Why don't these multiline regular expressions using a positive lookbehind work?

Question

EDIT: Don't waste time reading this question unless you're interested in a good example of why you should always check the :help first: a severe case of PEBKAC.

Consider the text:

foo
.
.
barbaz

Enter the following search:

/\(foo\_.\{-}bar\)baz
 \(             \)     # a group, containing
   foo                 #   'foo', followed by
         \{-}          #   as few as possible instances of
      \_.              #   anything, including newlines, followed by
             bar       #   'bar'
                  baz  # followed by 'baz'

It matches!

Now let's insert a positive lookbehind, so we only match the baz:

/\(foo\_.\{-}bar\)\@<=baz
                  \@<=     # positive lookbehind

E486: Pattern not found: \(foo\_.\{-}bar\)\@<=baz

What's going on?

N.B. To check I'm using the zero-width @<= correctly, I tried removing the multiline part of the regex:

/\(bar\)\@<=baz

This matches the baz, as expected.

Note also that I tried manually specifying both of Vim's regular expression engines, which didn't help.

Finally, note that using \zs in place of \@<= works here, but not in the context in which I actually want to use the expression.

Here's another, arguably even simpler, but even more confusing example:

foo
foo
barbaz

This matches:

/\(foo\nbar\)baz

This matches:

/\(foo\nbar\)\@<=baz

This doesn't match:

/\(foo\nfoo\nbar\)\@<=baz

Am I doing something wrong, is this a documented limitation of Vim's regular expressions, or is it just a bug?

Answer 1

Why spend 2 minutes reading :help /\@<= when you could spend 20 writing an exhaustive question for vi.se?

Theoretically these matches could start anywhere before this position. But to limit the time needed, only the line where what follows matches is searched, and one line before that (if there is one). This should be sufficient to match most things and not be too slow.

(emphasis mine)

Probably I should try actually reading the documentation before asking if something is documented.

As always, je suis le idiot. :endless-facepalm-emoji: