mysql udf alphanum function

<sql>
CREATE   FUNCTION `alphanum`( str CHAR(255) ) RETURNS char(255) CHARSET utf8
BEGIN 
  DECLARE i, len SMALLINT DEFAULT 1; 
  DECLARE ret CHAR(255) DEFAULT ''; 
  DECLARE c CHAR(1); 
  SET len = CHAR_LENGTH( str ); 
  REPEAT 
    BEGIN 
      SET c = MID( str, i, 1 ); 
      #(c REGEXP '[[:alnum:]]') and
      IF  ( (c BETWEEN 'a' AND 'z') OR (c BETWEEN 'A' AND 'Z') OR (c BETWEEN '0' AND '9') OR (c = '-' or c='_')  ) THEN
        SET ret=CONCAT(ret,c); 
      END IF; 
      SET i = i + 1; 
    END; 
  UNTIL i > len END REPEAT; 
  RETURN ret; 
END
</sql>