Tárolt eljárás PLSQL

CREATE OR REPLACE FUNCTION SF_CHECK_RENDSZAM(
    p_rendszam autok.rendszam%TYPE
) RETURN VARCHAR2
AS
BEGIN
    IF(LENGTH(p_rendszam) <> 7) THEN
        RETURN 'false';
    END IF;

    FOR i IN 1..7 LOOP
        IF(i < 4) THEN
            IF(NOT(ASCII(SUBSTR(p_rendszam,i,1)) BETWEEN ASCII('A') AND ASCII('Z'))) THEN
                RETURN 'false';
            END IF;
        END IF;

        IF(i = 4 AND SUBSTR(p_rendszam,i,1) <> '-') THEN
            RETURN 'false';
        END IF;

        IF(i > 4) THEN
            IF(NOT(ASCII(SUBSTR(p_rendszam,i,1)) BETWEEN ASCII('0') AND ASCII('9'))) THEN
                RETURN 'false';
            END IF;
        END IF;
    END LOOP;
    RETURN 'true';
END SF_CHECK_RENDSZAM;