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