402 lines
16 KiB
Groff
402 lines
16 KiB
Groff
.TH PCRE2CALLOUT 3 "29 September 2016" "PCRE2 10.23"
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.SH NAME
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PCRE2 - Perl-compatible regular expressions (revised API)
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.SH SYNOPSIS
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.rs
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.sp
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.B #include <pcre2.h>
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.PP
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.SM
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.nf
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.B int (*pcre2_callout)(pcre2_callout_block *, void *);
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.sp
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.B int pcre2_callout_enumerate(const pcre2_code *\fIcode\fP,
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.B " int (*\fIcallback\fP)(pcre2_callout_enumerate_block *, void *),"
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.B " void *\fIuser_data\fP);"
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.fi
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.
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.SH DESCRIPTION
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.rs
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.sp
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PCRE2 provides a feature called "callout", which is a means of temporarily
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passing control to the caller of PCRE2 in the middle of pattern matching. The
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caller of PCRE2 provides an external function by putting its entry point in
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a match context (see \fBpcre2_set_callout()\fP in the
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.\" HREF
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\fBpcre2api\fP
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.\"
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documentation).
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.P
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Within a regular expression, (?C<arg>) indicates a point at which the external
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function is to be called. Different callout points can be identified by putting
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a number less than 256 after the letter C. The default value is zero.
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Alternatively, the argument may be a delimited string. The starting delimiter
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must be one of ` ' " ^ % # $ { and the ending delimiter is the same as the
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start, except for {, where the ending delimiter is }. If the ending delimiter
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is needed within the string, it must be doubled. For example, this pattern has
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two callout points:
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.sp
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(?C1)abc(?C"some ""arbitrary"" text")def
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.sp
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If the PCRE2_AUTO_CALLOUT option bit is set when a pattern is compiled, PCRE2
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automatically inserts callouts, all with number 255, before each item in the
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pattern except for immediately before or after a callout item in the pattern.
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For example, if PCRE2_AUTO_CALLOUT is used with the pattern
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.sp
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A(?C3)B
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.sp
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it is processed as if it were
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.sp
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(?C255)A(?C3)B(?C255)
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.sp
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Here is a more complicated example:
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.sp
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A(\ed{2}|--)
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.sp
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With PCRE2_AUTO_CALLOUT, this pattern is processed as if it were
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.sp
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(?C255)A(?C255)((?C255)\ed{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)
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.sp
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Notice that there is a callout before and after each parenthesis and
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alternation bar. If the pattern contains a conditional group whose condition is
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an assertion, an automatic callout is inserted immediately before the
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condition. Such a callout may also be inserted explicitly, for example:
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.sp
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(?(?C9)(?=a)ab|de) (?(?C%text%)(?!=d)ab|de)
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.sp
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This applies only to assertion conditions (because they are themselves
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independent groups).
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.P
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Callouts can be useful for tracking the progress of pattern matching. The
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.\" HREF
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\fBpcre2test\fP
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.\"
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program has a pattern qualifier (/auto_callout) that sets automatic callouts.
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When any callouts are present, the output from \fBpcre2test\fP indicates how
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the pattern is being matched. This is useful information when you are trying to
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optimize the performance of a particular pattern.
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.
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.
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.SH "MISSING CALLOUTS"
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.rs
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.sp
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You should be aware that, because of optimizations in the way PCRE2 compiles
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and matches patterns, callouts sometimes do not happen exactly as you might
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expect.
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.
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.
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.SS "Auto-possessification"
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.rs
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.sp
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At compile time, PCRE2 "auto-possessifies" repeated items when it knows that
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what follows cannot be part of the repeat. For example, a+[bc] is compiled as
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if it were a++[bc]. The \fBpcre2test\fP output when this pattern is compiled
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with PCRE2_ANCHORED and PCRE2_AUTO_CALLOUT and then applied to the string
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"aaaa" is:
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.sp
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--->aaaa
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+0 ^ a+
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+2 ^ ^ [bc]
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No match
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.sp
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This indicates that when matching [bc] fails, there is no backtracking into a+
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(because it is being treated as a++) and therefore the callouts that would be
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taken for the backtracks do not occur. You can disable the auto-possessify
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feature by passing PCRE2_NO_AUTO_POSSESS to \fBpcre2_compile()\fP, or starting
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the pattern with (*NO_AUTO_POSSESS). In this case, the output changes to this:
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.sp
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--->aaaa
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+0 ^ a+
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+2 ^ ^ [bc]
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+2 ^ ^ [bc]
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+2 ^ ^ [bc]
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+2 ^^ [bc]
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No match
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.sp
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This time, when matching [bc] fails, the matcher backtracks into a+ and tries
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again, repeatedly, until a+ itself fails.
