From cba4af1b7643b0da036ff78fd152f74a906c4e97 Mon Sep 17 00:00:00 2001 From: Ian Jackson Date: Wed, 13 Jul 2016 21:24:43 +0100 Subject: Commit upstream pcre-8.39.tar.bz2 --- doc/html/pcrecallout.html | 286 ++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 286 insertions(+) create mode 100644 doc/html/pcrecallout.html (limited to 'doc/html/pcrecallout.html') diff --git a/doc/html/pcrecallout.html b/doc/html/pcrecallout.html new file mode 100644 index 0000000..53a937f --- /dev/null +++ b/doc/html/pcrecallout.html @@ -0,0 +1,286 @@ + + +pcrecallout specification + + +

pcrecallout man page

+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. +
+

SYNOPSIS +
DESCRIPTION +
MISSING CALLOUTS +
THE CALLOUT INTERFACE +
RETURN VALUES +
AUTHOR +
REVISION +

+
SYNOPSIS
+

+#include <pcre.h> +

+int (*pcre_callout)(pcre_callout_block *); +

+int (*pcre16_callout)(pcre16_callout_block *); +

+int (*pcre32_callout)(pcre32_callout_block *); +

+
DESCRIPTION
+

+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 (pcre16_callout for the 16-bit +library, pcre32_callout for the 32-bit library). 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 a pattern is compiled, 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 +
+
+(?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255) +
+
+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: +

+  (?(?C9)(?=a)ab|de)
+

+This applies only to assertion conditions (because they are themselves +independent groups). +

+Automatic callouts can be used for tracking the progress of pattern matching. +The +pcretest +program has a pattern qualifier (/C) that sets automatic callouts; when it is +used, the output indicates how the pattern is being matched. This is useful +information when you are trying to optimize the performance of a particular +pattern. +

+
MISSING CALLOUTS
+

+You should be aware that, because of optimizations in the way PCRE compiles and +matches patterns, callouts sometimes do not happen exactly as you might expect. +

+At compile time, PCRE "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 pcretest output when this pattern is anchored and +then applied with automatic callouts to the string "aaaa" is: +

+  --->aaaa
+   +0 ^        ^
+   +1 ^        a+
+   +3 ^   ^    [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 PCRE_NO_AUTO_POSSESS +to pcre_compile(), or starting the pattern with (*NO_AUTO_POSSESS). If +this is done in pcretest (using the /O qualifier), the output changes to +this: +

+  --->aaaa
+   +0 ^        ^
+   +1 ^        a+
+   +3 ^   ^    [bc]
+   +3 ^  ^     [bc]
+   +3 ^ ^      [bc]
+   +3 ^^       [bc]
+  No match
+

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

+Other optimizations that provide fast "no match" results also affect callouts. +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. +

+If the pattern is studied, PCRE 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 PCRE_NO_START_OPTIMIZE +option to the matching function, 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. +

+
THE CALLOUT INTERFACE
+

+During matching, when PCRE reaches a callout point, the external function +defined by pcre_callout or pcre[16|32]_callout is called (if it is +set). This applies to both normal and DFA matching. The only argument to the +callout function is a pointer to a pcre_callout or +pcre[16|32]_callout block. These structures contains the following +fields: +

+  int           version;
+  int           callout_number;
+  int          *offset_vector;
+  const char   *subject;           (8-bit version)
+  PCRE_SPTR16   subject;           (16-bit version)
+  PCRE_SPTR32   subject;           (32-bit version)
+  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;
+  const unsigned char *mark;       (8-bit version)
+  const PCRE_UCHAR16  *mark;       (16-bit version)
+  const PCRE_UCHAR32  *mark;       (32-bit version)
+

+The version field is an integer containing the version number of the +block format. The initial version was 0; the current version is 2. 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 the matching function. When pcre_exec() or +pcre[16|32]_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 the DFA matching +functions, 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 pcre_exec() or pcre[16|32]_exec() 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 -1. This is +always the case for the DFA matching functions. +

+The callout_data field contains a value that is passed to a matching +function specifically so that it can be passed back in callouts. It is passed +in the callout_data field of a pcre_extra or pcre[16|32]_extra +data structure. If no such data was passed, the value of callout_data in +a 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 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 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 mark field is present from version 2 of the callout structure. In +callouts from pcre_exec() or pcre[16|32]_exec() it 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 functions this field always +contains NULL. +

+
RETURN VALUES
+

+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, the matching function 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. +

+
AUTHOR
+

+Philip Hazel +
+University Computing Service +
+Cambridge CB2 3QH, England. +
+

+
REVISION
+

+Return to the PCRE index page. +

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