BUG: NFA-regexp discrepancy with \@<= and \1 backreference

[Ingo Karkat]

Hello Vim developers,

there's yet another discrepancy between the regular expression engines:

for n in [1,2] | echo match('",abc" in "abc,abc"', '\%#=' . n .
'\1\@<=,\([a-z]\+\)') | endfor

I actually found this while checking out an example from the help
(trying to understand why a regexp I use in a plugin didn't work):

,----[ :help /\@<= ]----
| The part of the pattern after "\@<=" and "\@<!" are checked for a
| match first, thus things like "\1" don't work to reference \(\) inside
| the preceding atom. It does work the other way around:
| Example matches ~
| \1\@<=,\([a-z]\+\) ",abc" in "abc,abc"
`----

The NFA engine somehow doesn't properly handle the backreference
assertion and finds a match where it shouldn't.

To reproduce, use above scriptlet or the identical attached script:

vim -N -u NONE -S bad-re5.vim
14
1

This is with a huge build of Vim 7.4.383, running in an Ubuntu 13.10 x64
VM.

-- regards, ingo

[Bram Moolenaar]

Thanks, I'll add it to the todo list.

Keep in mind that NFA does not actually support backreferences, that it
works at all in Vim is a bit of a miracle :-).

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