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From: Noam Postavsky <npostavs@users.sourceforge.net>
To: 6640@debbugs.gnu.org
Cc: Ryan Rix <ryan@whatthefuck.computer>, michael@cadilhac.name
Subject: bug#6640: 23.2; Why is this regexp search taking so long? (and will it end?)
Date: Fri, 10 Jun 2016 17:13:16 -0400	[thread overview]
Message-ID: <CAM-tV-9vsH6VMU_Mt0tG_sujVejwrf820pUq+h_BW5rqwXSruw@mail.gmail.com> (raw)
In-Reply-To: <yvzgk4own4ym.fsf@cadilhac.name>

I haven't actually debugged the regexp engine, but I believe the
problem is that this regexp contains several repetitions of [^:]*[^:]*
(which becomes apparent if you expand the \{9\}). The regexp engine
isn't smart enough to coalesce them so when the match fails (due to
$), it has to go back and retry with all the possible different
matches to see if it will work that way. There are A^n possible
matches to try, where A is the length of non-colon string in the
buffer, and n is the number of [^:]*[^:]* sequences in the regexp
(which is 8 if \{9\} is used).

A regexp which should match the same thing is ^\([^:]*:\)\{9\}[^:]*
and ^\([^:]*:\)\{9\}[^:]*$ will fail to match anything much faster.





      parent reply	other threads:[~2016-06-10 21:13 UTC|newest]

Thread overview: 2+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <yvzgk4own4ym.fsf@cadilhac.name>
2016-03-06  4:12 ` bug#6640: 23.2; Why is this regexp search taking so long? (and will it end?) Ryan Rix
2016-06-10 21:13 ` Noam Postavsky [this message]

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