Font-lock should ignore missing submatches

[Jorgen Schaefer <forcer@forcix.cx>]

Hello!
When using font-lock-mode to highlight complex expressions, the
font locking engine crashes when one of the subexpressions
specified was not matched.

Consider:

  (set (make-local-variable 'font-lock-keywords)
       '(("\\(?:\\(foo\\) *\\)?\\(bar\\)"
         (1 font-lock-comment-face)
         (2 font-lock-string-face))))

I.e. "possibly match the foo that is followed by whitespace, then
definitively match a bar."

This will (correctly) highlight these expressions:

  foobar
  foo bar
  etc.

But font-lock will error out if the first subexpression does not
match anything, even though the regular expression matches:

  oobar
  oo bar
  bar
  etc.

The code will give an appropriate error:

  Error during redisplay: (error No match 1 in highlight (1
  font-lock-comment-face))

Is there any chance to make font-lock just ignore unmatched
subexpressions?

Tested in:
- GNU Emacs 22.1.1
- GNU Emacs from CVS, approx. two weeks old

Sorry, no current CVS version here :-(

Regards,
        -- Jorgen