masking function

create or replace FUNCTION stfn(varchar_input IN VARCHAR2, shifting_index IN NUMBER DEFAULT NULL)
RETURN VARCHAR2
IS
  varchar_output VARCHAR2(255);
  inchar_len NUMBER;
  temp_char CHAR;
  v_shifting_index NUMBER;

BEGIN
  IF varchar_input IS NULL
    THEN RETURN NULL;
  ELSE
    IF shifting_index IS NULL
    THEN v_shifting_index := 10;
    ELSE v_shifting_index :=shifting_index;
    END IF;
    varchar_output := '';
    inchar_len := Length(varchar_input);

    FOR Lcntr IN 1..inchar_len
    LOOP
      temp_char := SubStr(varchar_input, Lcntr, 1);
      IF (temp_char NOT IN ('(',')'))  THEN
        temp_char := Chr(Ascii(temp_char)+v_shifting_index);
      END IF;
      varchar_output := varchar_output || temp_char;
    END LOOP;
  END IF;

  RETURN varchar_output;

END stfn;