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(Regexp Special): Clarify nested regexp warning.

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This commit is contained in:
Richard M. Stallman 2006-05-17 22:17:09 +00:00
parent 05327ca972
commit bc023640a3
1 changed files with 5 additions and 3 deletions
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8
lispref/searching.texi
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@ -306,9 +306,10 @@ matching @samp{ca*ar} against the string @samp{caaar}, the @samp{a*}
first tries to match all three @samp{a}s; but the rest of the pattern is
@samp{ar} and there is only @samp{r} left to match, so this try fails.
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
this choice, the rest of the regexp matches successfully.
Nested repetition operators take a long time, or even forever, if they
@strong{Warning:} Nested repetition operators take a long time,
or even forever, if they
lead to ambiguous matching. For example, trying to match the regular
expression @samp{\(x+y*\)*a} against the string
@samp{xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxz} could take hours before it
@ -316,7 +317,8 @@ ultimately fails. Emacs must try each way of grouping the 35
@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.
repetitions carefully, to make sure that they do not cause combinatorial
explosions in backtracking.
@item @samp{+}
@cindex @samp{+} in regexp