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Regexps cannot infloop; fix manual

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* doc/lispref/searching.texi (Regexp Special): Edit erroneous
statements about infinite looping in regexps.

(cherry picked from commit 63268253d2)
This commit is contained in:
Mattias Engdegård 2020-05-03 14:22:50 +02:00
parent cf0ee6f49b
commit 8ff5e346fe
1 changed files with 4 additions and 6 deletions
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doc/lispref/searching.texi
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@ -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