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.
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.
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.SS "Automatic .* anchoring"
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.rs
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.sp
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By default, an optimization is applied when .* is the first significant item in
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a pattern. If PCRE2_DOTALL is set, so that the dot can match any character, the
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pattern is automatically anchored. If PCRE2_DOTALL is not set, a match can
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start only after an internal newline or at the beginning of the subject, and
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\fBpcre2_compile()\fP remembers this. This optimization is disabled, however,
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if .* is in an atomic group or if there is a back reference to the capturing
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group in which it appears. It is also disabled if the pattern contains (*PRUNE)
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or (*SKIP). However, the presence of callouts does not affect it.
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.P
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For example, if the pattern .*\ed is compiled with PCRE2_AUTO_CALLOUT and
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applied to the string "aa", the \fBpcre2test\fP output is:
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.sp
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--->aa
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+0 ^ .*
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+2 ^ ^ \ed
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+2 ^^ \ed
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+2 ^ \ed
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No match
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.sp
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This shows that all match attempts start at the beginning of the subject. In
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other words, the pattern is anchored. You can disable this optimization by
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passing PCRE2_NO_DOTSTAR_ANCHOR to \fBpcre2_compile()\fP, or starting the
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pattern with (*NO_DOTSTAR_ANCHOR). In this case, the output changes to:
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.sp
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--->aa
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+0 ^ .*
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+2 ^ ^ \ed
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+2 ^^ \ed
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+2 ^ \ed
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+0 ^ .*
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+2 ^^ \ed
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+2 ^ \ed
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No match
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.sp
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This shows more match attempts, starting at the second subject character.
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Another optimization, described in the next section, means that there is no
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subsequent attempt to match with an empty subject.
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.P
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If a pattern has more than one top-level branch, automatic anchoring occurs if
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all branches are anchorable.
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.
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.
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.SS "Other optimizations"
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.rs
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.sp
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Other optimizations that provide fast "no match" results also affect callouts.
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For example, if the pattern is
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.sp
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ab(?C4)cd
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.sp
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PCRE2 knows that any matching string must contain the letter "d". If the
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subject string is "abyz", the lack of "d" means that matching doesn't ever
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start, and the callout is never reached. However, with "abyd", though the
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result is still no match, the callout is obeyed.
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.P
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PCRE2 also knows the minimum length of a matching string, and will immediately
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give a "no match" return without actually running a match if the subject is not
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long enough, or, for unanchored patterns, if it has been scanned far enough.
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.P
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You can disable these optimizations by passing the PCRE2_NO_START_OPTIMIZE
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option to \fBpcre2_compile()\fP, or by starting the pattern with
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(*NO_START_OPT). This slows down the matching process, but does ensure that
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callouts such as the example above are obeyed.
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.
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.
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.\" HTML <a name="calloutinterface"></a>
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.SH "THE CALLOUT INTERFACE"
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.rs
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.sp
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During matching, when PCRE2 reaches a callout point, if an external function is
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set in the match context, it is called. This applies to both normal and DFA
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matching. The first argument to the callout function is a pointer to a
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\fBpcre2_callout\fP block. The second argument is the void * callout data that
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was supplied when the callout was set up by calling \fBpcre2_set_callout()\fP
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(see the
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.\" HREF
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\fBpcre2api\fP
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.\"
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documentation). The callout block structure contains the following fields:
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.sp
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uint32_t \fIversion\fP;
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uint32_t \fIcallout_number\fP;
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uint32_t \fIcapture_top\fP;
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uint32_t \fIcapture_last\fP;
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PCRE2_SIZE *\fIoffset_vector\fP;
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PCRE2_SPTR \fImark\fP;
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PCRE2_SPTR \fIsubject\fP;
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PCRE2_SIZE \fIsubject_length\fP;
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PCRE2_SIZE \fIstart_match\fP;
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PCRE2_SIZE \fIcurrent_position\fP;
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PCRE2_SIZE \fIpattern_position\fP;
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PCRE2_SIZE \fInext_item_length\fP;
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PCRE2_SIZE \fIcallout_string_offset\fP;
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PCRE2_SIZE \fIcallout_string_length\fP;
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PCRE2_SPTR \fIcallout_string\fP;
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.sp
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The \fIversion\fP field contains the version number of the block format. The
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current version is 1; the three callout string fields were added for this
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version. If you are writing an application that might use an earlier release of
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PCRE2, you should check the version number before accessing any of these
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fields. The version number will increase in future if more fields are added,
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but the intention is never to remove any of the existing fields.
