481 lines
20 KiB
HTML
481 lines
20 KiB
HTML
<html>
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<head>
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<title>pcre2callout specification</title>
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</head>
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<body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
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<h1>pcre2callout man page</h1>
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<p>
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Return to the <a href="index.html">PCRE2 index page</a>.
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</p>
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<p>
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This page is part of the PCRE2 HTML documentation. It was generated
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automatically from the original man page. If there is any nonsense in it,
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please consult the man page, in case the conversion went wrong.
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<br>
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<ul>
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<li><a name="TOC1" href="#SEC1">SYNOPSIS</a>
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<li><a name="TOC2" href="#SEC2">DESCRIPTION</a>
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<li><a name="TOC3" href="#SEC3">MISSING CALLOUTS</a>
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<li><a name="TOC4" href="#SEC4">THE CALLOUT INTERFACE</a>
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<li><a name="TOC5" href="#SEC5">RETURN VALUES FROM CALLOUTS</a>
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<li><a name="TOC6" href="#SEC6">CALLOUT ENUMERATION</a>
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<li><a name="TOC7" href="#SEC7">AUTHOR</a>
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<li><a name="TOC8" href="#SEC8">REVISION</a>
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</ul>
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<br><a name="SEC1" href="#TOC1">SYNOPSIS</a><br>
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<P>
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<b>#include <pcre2.h></b>
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</P>
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<P>
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<b>int (*pcre2_callout)(pcre2_callout_block *, void *);</b>
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<br>
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<br>
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<b>int pcre2_callout_enumerate(const pcre2_code *<i>code</i>,</b>
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<b> int (*<i>callback</i>)(pcre2_callout_enumerate_block *, void *),</b>
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<b> void *<i>user_data</i>);</b>
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</P>
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<br><a name="SEC2" href="#TOC1">DESCRIPTION</a><br>
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<P>
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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 <b>pcre2_set_callout()</b> in the
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<a href="pcre2api.html"><b>pcre2api</b></a>
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documentation).
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</P>
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<P>
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When using the <b>pcre2_substitute()</b> function, an additional callout feature
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is available. This does a callout after each change to the subject string and
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is described in the
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<a href="pcre2api.html"><b>pcre2api</b></a>
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documentation; the rest of this document is concerned with callouts during
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pattern matching.
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</P>
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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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<pre>
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(?C1)abc(?C"some ""arbitrary"" text")def
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</pre>
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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 an explicit callout. For
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example, if PCRE2_AUTO_CALLOUT is used with the pattern
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<pre>
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A(?C3)B
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</pre>
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it is processed as if it were
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<pre>
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(?C255)A(?C3)B(?C255)
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</pre>
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Here is a more complicated example:
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<pre>
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A(\d{2}|--)
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</pre>
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With PCRE2_AUTO_CALLOUT, this pattern is processed as if it were
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<pre>
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(?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)
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</pre>
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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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<pre>
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(?(?C9)(?=a)ab|de) (?(?C%text%)(?!=d)ab|de)
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</pre>
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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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<P>
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Callouts can be useful for tracking the progress of pattern matching. The
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<a href="pcre2test.html"><b>pcre2test</b></a>
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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 <b>pcre2test</b> 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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</P>
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<br><a name="SEC3" href="#TOC1">MISSING CALLOUTS</a><br>
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<P>
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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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</P>
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<br><b>
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Auto-possessification
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</b><br>
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<P>
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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 <b>pcre2test</b> 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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<pre>
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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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</pre>
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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 <b>pcre2_compile()</b>, or starting
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the pattern with (*NO_AUTO_POSSESS). In this case, the output changes to this:
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<pre>
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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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</pre>
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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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</P>
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<br><b>
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Automatic .* anchoring
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</b><br>
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<P>
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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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<b>pcre2_compile()</b> remembers this. If a pattern has more than one top-level
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branch, automatic anchoring occurs if all branches are anchorable.
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</P>
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<P>
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This optimization is disabled, however, if .* is in an atomic group or if there
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is a backreference to the capture group in which it appears. It is also
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disabled if the pattern contains (*PRUNE) or (*SKIP). However, the presence of
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callouts does not affect it.
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</P>
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<P>
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For example, if the pattern .*\d is compiled with PCRE2_AUTO_CALLOUT and
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applied to the string "aa", the <b>pcre2test</b> output is:
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<pre>
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--->aa
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+0 ^ .*
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+2 ^ ^ \d
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+2 ^^ \d
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+2 ^ \d
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No match
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</pre>
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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 <b>pcre2_compile()</b>, or starting the
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pattern with (*NO_DOTSTAR_ANCHOR). In this case, the output changes to:
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<pre>
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--->aa
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+0 ^ .*
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+2 ^ ^ \d
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+2 ^^ \d
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+2 ^ \d
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+0 ^ .*
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+2 ^^ \d
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+2 ^ \d
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No match
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</pre>
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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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<br><b>
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Other optimizations
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</b><br>
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<P>
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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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<pre>
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ab(?C4)cd
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</pre>
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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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<P>
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For most patterns PCRE2 also knows the minimum length of a matching string, and
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will immediately give a "no match" return without actually running a match if
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the subject is not long enough, or, for unanchored patterns, if it has been
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scanned far enough.
