Auto-possessification

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

This 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 match

This time, when matching [bc] fails, the matcher backtracks into a+ and tries again, repeatedly, until a+ itself fails.

Automatic .* anchoring

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

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 pcre2_compile(), or starting the pattern with (*NO_DOTSTAR_ANCHOR). In this case, the output changes to:

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

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.

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

Other optimizations

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

ab(?C4)cd

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.

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.

Fields for numerical callouts

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 manual callouts; it is 255 for automatically generated callouts.

Fields for string 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.

Fields for all callouts

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 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, 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.