Function: isearch-fallback

Return point to previous successful match to allow regexp liberalization.

Respects C-s and C-r by stopping at `isearch-barrier' as needed.

Do nothing if a backslash is escaping the liberalizing character.
If WANT-BACKSLASH is non-nil, invert this behavior (for \} and \|).

Do nothing if regexp has recently been invalid unless optional

If optional TO-BARRIER non-nil, ignore previous matches and go exactly to the barrier.