+Return to the PCRE index page. +
+
+This page is part of the PCRE HTML documentation. It was generated automatically
+from the original man page. If there is any nonsense in it, please consult the
+man page, in case the conversion went wrong.
+
+
+int (*pcre_callout)(pcre_callout_block *); +
++PCRE provides a feature called "callout", which is a means of temporarily +passing control to the caller of PCRE in the middle of pattern matching. The +caller of PCRE provides an external function by putting its entry point in the +global variable pcre_callout. By default, this variable contains NULL, +which disables all calling out. +
++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)def ++If the PCRE_AUTO_CALLOUT option bit is set when pcre_compile() is called, +PCRE automatically inserts callouts, all with number 255, before each item in +the pattern. For example, if PCRE_AUTO_CALLOUT is used with the pattern +
+ A(\d{2}|--)
+
+it is processed as if it were
++You should be aware that, because of optimizations in the way PCRE matches +patterns, callouts sometimes do not happen. For example, if the pattern is +
+ ab(?C4)cd ++PCRE 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. + +
+During matching, when PCRE reaches a callout point, the external function +defined by pcre_callout is called (if it is set). This applies to both +the pcre_exec() and the pcre_dfa_exec() matching functions. The +only argument to the callout function is a pointer to a pcre_callout +block. This structure contains the following fields: +
+ int version; + int callout_number; + int *offset_vector; + const char *subject; + int subject_length; + int start_match; + int current_position; + int capture_top; + int capture_last; + void *callout_data; + int pattern_position; + int next_item_length; ++The version field is an integer containing the version number of the +block format. The initial version was 0; the current version is 1. The version +number will change again 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 offsets that was +passed by the caller to pcre_exec() or pcre_dfa_exec(). When +pcre_exec() 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 pcre_dfa_exec() this field is +not useful. +
++The subject and subject_length fields contain copies of the values +that were passed to pcre_exec(). +
++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 pcre_exec() function 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 pcre_dfa_exec() is used, because it +does not support captured substrings. +
++The capture_last field contains the number of the most recently captured +substring. If no substrings have been captured, its value is -1. This is always +the case when pcre_dfa_exec() is used. +
++The callout_data field contains a value that is passed to +pcre_exec() or pcre_dfa_exec() specifically so that it can be +passed back in callouts. It is passed in the pcre_callout field of the +pcre_extra data structure. If no such data was passed, the value of +callout_data in a pcre_callout block is NULL. There is a +description of the pcre_extra structure in the +pcreapi +documentation. +
++The pattern_position field is present from version 1 of the +pcre_callout structure. It contains the offset to the next item to be +matched in the pattern string. +
++The next_item_length field is present from version 1 of the +pcre_callout structure. It 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. +
++The external callout function returns an integer to PCRE. 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 pcre_exec() (or pcre_dfa_exec()) +returns the negative value. +
++Negative values should normally be chosen from the set of PCRE_ERROR_xxx +values. In particular, PCRE_ERROR_NOMATCH forces a standard "no match" failure. +The error number PCRE_ERROR_CALLOUT is reserved for use by callout functions; +it will never be used by PCRE itself. +
+
+Philip Hazel
+
+University Computing Service
+
+Cambridge CB2 3QH, England.
+
+
+Last updated: 29 May 2007
+
+Copyright © 1997-2007 University of Cambridge.
+
+
+Return to the PCRE index page. +