[vim/vim] Automatic regular expression engine selection matches incorrectly on long line (#867)

[fritzophrenic]

Using the following regular expression:

\%(|\u.*\)\@<=[^|\t]\+$

I get a match in this line with default settings, or when I force automatic selection with \%#=0:

        xxxxxxxxxxxx    xxxx xxxxxx xxxxxxx x xxxxxxxxx xx xxxxxx xxxxxx xxxxx xxxxxxx xx xxxx xxxxxxxx xxxx xxxxxxxxxxx xxx xxxxxxx xxxxxxxxx xx xxxxxx xx xxxxxxx xxxxxxxxxxxxxxxx xxxxxxxxx  xxx xxxxxxxx xxxxxxxxx xxxx xxx xxxx xxx xxx xxxxx xxxxxxxxxxxx xxxx xxxxxxxxx xxxxxxxxxxx xx xxxxx xxx xxxxxxxx xxxxxx xxx xxx xxxxxxxxx xxxxxxx x xxxxxxxxx xx xxxxxx xxxxxxx  xxxxxxxxxxxxxxxxxx xxxxxxx xxxxxxx xxx xxx xxxxxxxx xxxxxxx  xxxx xxx xxxxxx xxxxx xxxxx xx xxxxxx xxxxxxx xxx xxxxxxxxxxxx xxxx xxxxxxxxx xxxxxx xxxxxx xxxxx xxx xxxxxxx xxxxxxxxxxxxxxxx xxxxxxxxx  xxxxxxxxxx xxxx xx xxxxxxxx xxx xxxxxxxxxxx xxxxx

However, if I force the engine to either 1 or 2, there is no match, as expected.

Tested in 7.4.1862, with patch 1848 reverted. I will try again in a more recent Vim when I get time to compile it later today, but I took a quick look at the patch list and didn't see anything that appears relevant since then.

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[fritzophrenic]

Confirmed, this also affects 7.4.1940, both on 64-bit Windows 7, and on Solaris. Setting the 'regexpengine' to either 1 or 2 also causes the pattern to correctly not match.

[Christian Brabandt]

Am 2016-06-15 23:39, schrieb fritzophrenic:
> Confirmed, this also affects 7.4.1940, both on 64-bit Windows 7, and
> on Solaris. Setting the 'regexpengine' to either 1 or 2 also causes
> the pattern to correctly not match.
>

Can you please set verbose before searching?

I wonder if this happens, because the NFA engine stops because it
returns NFA_TOO_EXPENSIVE
and then the retry with the old engine is somehow confused? There should
be a message
about switching the engine, if verbose is set.

Best,
Christian

[fritzophrenic]

Can you please set verbose before searching?

I did "gvim -N -u NONE -i NONE" then ":set verbose=15". Searching for "\%(|\u.*)\@<=[^|\t]+$" in a long line with no uppercase characters still matches, and I get no messages at all.

I wonder if this happens, because the NFA engine stops because it returns NFA_TOO_EXPENSIVE and then the retry with the old engine is somehow confused?

I don't think so (and I guess I proved it with the verbose setting, unless I did that wrong), because I can force the new engine with \%#=2 or by setting 'regexpengine', and the pattern works as expected.

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[Christian Brabandt]

On Do, 16 Jun 2016, fritzophrenic wrote:

> > Can you please set verbose before searching?
>

> I did "gvim -N -u NONE -i NONE" then ":set verbose=15". Searching for "\%(|\u.*\)\@<=[^|\t]\+$" in a long line with no uppercase characters still matches, and I get no messages at all.

>
> > I wonder if this happens, because the NFA engine stops because it returns NFA_TOO_EXPENSIVE and then the retry with the old engine is somehow confused?
>
> I don't think so (and I guess I proved it with the verbose setting, unless I did that wrong), because I can force the new engine with \%#=2 or by setting 'regexpengine', and the pattern works as expected.

Okay, I checked the problem. My suspicion was correct, that patch
7.4.494 causes this problem, because after recursive_regmatch() returns
NFA_TOO_EXPENSIVE nfa_listid will be restored from save_nfa_listid and
then the exit condition in nfa_regmatch does not match anymore.

Attached is a patch, including a test.
(attachment won't be visible in github, see the thread
https://groups.google.com/d/msg/vim_dev/BE0ISqcD3aI/ZGeOMrFQJgAJ for the
patch)

Best,
Christian
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[Bram Moolenaar]

Closed #867 via 6747fab.

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[Bram Moolenaar]

Glad you could locate and fix it!

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