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Warn about nested repetition.

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Richard M. Stallman 1995-05-02 03:47:29 +00:00
parent 698e4c1c47
commit 7303160369
1 changed files with 8 additions and 0 deletions
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lispref/searching.texi
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@ -225,6 +225,14 @@ 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.@refill
Nested repetition operators can be extremely slow if they specify
backtracking loops. For example, @samp{\(x+y*\)*a} could take hours to
match the sequence @samp{xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxz}. The
slowness is because Emacs must try each imaginable way of grouping the
35 @samp{x}'s before concluding that none of them can work. To make
sure your regular expressions run fast, check nested repetitions
carefully.
@item +
@cindex @samp{+} in regexp
is a suffix operator similar to @samp{*} except that the preceding