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author | Mattias EngdegÄrd <mattiase@acm.org> | 2020-05-03 14:22:50 +0200 |
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committer | Mattias EngdegÄrd <mattiase@acm.org> | 2020-05-03 14:27:51 +0200 |
commit | 63268253d21c57d991cba3f3b083d74f154a26fe (patch) | |
tree | 71a8e85776ec60d3a68e2c8a9cad4b2fa37e2719 /doc/lispref/searching.texi | |
parent | 7f51ab5163635aa6bc369b5173af955c6a27f927 (diff) | |
download | emacs-63268253d21c57d991cba3f3b083d74f154a26fe.tar.gz |
Regexps cannot infloop; fix manual
* doc/lispref/searching.texi (Regexp Special): Edit erroneous
statements about infinite looping in regexps.
Diffstat (limited to 'doc/lispref/searching.texi')
-rw-r--r-- | doc/lispref/searching.texi | 10 |
1 files changed, 4 insertions, 6 deletions
diff --git a/doc/lispref/searching.texi b/doc/lispref/searching.texi index 83c154938cd..c8a12bdd66b 100644 --- a/doc/lispref/searching.texi +++ b/doc/lispref/searching.texi @@ -338,16 +338,14 @@ first tries to match all three @samp{a}s; but the rest of the pattern is The next alternative is for @samp{a*} to match only two @samp{a}s. With this choice, the rest of the regexp matches successfully. -@strong{Warning:} Nested repetition operators can run for an -indefinitely long time, if they lead to ambiguous matching. For +@strong{Warning:} Nested repetition operators can run for a very +long time, if they lead to ambiguous matching. For example, trying to match the regular expression @samp{\(x+y*\)*a} against the string @samp{xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxz} could take hours before it ultimately fails. Emacs must try each way of grouping the @samp{x}s before concluding that none of them can work. -Even worse, @samp{\(x*\)*} can match the null string in infinitely -many ways, so it causes an infinite loop. To avoid these problems, -check nested repetitions carefully, to make sure that they do not -cause combinatorial explosions in backtracking. +In general, avoid expressions that can match the same string in +multiple ways. @item @samp{+} @cindex @samp{+} in regexp |