Tristan Matthews | 0461646 | 2013-11-14 16:09:34 -0500 | [diff] [blame] | 1 | .TH PCREPERFORM 3 |
| 2 | .SH NAME |
| 3 | PCRE - Perl-compatible regular expressions |
| 4 | .SH "PCRE PERFORMANCE" |
| 5 | .rs |
| 6 | .sp |
| 7 | Two aspects of performance are discussed below: memory usage and processing |
| 8 | time. The way you express your pattern as a regular expression can affect both |
| 9 | of them. |
| 10 | . |
| 11 | .SH "COMPILED PATTERN MEMORY USAGE" |
| 12 | .rs |
| 13 | .sp |
| 14 | Patterns are compiled by PCRE into a reasonably efficient byte code, so that |
| 15 | most simple patterns do not use much memory. However, there is one case where |
| 16 | the memory usage of a compiled pattern can be unexpectedly large. If a |
| 17 | parenthesized subpattern has a quantifier with a minimum greater than 1 and/or |
| 18 | a limited maximum, the whole subpattern is repeated in the compiled code. For |
| 19 | example, the pattern |
| 20 | .sp |
| 21 | (abc|def){2,4} |
| 22 | .sp |
| 23 | is compiled as if it were |
| 24 | .sp |
| 25 | (abc|def)(abc|def)((abc|def)(abc|def)?)? |
| 26 | .sp |
| 27 | (Technical aside: It is done this way so that backtrack points within each of |
| 28 | the repetitions can be independently maintained.) |
| 29 | .P |
| 30 | For regular expressions whose quantifiers use only small numbers, this is not |
| 31 | usually a problem. However, if the numbers are large, and particularly if such |
| 32 | repetitions are nested, the memory usage can become an embarrassment. For |
| 33 | example, the very simple pattern |
| 34 | .sp |
| 35 | ((ab){1,1000}c){1,3} |
| 36 | .sp |
| 37 | uses 51K bytes when compiled. When PCRE is compiled with its default internal |
| 38 | pointer size of two bytes, the size limit on a compiled pattern is 64K, and |
| 39 | this is reached with the above pattern if the outer repetition is increased |
| 40 | from 3 to 4. PCRE can be compiled to use larger internal pointers and thus |
| 41 | handle larger compiled patterns, but it is better to try to rewrite your |
| 42 | pattern to use less memory if you can. |
| 43 | .P |
| 44 | One way of reducing the memory usage for such patterns is to make use of PCRE's |
| 45 | .\" HTML <a href="pcrepattern.html#subpatternsassubroutines"> |
| 46 | .\" </a> |
| 47 | "subroutine" |
| 48 | .\" |
| 49 | facility. Re-writing the above pattern as |
| 50 | .sp |
| 51 | ((ab)(?2){0,999}c)(?1){0,2} |
| 52 | .sp |
| 53 | reduces the memory requirements to 18K, and indeed it remains under 20K even |
| 54 | with the outer repetition increased to 100. However, this pattern is not |
| 55 | exactly equivalent, because the "subroutine" calls are treated as |
| 56 | .\" HTML <a href="pcrepattern.html#atomicgroup"> |
| 57 | .\" </a> |
| 58 | atomic groups |
| 59 | .\" |
| 60 | into which there can be no backtracking if there is a subsequent matching |
| 61 | failure. Therefore, PCRE cannot do this kind of rewriting automatically. |
| 62 | Furthermore, there is a noticeable loss of speed when executing the modified |
| 63 | pattern. Nevertheless, if the atomic grouping is not a problem and the loss of |
| 64 | speed is acceptable, this kind of rewriting will allow you to process patterns |
| 65 | that PCRE cannot otherwise handle. |
| 66 | . |
| 67 | . |
| 68 | .SH "STACK USAGE AT RUN TIME" |
| 69 | .rs |
| 70 | .sp |
| 71 | When \fBpcre_exec()\fP is used for matching, certain kinds of pattern can cause |
| 72 | it to use large amounts of the process stack. In some environments the default |
| 73 | process stack is quite small, and if it runs out the result is often SIGSEGV. |
| 74 | This issue is probably the most frequently raised problem with PCRE. Rewriting |
| 75 | your pattern can often help. The |
| 76 | .\" HREF |
| 77 | \fBpcrestack\fP |
| 78 | .\" |
| 79 | documentation discusses this issue in detail. |
| 80 | . |
| 81 | . |
| 82 | .SH "PROCESSING TIME" |
| 83 | .rs |
| 84 | .sp |
| 85 | Certain items in regular expression patterns are processed more efficiently |
| 86 | than others. It is more efficient to use a character class like [aeiou] than a |
| 87 | set of single-character alternatives such as (a|e|i|o|u). In general, the |
| 88 | simplest construction that provides the required behaviour is usually the most |
| 89 | efficient. Jeffrey Friedl's book contains a lot of useful general discussion |
| 90 | about optimizing regular expressions for efficient performance. This document |
