Accepts a regular expression - either as a
parse-tree or as a string - and returns a scan closure which will scan
strings for this regular expression and a list mapping registers to
their names (NIL stands for unnamed ones). The “mode” keyboard
arguments are equivalent to the imsx modifiers in Perl. If DESTRUCTIVE
is not NIL the function is allowed to destructively modify its first
argument (but only if it's a parse tree).