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diff --git a/doc/pcreperform.3 b/doc/pcreperform.3 new file mode 100644 index 0000000..fb2aa95 --- /dev/null +++ b/doc/pcreperform.3 @@ -0,0 +1,177 @@ +.TH PCREPERFORM 3 "09 January 2012" "PCRE 8.30" +.SH NAME +PCRE - Perl-compatible regular expressions +.SH "PCRE PERFORMANCE" +.rs +.sp +Two aspects of performance are discussed below: memory usage and processing +time. The way you express your pattern as a regular expression can affect both +of them. +. +.SH "COMPILED PATTERN MEMORY USAGE" +.rs +.sp +Patterns are compiled by PCRE into a reasonably efficient interpretive code, so +that most simple patterns do not use much memory. However, there is one case +where the memory usage of a compiled pattern can be unexpectedly large. If a +parenthesized subpattern has a quantifier with a minimum greater than 1 and/or +a limited maximum, the whole subpattern is repeated in the compiled code. For +example, the pattern +.sp + (abc|def){2,4} +.sp +is compiled as if it were +.sp + (abc|def)(abc|def)((abc|def)(abc|def)?)? +.sp +(Technical aside: It is done this way so that backtrack points within each of +the repetitions can be independently maintained.) +.P +For regular expressions whose quantifiers use only small numbers, this is not +usually a problem. However, if the numbers are large, and particularly if such +repetitions are nested, the memory usage can become an embarrassment. For +example, the very simple pattern +.sp + ((ab){1,1000}c){1,3} +.sp +uses 51K bytes when compiled using the 8-bit library. When PCRE is compiled +with its default internal pointer size of two bytes, the size limit on a +compiled pattern is 64K data units, and this is reached with the above pattern +if the outer repetition is increased from 3 to 4. PCRE can be compiled to use +larger internal pointers and thus handle larger compiled patterns, but it is +better to try to rewrite your pattern to use less memory if you can. +.P +One way of reducing the memory usage for such patterns is to make use of PCRE's +.\" HTML <a href="pcrepattern.html#subpatternsassubroutines"> +.\" </a> +"subroutine" +.\" +facility. Re-writing the above pattern as +.sp + ((ab)(?2){0,999}c)(?1){0,2} +.sp +reduces the memory requirements to 18K, and indeed it remains under 20K even +with the outer repetition increased to 100. However, this pattern is not +exactly equivalent, because the "subroutine" calls are treated as +.\" HTML <a href="pcrepattern.html#atomicgroup"> +.\" </a> +atomic groups +.\" +into which there can be no backtracking if there is a subsequent matching +failure. Therefore, PCRE cannot do this kind of rewriting automatically. +Furthermore, there is a noticeable loss of speed when executing the modified +pattern. Nevertheless, if the atomic grouping is not a problem and the loss of +speed is acceptable, this kind of rewriting will allow you to process patterns +that PCRE cannot otherwise handle. +. +. +.SH "STACK USAGE AT RUN TIME" +.rs +.sp +When \fBpcre_exec()\fP or \fBpcre[16|32]_exec()\fP is used for matching, certain +kinds of pattern can cause it to use large amounts of the process stack. In +some environments the default process stack is quite small, and if it runs out +the result is often SIGSEGV. This issue is probably the most frequently raised +problem with PCRE. Rewriting your pattern can often help. The +.\" HREF +\fBpcrestack\fP +.\" +documentation discusses this issue in detail. +. +. +.SH "PROCESSING TIME" +.rs +.sp +Certain items in regular expression patterns are processed more efficiently +than others. It is more efficient to use a character class like [aeiou] than a +set of single-character 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 useful general discussion +about optimizing regular expressions for efficient performance. This document +contains a few observations about PCRE. +.P +Using Unicode character properties (the \ep, \eP, and \eX escapes) is slow, +because PCRE has to use a multi-stage table lookup whenever it needs a +character's property. If you can find an alternative pattern that does not use +character properties, it will probably be faster. +.P +By default, the escape sequences \eb, \ed, \es, and \ew, and the POSIX +character classes such as [:alpha:] do not use Unicode properties, partly for +backwards compatibility, and partly for performance reasons. However, you can +set PCRE_UCP if you want Unicode character properties to be used. This can +double the matching time for items such as \ed, when matched with +a traditional matching function; the performance loss is less with +a DFA matching function, and in both cases there is not much difference for +\eb. +.P +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 +.sp + .*second +.sp +matches the subject "first\enand second" (where \en 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. +.P +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 ^.* or ^.*? to indicate explicit anchoring. That saves PCRE +from having to scan along the subject looking for a newline to restart at. +.P +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 +.sp + ^(a+)* +.sp +This can match "aaaa" in 16 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 or 4, 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, even for relatively short +strings. +.P +An optimization catches some of the more simple cases such as +.sp + (a+)*b +.sp +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 +.sp + (a+)*\ed +.sp +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. +.P +In many cases, the solution to this kind of performance issue is to use an +atomic group or a possessive quantifier. +. +. +.SH AUTHOR +.rs +.sp +.nf +Philip Hazel +University Computing Service +Cambridge CB2 3QH, England. +.fi +. +. +.SH REVISION +.rs +.sp +.nf +Last updated: 25 August 2012 +Copyright (c) 1997-2012 University of Cambridge. +.fi |