Executes BODY with the variables in VAR-LIST bound to the
corresponding register groups after TARGET-STRING has been matched
against REGEX, i.e. each variable is either bound to a string or to
NIL. If there is no match, BODY is not executed. For each element of
VAR-LIST which is NIL there's no binding to the corresponding register
group. The number of variables in VAR-LIST must not be greater than
the number of register groups. If SHAREDP is true, the substrings may
share structure with TARGET-STRING.