How to anchor matchlist after searchpos?

Question

I'm trying to find words before the cursor using searchpos, then use matchlist to get the captures. I can't find a reliable way to anchor the pattern for matchlist. For example, if the cursor is positioned on [cursor]

" 3456789012345
" !!foobar !!foobaz [cursor]  !!foobat

let pat = '\%(!!\)\@<=fooba\(.\)'
let [l, c] = searchpos(pat, 'bWn')    " returns [2, 14]

echo 'pat1'
echo l c matchlist(getline(l), pat)                     | " bad
echo l c matchlist(getline(l), pat, c-3)                | " ok
echo l c matchlist(getline(l), '\%'.c.'c\%('.pat.'\)')  | " ok

searchpos finds the column 14, since the look-behind is not included in the match. The first matchlist finds the wrong fooba.. Thus, I need to anchor the pattern using the column number. The second alternative is not an option because it would require me to know how many characters were zero-width'd. The third one works great, until...

let pat = '!!\zsfooba\(.\)'
let [l, c] = searchpos(pat, 'bWn')    " also returns [2, 14]

echo 'pat2'
echo l c matchlist(getline(l), pat)                     | " bad
echo l c matchlist(getline(l), pat, c-3)                | " ok
echo l c matchlist(getline(l), '\%'.c.'c\%('.pat.'\)')  | " bad

Here, prepending the column number doesn't work because although \zs is zero-width, the stuff before it is not.

Note:

• I need a general-purpose solution since this is for a plugin and pat is supplied by the user.
• I can't simply replace \zs with \%14c\zs, because there may be alternations \| in the pattern. For example foo\%14c\zsbar\|foobaz will allow foobaz in any position.
• This is not actually peculiar to searchpos- the same would be seen with e.g., the character index from match.
• Negative look-behinds in pat are possible.
• A perfect answer would include the possibility of \ze.

Answer 1

The problem seems to be that is that searchpos is searching backwards (from the position of the cursor) and matchlist is searching forwards (from the start of the line).

How about altering the way we call matchlist to search backwards too, via the time honoured technique of brute force?

let line1 = getline(l)
let line2 = getline(l)[c:]
let line1 = line1[:c-1]

" We want to ignore matches found *after* the position returned by 
" searchpos()
let boo = matchlist(line2, pat)

while len(line1) > 0
  let line2 = line1[-1:] . line2
  let line1 = line1[:-2]
  let yay = matchlist(line2, pat)
  if len(yay) > 0 && yay != boo
    echo l c yay
    break
  endif
endwhile

It's ugly and inefficient, and I have a feeling it might be possible to construct a complicated regular expression that breaks it, but it works for the test cases given.