only to assertion conditions (because they are themselves independent groups).
Automatic callouts can be used for tracking the progress of pattern matching. The pcre2test program has a pattern qualifier (/auto_callout) that sets automatic callouts; when it is used, the output indicates how the pattern is being matched. This is useful information when you are trying to optimize the performance of a particular pattern.
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.
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 pcre2test output when this pattern is compiled with PCRE2_ANCHORED and PCRE2_AUTO_CALLOUT and then applied to the string "aaaa" is:
--->aaaa +0 ^ a+ +2 ^ ^ [bc] No matchThis indicates that when matching [bc] fails, there is no backtracking into 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 pcre2_compile(), or starting the pattern with (*NO_AUTO_POSSESS). In this case, the output changes to this:
--->aaaa +0 ^ a+ +2 ^ ^ [bc] +2 ^ ^ [bc] +2 ^ ^ [bc] +2 ^^ [bc] No matchThis time, when matching [bc] fails, the matcher backtracks into a+ and tries again, repeatedly, until a+ itself fails.
Other optimizations that provide fast "no match" results also affect callouts. For example, if the pattern is
ab(?C4)cdPCRE2 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.
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.
You can disable these optimizations by passing the PCRE2_NO_START_OPTIMIZE option to pcre2_compile(), 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.
During matching, when PCRE2 reaches a callout point, if an external function is set in the match context, it is called. This applies to both normal and DFA matching. The only argument to the callout function is a pointer to a pcre2_callout block. This structure contains the following fields:
uint32_t version; uint32_t callout_number; uint32_t capture_top; uint32_t capture_last; void *callout_data; PCRE2_SIZE *offset_vector; PCRE2_SPTR mark; PCRE2_SPTR subject; PCRE2_SIZE subject_length; PCRE2_SIZE start_match; PCRE2_SIZE current_position; PCRE2_SIZE pattern_position; PCRE2_SIZE next_item_length;The version field contains the version number of the block format. The current version is 0. The version number will change in future if additional fields are added, but the intention is never to remove any of the existing fields.
The callout_number field contains the number of the callout, as compiled into the pattern (that is, the number after ?C for manual callouts, and 255 for automatically generated callouts).
The offset_vector field is a pointer to the vector of capturing offsets (the "ovector") that was passed to the matching function in the match data block. When pcre2_match() is used, the contents can be inspected in order to extract substrings that have been matched so far, in the same way as for extracting substrings after a match has completed. For the DFA matching function, this field is not useful.
The subject and subject_length fields contain copies of the values that were passed to the matching function.
The start_match field normally contains the offset within the subject at which the current match attempt started. However, if the escape sequence \K 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.
The current_position field contains the offset within the subject of the current match pointer.
When the pcre2_match() is used, the capture_top field contains one more than the number of the highest numbered captured substring so far. If no substrings have been captured, the value of capture_top is one. This is always the case when the DFA functions are used, because they do not support captured substrings.
The capture_last field contains the number of the most recently captured substring. However, when a recursion exits, the value reverts to what it was outside the recursion, as do the values of all captured substrings. If no substrings have been captured, the value of capture_last is 0. This is always the case for the DFA matching functions.
The callout_data field contains a value that is passed to a matching function specifically so that it can be passed back in callouts. It is set in the match context when the callout is set up by calling pcre2_set_callout() (see the pcre2api documentation).
The pattern_position field contains the offset to the next item to be matched in the pattern string.
The next_item_length field contains the length of the next item to be matched in the pattern string. When the callout immediately precedes an alternation bar, a closing parenthesis, or the end of the pattern, the length is zero. When the callout precedes an opening parenthesis, the length is that of the entire subpattern.
The pattern_position and next_item_length 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.
In callouts from pcre2_match() the mark 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.
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.
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.
Philip Hazel
University Computing Service
Cambridge, England.
Last updated: 23 November 2014
Copyright © 1997-2014 University of Cambridge.
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