pcreperform.3 7.1 KB

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  1. .TH PCREPERFORM 3 "09 January 2012" "PCRE 8.30"
  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 interpretive code, so
  15. that most simple patterns do not use much memory. However, there is one case
  16. where 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 using the 8-bit library. When PCRE is compiled
  38. with its default internal pointer size of two bytes, the size limit on a
  39. compiled pattern is 64K data units, and this is reached with the above pattern
  40. if the outer repetition is increased from 3 to 4. PCRE can be compiled to use
  41. larger internal pointers and thus handle larger compiled patterns, but it is
  42. better to try to rewrite your 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 or \fBpcre[16|32]_exec()\fP is used for matching, certain
  72. kinds of pattern can cause it to use large amounts of the process stack. In
  73. some environments the default process stack is quite small, and if it runs out
  74. the result is often SIGSEGV. This issue is probably the most frequently raised
  75. problem with PCRE. Rewriting 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 use a multi-stage table lookup whenever it needs a
  95. character's property. If you can find an alternative pattern that does not use
  96. character properties, it will probably be faster.
  97. .P
  98. By default, the escape sequences \eb, \ed, \es, and \ew, and the POSIX
  99. character classes such as [:alpha:] do not use Unicode properties, partly for
  100. backwards compatibility, and partly for performance reasons. However, you can
  101. set PCRE_UCP if you want Unicode character properties to be used. This can
  102. double the matching time for items such as \ed, when matched with
  103. a traditional matching function; the performance loss is less with
  104. a DFA matching function, and in both cases there is not much difference for
  105. \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: 25 August 2012
  176. Copyright (c) 1997-2012 University of Cambridge.
  177. .fi