From: Noam Postavsky <npostavs@users.sourceforge.net>
To: 20230@debbugs.gnu.org
Cc: Nicolas Richard <theonewiththeevillook@yahoo.fr>
Subject: bug#20230: Bug #20230: 24.4.91; slow regexp
Date: Sun, 26 Jun 2016 14:49:09 -0400 [thread overview]
Message-ID: <CAM-tV-8-jLvPxhoxAayXhuX55-SeW5QYd0xHxHoHwLT-v11RJw@mail.gmail.com> (raw)
In-Reply-To: <87619iv8yy.fsf@yahoo.fr>
merge 6640 20230
quit
Same problem as http://debbugs.gnu.org/cgi/bugreport.cgi?bug=6640:
Emacs uses backtracking regexp engine, so when then you have a failing
regexp with repeated sub-parts that can match in many different ways,
you hit exponential behaviour. In this case
\\(?: .*\\)?[ ]*
can match a stretch of n spaces in n different ways, and since that
part is itself inside a * repetition, each those n ways has to be
tried on each line giving n^L runtime (where L is number of lines). A
faster regexp which should match the same is
(looking-at "^[ \t]*:PROPERTIES:[ \t]*
\\(?:[ \t]*:\\S-+:[^\n]*
\\)*[ \t]*:END:[ \t]*$")
prev parent reply other threads:[~2016-06-26 18:49 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-03-30 14:46 bug#20230: 24.4.91; slow regexp Nicolas Richard
2016-06-26 18:49 ` Noam Postavsky [this message]
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