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.
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.
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.SS "Fields for numerical callouts"
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.rs
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.sp
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For a numerical callout, \fIcallout_string\fP is NULL, and \fIcallout_number\fP
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contains the number of the callout, in the range 0-255. This is the number
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that follows (?C for callouts that part of the pattern; it is 255 for
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automatically generated callouts.
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.
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.
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.SS "Fields for string callouts"
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.rs
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.sp
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For callouts with string arguments, \fIcallout_number\fP is always zero, and
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\fIcallout_string\fP points to the string that is contained within the compiled
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pattern. Its length is given by \fIcallout_string_length\fP. Duplicated ending
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delimiters that were present in the original pattern string have been turned
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into single characters, but there is no other processing of the callout string
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argument. An additional code unit containing binary zero is present after the
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string, but is not included in the length. The delimiter that was used to start
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the string is also stored within the pattern, immediately before the string
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itself. You can access this delimiter as \fIcallout_string\fP[-1] if you need
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it.
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.P
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The \fIcallout_string_offset\fP field is the code unit offset to the start of
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the callout argument string within the original pattern string. This is
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provided for the benefit of applications such as script languages that might
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need to report errors in the callout string within the pattern.
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.
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.
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.SS "Fields for all callouts"
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.rs
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.sp
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The remaining fields in the callout block are the same for both kinds of
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callout.
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.P
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The \fIoffset_vector\fP field is a pointer to the vector of capturing offsets
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(the "ovector") that was passed to the matching function in the match data
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block. When \fBpcre2_match()\fP is used, the contents can be inspected in
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order to extract substrings that have been matched so far, in the same way as
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for extracting substrings after a match has completed. For the DFA matching
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function, this field is not useful.
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.P
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The \fIsubject\fP and \fIsubject_length\fP fields contain copies of the values
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that were passed to the matching function.
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.P
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The \fIstart_match\fP field normally contains the offset within the subject at
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which the current match attempt started. However, if the escape sequence \eK
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has been encountered, this value is changed to reflect the modified starting
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point. If the pattern is not anchored, the callout function may be called
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several times from the same point in the pattern for different starting points
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in the subject.
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.P
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The \fIcurrent_position\fP field contains the offset within the subject of the
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current match pointer.
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.P
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When the \fBpcre2_match()\fP is used, the \fIcapture_top\fP field contains one
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more than the number of the highest numbered captured substring so far. If no
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substrings have been captured, the value of \fIcapture_top\fP is one. This is
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always the case when the DFA functions are used, because they do not support
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captured substrings.
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.P
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The \fIcapture_last\fP field contains the number of the most recently captured
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substring. However, when a recursion exits, the value reverts to what it was
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outside the recursion, as do the values of all captured substrings. If no
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substrings have been captured, the value of \fIcapture_last\fP is 0. This is
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always the case for the DFA matching functions.
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.P
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The \fIpattern_position\fP field contains the offset in the pattern string to
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the next item to be matched.
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.P
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The \fInext_item_length\fP field contains the length of the next item to be
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processed in the pattern string. When the callout is at the end of the pattern,
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the length is zero. When the callout precedes an opening parenthesis, the
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length includes meta characters that follow the parenthesis. For example, in a
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callout before an assertion such as (?=ab) the length is 3. For an an
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alternation bar or a closing parenthesis, the length is one, unless a closing
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parenthesis is followed by a quantifier, in which case its length is included.
