re.c:1687
static VALUE
rb_reg_s_last_match(argc, argv)
    int argc;
    VALUE *argv;
{
    VALUE nth;

    if (rb_scan_args(argc, argv, "01", &nth) == 1) {
	return rb_reg_nth_match(NUM2INT(nth), rb_backref_get());
    }
    return match_getter();
}
