Return non-nil if text before point matches regular expression REGEXP.
Like `looking-at' except matches before point, and is slower.
LIMIT if non-nil speeds up the search by specifying a minimum
starting position, to avoid checking matches that would start
before LIMIT.
If GREEDY is non-nil, extend the match backwards as far as
possible, stopping when a single additional previous character
cannot be part of a match for REGEXP. When the match is
extended, its starting position is allowed to occur before
LIMIT.