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</P>
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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 <b>pcre2_compile()</b>, 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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<a name="calloutinterface"></a></P>
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<br><a name="SEC4" href="#TOC1">THE CALLOUT INTERFACE</a><br>
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<P>
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During matching, when PCRE2 reaches a callout point, if an external function is
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provided in the match context, it is called. This applies to both normal,
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DFA, and JIT matching. The first argument to the callout function is a pointer
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to a <b>pcre2_callout</b> block. The second argument is the void * callout data
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that was supplied when the callout was set up by calling
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<b>pcre2_set_callout()</b> (see the
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<a href="pcre2api.html"><b>pcre2api</b></a>
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documentation). The callout block structure contains the following fields, not
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necessarily in this order:
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<pre>
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uint32_t <i>version</i>;
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uint32_t <i>callout_number</i>;
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uint32_t <i>capture_top</i>;
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uint32_t <i>capture_last</i>;
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uint32_t <i>callout_flags</i>;
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PCRE2_SIZE *<i>offset_vector</i>;
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PCRE2_SPTR <i>mark</i>;
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PCRE2_SPTR <i>subject</i>;
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PCRE2_SIZE <i>subject_length</i>;
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PCRE2_SIZE <i>start_match</i>;
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PCRE2_SIZE <i>current_position</i>;
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PCRE2_SIZE <i>pattern_position</i>;
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PCRE2_SIZE <i>next_item_length</i>;
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PCRE2_SIZE <i>callout_string_offset</i>;
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PCRE2_SIZE <i>callout_string_length</i>;
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PCRE2_SPTR <i>callout_string</i>;
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</pre>
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The <i>version</i> field contains the version number of the block format. The
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current version is 2; the three callout string fields were added for version 1,
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and the <i>callout_flags</i> field for version 2. If you are writing an
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application that might use an earlier release of PCRE2, you should check the
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version number before accessing any of these fields. The version number will
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increase in future if more fields are added, but the intention is never to
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remove any of the existing fields.
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</P>
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<br><b>
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Fields for numerical callouts
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</b><br>
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<P>
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For a numerical callout, <i>callout_string</i> is NULL, and <i>callout_number</i>
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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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</P>
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<br><b>
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Fields for string callouts
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</b><br>
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<P>
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For callouts with string arguments, <i>callout_number</i> is always zero, and
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<i>callout_string</i> points to the string that is contained within the compiled
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pattern. Its length is given by <i>callout_string_length</i>. 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 <i>callout_string</i>[-1] if you need
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it.
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</P>
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<P>
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The <i>callout_string_offset</i> 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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</P>
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<br><b>
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Fields for all callouts
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</b><br>
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<P>
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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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<P>
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The <i>offset_vector</i> field is a pointer to a vector of capturing offsets
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(the "ovector"). You may read the elements in this vector, but you must not
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change any of them.
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</P>
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<P>
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For calls to <b>pcre2_match()</b>, the <i>offset_vector</i> field is not (since
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release 10.30) a pointer to the actual ovector that was passed to the matching
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function in the match data block. Instead it points to an internal ovector of a
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size large enough to hold all possible captured substrings in the pattern. Note
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that whenever a recursion or subroutine call within a pattern completes, the
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capturing state is reset to what it was before.
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</P>
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<P>
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The <i>capture_last</i> field contains the number of the most recently captured
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substring, and the <i>capture_top</i> field contains one more than the number of
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the highest numbered captured substring so far. If no substrings have yet been
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captured, the value of <i>capture_last</i> is 0 and the value of
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<i>capture_top</i> is 1. The values of these fields do not always differ by one;
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for example, when the callout in the pattern ((a)(b))(?C2) is taken,
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<i>capture_last</i> is 1 but <i>capture_top</i> is 4.
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</P>
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<P>
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The contents of ovector[2] to ovector[<capture_top>*2-1] 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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extracting substrings after a match has completed. The values in ovector[0] and
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ovector[1] are always PCRE2_UNSET because the match is by definition not
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complete. Substrings that have not been captured but whose numbers are less
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than <i>capture_top</i> also have both of their ovector slots set to
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PCRE2_UNSET.
