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From: Stefan Monnier <monnier@IRO.UMontreal.CA>
To: Andreas Schwab <schwab@linux-m68k.org>
Cc: Alan Mackenzie <acm@muc.de>, 18577@debbugs.gnu.org
Subject: bug#18577: Regexp I-search: [(error Stack overflow in regexp matcher)]
Date: Sun, 28 Sep 2014 13:35:30 -0400	[thread overview]
Message-ID: <jwv38bbsme9.fsf-monnier+emacsbugs@gnu.org> (raw)
In-Reply-To: <87d2afud0p.fsf@igel.home> (Andreas Schwab's message of "Sun, 28 Sep 2014 14:48:22 +0200")

>> Is this a defect in my regexp or in the regexp engine?
> It is fundamental to the way regexp matching works.

To clarify: it is fundamental to the way *our* regexp engine works.

As long as the regexp doesn't use backrefs, it can be matched
efficiently, without backtracking.  Of course using \(..\) (as opposed
to using \(?:..\)) can also make the problem harder since the various
different (but largely equivalent) ways to match might need to be
distinguishable via match-data.

But even tho your regexp doesn't use backrefs, and even if you replace
all \(..\) with \(?:..\), your regexp will still cause problems because
our regexp engine does not try to optimize these kinds of cases.

So you have to do it by hand.

>> If the former, how could I rewrite the regexp so that it would not hit
>> these problems?

Maybe something like:

/\*\(<insidecomment>\)*\*+/

where <insidecomment> is something like

   [^'*]\|\*+\([^/'*]\|'<afterquote>\)\|'<afterquote>

where <afterquote> is something like

   \([^'*]\|\*+[^/'*]\)*'

Tho this will still push a backtrack point for every character.
Maybe better would be something like

/\*[^'*]*\(<insidecomment>\)*\*+/

where <insidecomment> is something like

   \(\*+[^/'*]\|\**'<afterquote>\)[^'*]*

where <afterquote> is still something like

   \([^'*]\|\*+[^/'*]\)*'

so that we should only push a backtrace point when we see a * or a ' in
the comment.


        Stefan





  reply	other threads:[~2014-09-28 17:35 UTC|newest]

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2014-09-28  8:55 bug#18577: Regexp I-search: [(error Stack overflow in regexp matcher)] Alan Mackenzie
2014-09-28 10:56 ` Andreas Schwab
2014-09-28 12:37   ` Alan Mackenzie
2014-09-28 12:48     ` Andreas Schwab
2014-09-28 17:35       ` Stefan Monnier [this message]
2014-11-27  8:44         ` Tassilo Horn
2021-10-23  2:47         ` Stefan Kangas
2021-10-23  7:32           ` Eli Zaretskii
2021-10-23  8:30             ` Stefan Kangas
2021-10-23  8:39               ` Eli Zaretskii
2021-10-23  9:32                 ` Stefan Kangas
2021-10-23 11:27                   ` Eli Zaretskii
2021-10-24 22:08                     ` Stefan Kangas

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