From 582b500cd996c96054615870fd13d6ab0ea77428 Mon Sep 17 00:00:00 2001 From: Jay Berkenbilt Date: Sat, 10 Oct 2009 15:10:05 +0000 Subject: start integrating windows port git-svn-id: svn+q:///qpdf/trunk@757 71b93d88-0707-0410-a8cf-f5a4172ac649 --- external-libs/pcre/doc/pcreperform.3 | 66 ++++++++++++++++++++++++++++++++++++ 1 file changed, 66 insertions(+) create mode 100644 external-libs/pcre/doc/pcreperform.3 (limited to 'external-libs/pcre/doc/pcreperform.3') diff --git a/external-libs/pcre/doc/pcreperform.3 b/external-libs/pcre/doc/pcreperform.3 new file mode 100644 index 00000000..52a332fc --- /dev/null +++ b/external-libs/pcre/doc/pcreperform.3 @@ -0,0 +1,66 @@ +.TH PCRE 3 +.SH NAME +PCRE - Perl-compatible regular expressions +.SH PCRE PERFORMANCE +.rs +.sp +Certain items that may appear in regular expression patterns are more efficient +than others. It is more efficient to use a character class like [aeiou] than a +set of alternatives such as (a|e|i|o|u). In general, the simplest construction +that provides the required behaviour is usually the most efficient. Jeffrey +Friedl's book contains a lot of discussion about optimizing regular expressions +for efficient performance. + +When a pattern begins with .* not in parentheses, or in parentheses that are +not the subject of a backreference, and the PCRE_DOTALL option is set, the +pattern is implicitly anchored by PCRE, since it can match only at the start of +a subject string. However, if PCRE_DOTALL is not set, PCRE cannot make this +optimization, because the . metacharacter does not then match a newline, and if +the subject string contains newlines, the pattern may match from the character +immediately following one of them instead of from the very start. For example, +the pattern + + .*second + +matches the subject "first\\nand second" (where \\n stands for a newline +character), with the match starting at the seventh character. In order to do +this, PCRE has to retry the match starting after every newline in the subject. + +If you are using such a pattern with subject strings that do not contain +newlines, the best performance is obtained by setting PCRE_DOTALL, or starting +the pattern with ^.* to indicate explicit anchoring. That saves PCRE from +having to scan along the subject looking for a newline to restart at. + +Beware of patterns that contain nested indefinite repeats. These can take a +long time to run when applied to a string that does not match. Consider the +pattern fragment + + (a+)* + +This can match "aaaa" in 33 different ways, and this number increases very +rapidly as the string gets longer. (The * repeat can match 0, 1, 2, 3, or 4 +times, and for each of those cases other than 0, the + repeats can match +different numbers of times.) When the remainder of the pattern is such that the +entire match is going to fail, PCRE has in principle to try every possible +variation, and this can take an extremely long time. + +An optimization catches some of the more simple cases such as + + (a+)*b + +where a literal character follows. Before embarking on the standard matching +procedure, PCRE checks that there is a "b" later in the subject string, and if +there is not, it fails the match immediately. However, when there is no +following literal this optimization cannot be used. You can see the difference +by comparing the behaviour of + + (a+)*\\d + +with the pattern above. The former gives a failure almost instantly when +applied to a whole line of "a" characters, whereas the latter takes an +appreciable time with strings longer than about 20 characters. + +.in 0 +Last updated: 03 February 2003 +.br +Copyright (c) 1997-2003 University of Cambridge. -- cgit v1.2.3-54-g00ecf