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</P>
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<P>
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For DFA matching, the <i>offset_vector</i> field points to the ovector that was
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passed to the matching function in the match data block for callouts at the top
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level, but to an internal ovector during the processing of pattern recursions,
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lookarounds, and atomic groups. However, these ovectors hold no useful
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information because <b>pcre2_dfa_match()</b> does not support substring
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capturing. The value of <i>capture_top</i> is always 1 and the value of
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<i>capture_last</i> is always 0 for DFA matching.
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</P>
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<P>
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The <i>subject</i> and <i>subject_length</i> 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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<P>
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The <i>start_match</i> field normally contains the offset within the subject at
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which the current match attempt started. However, if the escape sequence \K
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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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<P>
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The <i>current_position</i> 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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<P>
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The <i>pattern_position</i> 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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<P>
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The <i>next_item_length</i> 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 group, and before an alternation
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bar or a closing parenthesis the length was zero.)
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</P>
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<P>
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The <i>pattern_position</i> and <i>next_item_length</i> 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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<b>pcre2test</b> 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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<P>
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In callouts from <b>pcre2_match()</b> the <i>mark</i> 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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</P>
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<P>
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The <i>callout_flags</i> field is always zero in callouts from
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<b>pcre2_dfa_match()</b> or when JIT is being used. When <b>pcre2_match()</b>
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without JIT is used, the following bits may be set:
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<pre>
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PCRE2_CALLOUT_STARTMATCH
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</pre>
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This is set for the first callout after the start of matching for each new
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starting position in the subject.
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<pre>
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PCRE2_CALLOUT_BACKTRACK
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</pre>
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This is set if there has been a matching backtrack since the previous callout,
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or since the start of matching if this is the first callout from a
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<b>pcre2_match()</b> run.
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</P>
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<P>
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Both bits are set when a backtrack has caused a "bumpalong" to a new starting
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position in the subject. Output from <b>pcre2test</b> does not indicate the
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presence of these bits unless the <b>callout_extra</b> modifier is set.
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</P>
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<P>
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The information in the <b>callout_flags</b> field is provided so that
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applications can track and tell their users how matching with backtracking is
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done. This can be useful when trying to optimize patterns, or just to
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understand how PCRE2 works. There is no support in <b>pcre2_dfa_match()</b>
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because there is no backtracking in DFA matching, and there is no support in
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JIT because JIT is all about maximimizing matching performance. In both these
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cases the <b>callout_flags</b> field is always zero.
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</P>
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<br><a name="SEC5" href="#TOC1">RETURN VALUES FROM CALLOUTS</a><br>
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<P>
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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.
|
|
</P>
|
|
<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.
|
|
</P>
|
|
<br><a name="SEC6" href="#TOC1">CALLOUT ENUMERATION</a><br>
|
|
<P>
|
|
<b>int pcre2_callout_enumerate(const pcre2_code *<i>code</i>,</b>
|
|
<b> int (*<i>callback</i>)(pcre2_callout_enumerate_block *, void *),</b>
|
|
<b> void *<i>user_data</i>);</b>
|
|
<br>
|
|
<br>
|
|
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 <b>pcre2_callout_enumerate()</b>. 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
|
|
<i>user_data</i> value that was passed to <b>pcre2_callout_enumerate()</b>. The
|
|
data block contains the following fields:
|
|
<pre>
|
|
<i>version</i> Block version number
|
|
<i>pattern_position</i> Offset to next item in pattern
|
|
<i>next_item_length</i> Length of next item in pattern
|
|
<i>callout_number</i> Number for numbered callouts
|
|
<i>callout_string_offset</i> Offset to string within pattern
|
|
<i>callout_string_length</i> Length of callout string
|
|
<i>callout_string</i> Points to callout string or is NULL
|
|
</pre>
|
|
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
|
|
<b>pcre2_callout</b> block that is used for callouts during matching, as
|
|
described
|
|
<a href="#calloutinterface">above.</a>
|
|
</P>
|
|
<P>
|
|
Note that the value of <i>pattern_position</i> 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 <i>pattern_position</i> in each case.
|
|
</P>
|
|
<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
|
|
<b>pcre2_callout_enumerate()</b>.
|
|
</P>
|
|
<br><a name="SEC7" href="#TOC1">AUTHOR</a><br>
|
|
<P>
|
|
Philip Hazel
|
|
<br>
|
|
University Computing Service
|
|
<br>
|
|
Cambridge, England.
|
|
<br>
|
|
</P>
|
|
<br><a name="SEC8" href="#TOC1">REVISION</a><br>
|
|
<P>
|
|
Last updated: 03 February 2019
|
|
<br>
|
|
Copyright © 1997-2019 University of Cambridge.
|
|
<br>
|
|
<p>
|
|
Return to the <a href="index.html">PCRE2 index page</a>.
|
|
</p>
|