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(This changed in release 10.23. In earlier releases, before an opening
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parenthesis the length was that of the entire subpattern, and before an
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alternation bar or a closing parenthesis the length was zero.)
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.P
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The \fIpattern_position\fP and \fInext_item_length\fP fields are intended to
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help in distinguishing between different automatic callouts, which all have the
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same callout number. However, they are set for all callouts, and are used by
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\fBpcre2test\fP to show the next item to be matched when displaying callout
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information.
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.P
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In callouts from \fBpcre2_match()\fP the \fImark\fP field contains a pointer to
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the zero-terminated name of the most recently passed (*MARK), (*PRUNE), or
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(*THEN) item in the match, or NULL if no such items have been passed. Instances
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of (*PRUNE) or (*THEN) without a name do not obliterate a previous (*MARK). In
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callouts from the DFA matching function this field always contains NULL.
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.
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.
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.SH "RETURN VALUES FROM CALLOUTS"
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.rs
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.sp
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The external callout function returns an integer to PCRE2. If the value is
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zero, matching proceeds as normal. If the value is greater than zero, matching
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fails at the current point, but the testing of other matching possibilities
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goes ahead, just as if a lookahead assertion had failed. If the value is less
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than zero, the match is abandoned, and the matching function returns the
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negative value.
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.P
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Negative values should normally be chosen from the set of PCRE2_ERROR_xxx
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values. In particular, PCRE2_ERROR_NOMATCH forces a standard "no match"
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failure. The error number PCRE2_ERROR_CALLOUT is reserved for use by callout
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functions; it will never be used by PCRE2 itself.
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.
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.
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.SH "CALLOUT ENUMERATION"
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.rs
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.sp
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.nf
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.B int pcre2_callout_enumerate(const pcre2_code *\fIcode\fP,
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.B " int (*\fIcallback\fP)(pcre2_callout_enumerate_block *, void *),"
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.B " void *\fIuser_data\fP);"
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.fi
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.sp
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A script language that supports the use of string arguments in callouts might
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like to scan all the callouts in a pattern before running the match. This can
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be done by calling \fBpcre2_callout_enumerate()\fP. The first argument is a
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pointer to a compiled pattern, the second points to a callback function, and
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the third is arbitrary user data. The callback function is called for every
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callout in the pattern in the order in which they appear. Its first argument is
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a pointer to a callout enumeration block, and its second argument is the
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\fIuser_data\fP value that was passed to \fBpcre2_callout_enumerate()\fP. The
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data block contains the following fields:
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.sp
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\fIversion\fP Block version number
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\fIpattern_position\fP Offset to next item in pattern
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\fInext_item_length\fP Length of next item in pattern
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\fIcallout_number\fP Number for numbered callouts
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\fIcallout_string_offset\fP Offset to string within pattern
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\fIcallout_string_length\fP Length of callout string
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\fIcallout_string\fP Points to callout string or is NULL
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.sp
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The version number is currently 0. It will increase if new fields are ever
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added to the block. The remaining fields are the same as their namesakes in the
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\fBpcre2_callout\fP block that is used for callouts during matching, as
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described
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.\" HTML <a href="#calloutinterface">
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.\" </a>
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above.
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.\"
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.P
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Note that the value of \fIpattern_position\fP is unique for each callout.
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However, if a callout occurs inside a group that is quantified with a non-zero
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minimum or a fixed maximum, the group is replicated inside the compiled
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pattern. For example, a pattern such as /(a){2}/ is compiled as if it were
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/(a)(a)/. This means that the callout will be enumerated more than once, but
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with the same value for \fIpattern_position\fP in each case.
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.P
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The callback function should normally return zero. If it returns a non-zero
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value, scanning the pattern stops, and that value is returned from
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\fBpcre2_callout_enumerate()\fP.
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.
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.
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.SH AUTHOR
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.rs
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.sp
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.nf
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Philip Hazel
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University Computing Service
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Cambridge, England.
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.fi
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.
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.
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.SH REVISION
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.rs
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.sp
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.nf
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Last updated: 29 September 2016
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Copyright (c) 1997-2016 University of Cambridge.
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.fi
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