diff options
Diffstat (limited to 'lispref/searching.texi')
| -rw-r--r-- | lispref/searching.texi | 16 |
1 files changed, 9 insertions, 7 deletions
diff --git a/lispref/searching.texi b/lispref/searching.texi index 9e26363a43a..d18587ccecb 100644 --- a/lispref/searching.texi +++ b/lispref/searching.texi @@ -244,13 +244,15 @@ 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.@refill -Nested repetition operators can be extremely slow if they specify -backtracking loops. For example, it could take hours for the regular -expression @samp{\(x+y*\)*a} to try to match the sequence -@samp{xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxz}, before it ultimately fails. -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 +Nested repetition operators can be extremely slow or loop infinitely +if they use repetition operators inside repetition operators. For +example, it could take hours for the regular expression +@samp{\(x+y*\)*a} to try to match the sequence +@samp{xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxz}, before it 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. @item @samp{+} |