X-Git-Url: http://ftp.carnet.hr/carnet-debian/scm?a=blobdiff_plain;ds=sidebyside;f=src%2Fexternal%2Fpcre2-10.32%2Fdoc%2Fpcre2callout.3;fp=src%2Fexternal%2Fpcre2-10.32%2Fdoc%2Fpcre2callout.3;h=c815c7229af8db2849e01c865d12f80f458c65d7;hb=3f728675941dc69d4e544d3a880a56240a6e394a;hp=0000000000000000000000000000000000000000;hpb=927951d1c1ad45ba9e7325f07d996154a91c911b;p=ossec-hids.git diff --git a/src/external/pcre2-10.32/doc/pcre2callout.3 b/src/external/pcre2-10.32/doc/pcre2callout.3 new file mode 100644 index 0000000..c815c72 --- /dev/null +++ b/src/external/pcre2-10.32/doc/pcre2callout.3 @@ -0,0 +1,448 @@ +.TH PCRE2CALLOUT 3 "26 April 2018" "PCRE2 10.32" +.SH NAME +PCRE2 - Perl-compatible regular expressions (revised API) +.SH SYNOPSIS +.rs +.sp +.B #include +.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) 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 except for immediately before or after an explicit callout. For +example, if PCRE2_AUTO_CALLOUT is used with the pattern +.sp + A(?C3)B +.sp +it is processed as if it were +.sp + (?C255)A(?C3)B(?C255) +.sp +Here is a more complicated example: +.sp + A(\ed{2}|--) +.sp +With PCRE2_AUTO_CALLOUT, this pattern 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+ +(because it is being treated as 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. If a pattern has more than one top-level +branch, automatic anchoring occurs if all branches are anchorable. +.P +This optimization is disabled, however, if .* is in an atomic group or if there +is a backreference 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. +. +. +.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 +For most patterns 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 +.SH "THE CALLOUT INTERFACE" +.rs +.sp +During matching, when PCRE2 reaches a callout point, if an external function is +provided in the match context, it is called. This applies to both normal, +DFA, and JIT 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, not +necessarily in this order: +.sp + uint32_t \fIversion\fP; + uint32_t \fIcallout_number\fP; + uint32_t \fIcapture_top\fP; + uint32_t \fIcapture_last\fP; + uint32_t \fIcallout_flags\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 2; the three callout string fields were added for version 1, +and the \fIcallout_flags\fP field for version 2. 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 callouts that part of the pattern; 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 a vector of capturing offsets +(the "ovector"). You may read the elements in this vector, but you must not +change any of them. +.P +For calls to \fBpcre2_match()\fP, the \fIoffset_vector\fP field is not (since +release 10.30) a pointer to the actual ovector that was passed to the matching +function in the match data block. Instead it points to an internal ovector of a +size large enough to hold all possible captured substrings in the pattern. Note +that whenever a recursion or subroutine call within a pattern completes, the +capturing state is reset to what it was before. +.P +The \fIcapture_last\fP field contains the number of the most recently captured +substring, and the \fIcapture_top\fP field contains one more than the number of +the highest numbered captured substring so far. If no substrings have yet been +captured, the value of \fIcapture_last\fP is 0 and the value of +\fIcapture_top\fP is 1. The values of these fields do not always differ by one; +for example, when the callout in the pattern ((a)(b))(?C2) is taken, +\fIcapture_last\fP is 1 but \fIcapture_top\fP is 4. +.P +The contents of ovector[2] to ovector[*2-1] can be inspected in +order to extract substrings that have been matched so far, in the same way as +extracting substrings after a match has completed. The values in ovector[0] and +ovector[1] are always PCRE2_UNSET because the match is by definition not +complete. Substrings that have not been captured but whose numbers are less +than \fIcapture_top\fP also have both of their ovector slots set to +PCRE2_UNSET. +.P +For DFA matching, the \fIoffset_vector\fP field points to the ovector that was +passed to the matching function in the match data block for callouts at the top +level, but to an internal ovector during the processing of pattern recursions, +lookarounds, and atomic groups. However, these ovectors hold no useful +information because \fBpcre2_dfa_match()\fP does not support substring +capturing. The value of \fIcapture_top\fP is always 1 and the value of +\fIcapture_last\fP is always 0 for DFA matching. +.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 +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 +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.) +.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. +.P +The \fIcallout_flags\fP field is always zero in callouts from +\fBpcre2_dfa_match()\fP or when JIT is being used. When \fBpcre2_match()\fP +without JIT is used, the following bits may be set: +.sp + PCRE2_CALLOUT_STARTMATCH +.sp +This is set for the first callout after the start of matching for each new +starting position in the subject. +.sp + PCRE2_CALLOUT_BACKTRACK +.sp +This is set if there has been a matching backtrack since the previous callout, +or since the start of matching if this is the first callout from a +\fBpcre2_match()\fP run. +.P +Both bits are set when a backtrack has caused a "bumpalong" to a new starting +position in the subject. Output from \fBpcre2test\fP does not indicate the +presence of these bits unless the \fBcallout_extra\fP modifier is set. +.P +The information in the \fBcallout_flags\fP field is provided so that +applications can track and tell their users how matching with backtracking is +done. This can be useful when trying to optimize patterns, or just to +understand how PCRE2 works. There is no support in \fBpcre2_dfa_match()\fP +because there is no backtracking in DFA matching, and there is no support in +JIT because JIT is all about maximimizing matching performance. In both these +cases the \fBcallout_flags\fP field is always zero. +. +. +.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 +.\" +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: 26 April 2018 +Copyright (c) 1997-2018 University of Cambridge. +.fi