GNU bug report logs - #58726
29.0.50; Bug in regexp matching with shy groups

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Package: emacs;

Reported by: Michael Heerdegen <michael_heerdegen <at> web.de>

Date: Sun, 23 Oct 2022 01:42:02 UTC

Severity: normal

Found in version 29.0.50

Done: Mattias Engdegård <mattiase <at> acm.org>

Bug is archived. No further changes may be made.

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From: Mattias Engdegård <mattiase <at> acm.org>
To: Michael Heerdegen <michael_heerdegen <at> web.de>
Cc: 58726 <at> debbugs.gnu.org
Subject: bug#58726: 29.0.50; Bug in regexp matching with shy groups
Date: Sun, 23 Oct 2022 15:50:41 +0200

[Message part 1 (text/plain, inline)]
Michael, thank you for finding this amusing bug!

>   (string-match-p "\\`\\(?:ab\\)*\\'" "a") ==> 0

With a bit of help from the regexp-disasm package, we see that this compiles to

    0  begbuf
    1  on-failure-jump-smart to 11
    4  exact "ab"
    8  jump to 1
   11  endbuf
   12  succeed

where the on-failure-jump-smart op turns into on-failure-keep-string-jump the first time it's executed.

This gives us a clue about what is wrong: when there is a failure inside an 'exact' string match, the target pointer should be reset to the start of that string ("ab" here) before jumping to the failure location.

Reading the source it becomes clear that this is done correctly when there is a mismatch, but not if the target string ends prematurely because PREFETCH() has no idea that it should reset the target pointer! Easy enough to fix.

Please try the attached patch. (The patch takes care of counted repetitions for good measure although I wasn't able to provoke a failure directly.)


[0001-Fix-regexp-matching-with-atomic-strings-and-optimise.patch (application/octet-stream, attachment)]
This bug report was last modified 2 years and 267 days ago.

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