Iterates over TARGET-STRING and tries to match REGEX as often as
possible evaluating BODY with MATCH-START and MATCH-END bound to the
start/end positions of each match in turn. After the last match,
returns RESULT-FORM if provided or NIL otherwise. An implicit block
named NIL surrounds DO-MATCHES; RETURN may be used to terminate the
loop immediately. If REGEX matches an empty string the scan is
continued one position behind this match. BODY may start with
declarations.