pcre2callout man page

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• SYNOPSIS
• DESCRIPTION
• MISSING CALLOUTS
• THE CALLOUT INTERFACE
• RETURN VALUES FROM CALLOUTS
• CALLOUT ENUMERATION
• AUTHOR
• REVISION

#include <pcre2.h>

int (*pcre2_callout)(pcre2_callout_block *, void *);

int pcre2_callout_enumerate(const pcre2_code *code, int (*callback)(pcre2_callout_enumerate_block *, void *), void *user_data);

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<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:

  (?C1)abc(?C"some ""arbitrary"" text")def

  A(?C3)B

  (?C255)A(?C3)B(?C255)   

  A(\d{2}|--)

  (?(?C9)(?=a)ab|de)  (?(?C%text%)(?!=d)ab|de)

Callouts can be useful for tracking the progress of pattern matching. The pcre2test program has a pattern qualifier (/auto_callout) that sets automatic callouts. When any callouts are present, the output from pcre2test 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 match

  --->aaaa
   +0 ^        a+
   +2 ^   ^    [bc]
   +2 ^  ^     [bc]
   +2 ^ ^      [bc]
   +2 ^^       [bc]
  No match

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 pcre2_compile() remembers this. This optimization is disabled, however, if .* is in an atomic group or if there is a back reference to the capturing 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.

For example, if the pattern .*\d is compiled with PCRE2_AUTO_CALLOUT and applied to the string "aa", the pcre2test output is:

  --->aa
   +0 ^      .*
   +2 ^ ^    \d
   +2 ^^     \d
   +2 ^      \d
  No match

  --->aa
   +0 ^      .*
   +2 ^ ^    \d
   +2 ^^     \d
   +2 ^      \d
   +0  ^     .*
   +2  ^^    \d
   +2  ^     \d
  No match

If a pattern has more than one top-level branch, automatic anchoring occurs if all branches are anchorable.

Other optimizations that provide fast "no match" results also affect callouts. For example, if the pattern is

  ab(?C4)cd

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 first argument to the callout function is a pointer to a pcre2_callout block. The second argument is the void * callout data that was supplied when the callout was set up by calling pcre2_set_callout() (see the pcre2api documentation). The callout block structure contains the following fields:

  uint32_t      version;
  uint32_t      callout_number;
  uint32_t      capture_top;
  uint32_t      capture_last;
  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;
  PCRE2_SIZE    callout_string_offset;
  PCRE2_SIZE    callout_string_length;
  PCRE2_SPTR    callout_string;

For a numerical callout, callout_string is NULL, and callout_number 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.

For callouts with string arguments, callout_number is always zero, and callout_string points to the string that is contained within the compiled pattern. Its length is given by callout_string_length. 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 callout_string[-1] if you need it.

The callout_string_offset 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.

The remaining fields in the callout block are the same for both kinds of callout.

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 pattern_position field contains the offset in the pattern string to the next item to be matched.

The next_item_length 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 subpattern, and before an alternation bar or a closing parenthesis the length was zero.)

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, and are used by pcre2test to show the next item to be matched when displaying callout information.

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.

int pcre2_callout_enumerate(const pcre2_code *code, int (*callback)(pcre2_callout_enumerate_block *, void *), void *user_data);

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 pcre2_callout_enumerate(). 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 user_data value that was passed to pcre2_callout_enumerate(). The data block contains the following fields:

  version                Block version number
  pattern_position       Offset to next item in pattern
  next_item_length       Length of next item in pattern
  callout_number         Number for numbered callouts
  callout_string_offset  Offset to string within pattern
  callout_string_length  Length of callout string
  callout_string         Points to callout string or is NULL

Note that the value of pattern_position 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 pattern_position in each case.

The callback function should normally return zero. If it returns a non-zero value, scanning the pattern stops, and that value is returned from pcre2_callout_enumerate().

Philip Hazel
University Computing Service
Cambridge, England.

Last updated: 29 September 2016
Copyright © 1997-2016 University of Cambridge.

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