From 63268253d21c57d991cba3f3b083d74f154a26fe Mon Sep 17 00:00:00 2001 From: =?utf8?q?Mattias=20Engdeg=C3=A5rd?= Date: Sun, 3 May 2020 14:22:50 +0200 Subject: [PATCH] Regexps cannot infloop; fix manual * doc/lispref/searching.texi (Regexp Special): Edit erroneous statements about infinite looping in regexps. --- doc/lispref/searching.texi | 10 ++++------ 1 file 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 -- 2.39.2