| 91 | contains a few observations about PCRE. |
| 92 | .P |
| 93 | Using Unicode character properties (the \ep, \eP, and \eX escapes) is slow, |
| 94 | because PCRE has to scan a structure that contains data for over fifteen |
| 95 | thousand characters whenever it needs a character's property. If you can find |
| 96 | an alternative pattern that does not use character properties, it will probably |
| 97 | be faster. |
| 98 | .P |
| 99 | By default, the escape sequences \eb, \ed, \es, and \ew, and the POSIX |
| 100 | character classes such as [:alpha:] do not use Unicode properties, partly for |
| 101 | backwards compatibility, and partly for performance reasons. However, you can |
| 102 | set PCRE_UCP if you want Unicode character properties to be used. This can |
| 103 | double the matching time for items such as \ed, when matched with |
| 104 | \fBpcre_exec()\fP; the performance loss is less with \fBpcre_dfa_exec()\fP, and |
| 105 | in both cases there is not much difference for \eb. |
| 106 | .P |
| 107 | When a pattern begins with .* not in parentheses, or in parentheses that are |
| 108 | not the subject of a backreference, and the PCRE_DOTALL option is set, the |
| 109 | pattern is implicitly anchored by PCRE, since it can match only at the start of |
| 110 | a subject string. However, if PCRE_DOTALL is not set, PCRE cannot make this |
| 111 | optimization, because the . metacharacter does not then match a newline, and if |
| 112 | the subject string contains newlines, the pattern may match from the character |
| 113 | immediately following one of them instead of from the very start. For example, |
| 114 | the pattern |
| 115 | .sp |
| 116 | .*second |
| 117 | .sp |
| 118 | matches the subject "first\enand second" (where \en stands for a newline |
| 119 | character), with the match starting at the seventh character. In order to do |
| 120 | this, PCRE has to retry the match starting after every newline in the subject. |
| 121 | .P |
| 122 | If you are using such a pattern with subject strings that do not contain |
| 123 | newlines, the best performance is obtained by setting PCRE_DOTALL, or starting |
| 124 | the pattern with ^.* or ^.*? to indicate explicit anchoring. That saves PCRE |
| 125 | from having to scan along the subject looking for a newline to restart at. |
| 126 | .P |
| 127 | Beware of patterns that contain nested indefinite repeats. These can take a |
| 128 | long time to run when applied to a string that does not match. Consider the |
| 129 | pattern fragment |
| 130 | .sp |
| 131 | ^(a+)* |
| 132 | .sp |
| 133 | This can match "aaaa" in 16 different ways, and this number increases very |
| 134 | rapidly as the string gets longer. (The * repeat can match 0, 1, 2, 3, or 4 |
| 135 | times, and for each of those cases other than 0 or 4, the + repeats can match |
| 136 | different numbers of times.) When the remainder of the pattern is such that the |
| 137 | entire match is going to fail, PCRE has in principle to try every possible |
| 138 | variation, and this can take an extremely long time, even for relatively short |
| 139 | strings. |
| 140 | .P |
| 141 | An optimization catches some of the more simple cases such as |
| 142 | .sp |
| 143 | (a+)*b |
| 144 | .sp |
| 145 | where a literal character follows. Before embarking on the standard matching |
| 146 | procedure, PCRE checks that there is a "b" later in the subject string, and if |
| 147 | there is not, it fails the match immediately. However, when there is no |
| 148 | following literal this optimization cannot be used. You can see the difference |
| 149 | by comparing the behaviour of |
| 150 | .sp |
| 151 | (a+)*\ed |
| 152 | .sp |
| 153 | with the pattern above. The former gives a failure almost instantly when |
| 154 | applied to a whole line of "a" characters, whereas the latter takes an |
| 155 | appreciable time with strings longer than about 20 characters. |
| 156 | .P |
| 157 | In many cases, the solution to this kind of performance issue is to use an |
| 158 | atomic group or a possessive quantifier. |
| 159 | . |
| 160 | . |
| 161 | .SH AUTHOR |
| 162 | .rs |
| 163 | .sp |
| 164 | .nf |
| 165 | Philip Hazel |
| 166 | University Computing Service |
| 167 | Cambridge CB2 3QH, England. |
| 168 | .fi |
| 169 | . |
| 170 | . |
| 171 | .SH REVISION |
| 172 | .rs |
| 173 | .sp |
| 174 | .nf |
| 175 | Last updated: 16 May 2010 |
| 176 | Copyright (c) 1997-2010 University of Cambridge. |
| 177 | .fi |