| .TH PCRE2CALLOUT 3 "23 March 2015" "PCRE2 10.20" |
| .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 |
| 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. For example, if PCRE2_AUTO_CALLOUT is used with the pattern |
| .sp |
| A(\ed{2}|--) |
| .sp |
| it 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+ |
| 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. 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. |
| .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. |
| .P |
| If a pattern has more than one top-level branch, automatic anchoring occurs if |
| all branches are anchorable. |
| . |
| . |
| .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 |
| 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 |
| 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 |
| \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: |
| .sp |
| uint32_t \fIversion\fP; |
| uint32_t \fIcallout_number\fP; |
| uint32_t \fIcapture_top\fP; |
| uint32_t \fIcapture_last\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 1; the three callout string fields were added for this |
| version. 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 manual callouts; 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 the vector of capturing offsets |
| (the "ovector") that was passed to the matching function in the match data |
| block. When \fBpcre2_match()\fP 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. |
| .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 |
| When the \fBpcre2_match()\fP is used, the \fIcapture_top\fP field contains one |
| more than the number of the highest numbered captured substring so far. If no |
| substrings have been captured, the value of \fIcapture_top\fP is one. This is |
| always the case when the DFA functions are used, because they do not support |
| captured substrings. |
| .P |
| The \fIcapture_last\fP 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 \fIcapture_last\fP is 0. This is |
| always the case for the DFA matching functions. |
| .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 |
| 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. |
| .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. |
| . |
| . |
| .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: 23 March 2015 |
| Copyright (c) 1997-2015 University of Cambridge. |
| .fi |