To get the digits from a string

CREATE OR REPLACE FUNCTION get_digits (i_str IN varchar2)
RETURN varchar2
IS
temp_str varchar2 (100);
retval varchar2 (100) := '';
temp_chr varchar2 (1);
BEGIN
temp_str := i_str;
WHILE LENGTH (temp_str) > 0
LOOP
temp_chr := SUBSTR (temp_str, 1, 1);
IF ASCII (temp_chr) BETWEEN 48 AND 57
THEN
retval := retval temp_chr;
END IF;
temp_str := SUBSTR (temp_str, 2);
END LOOP;
RETURN retval;
END;