rpartition

/*
 *  call-seq:
 *     str.rpartition(sep)            => [head, sep, tail]
 *  
 *  Searches <i>sep</i> in the string from the end of the string, and
 *  returns the part before it, the <i>sep</i>, and the part after it.
 *  If <i>sep</i> is not found, returns two empty strings and
 *  <i>str</i>.
 *     
 *     "hello".rpartition("l")         #=> ["hel", "l", "o"]
 *     "hello".rpartition("x")         #=> ["", "", "hello"]
 */

static VALUE
rb_str_rpartition(str, sep)
    VALUE str;
    VALUE sep;
{
    long pos = RSTRING(str)->len;

    if (TYPE(sep) != T_REGEXP) {
        VALUE tmp;

        tmp = rb_check_string_type(sep);
        if (NIL_P(tmp)) {
            rb_raise(rb_eTypeError, "type mismatch: %s given",
                     rb_obj_classname(sep));
        }
        sep = get_arg_pat(tmp);
    }
    pos = rb_reg_search(sep, str, pos, 1);
    if (pos < 0) {
        return rb_ary_new3(3, rb_str_new(0,0),rb_str_new(0,0), str);
    }
    sep = rb_reg_nth_match(0, rb_backref_get());
    return rb_ary_new3(3, rb_str_substr(str, 0, pos),
                          sep,
                          rb_str_substr(str, pos+RSTRING(sep)->len,
                                             RSTRING(str)->len-pos-RSTRING(sep)->len));
}