SQL & PL/SQL :: Inherit From Varchar 2 / Number

I see you can build an address object consisting of street name and number, city and so on, and use this in CREATE TABLE, so all tables use the same fields for addresses. And moreover you can add check functions to prevent street numbers to be negative for instance in the constructor. You can even use inheritance. I like that.

However, all teaching on this subject I found on the Internet starts with structs (points consisting of x and y, persons consisting of name and address, etc.). Then they go to inheritance and show how to build an employee who is just a person plus an employee number. They never start with simple types.

What I would like to do would be rather to start with basic types such as number or varchar2 and inherit from them. An EAN is a VARCHAR2 holding only digits where the last digit is a check sum that must be matched. An item number may be a VARCHAR2 matching 'NNN-AAAA'. A simple string may be a VARCHAR2 where no functional characters such as carriage return, etc. occur.

I would then write my create table statement thus:

CREATE TABLE item (itemno T_ITEMNO, ean T_EAN, itemname T_SIMPLESTRING); and inserts like INSERT INTO item values('123--BCD', '1234567890abc', 'toy') would fail with exceptions like 'item numbers must be formatted NNN-AAAA' or '1234567890abc is not a valid EAN'.

However this seems not possible:

create or replace type T_EAN under varchar2
(
constructor function T_EAN(p_string in varchar2) return self as result
);

fails with "PLS-00580: supertype must be an object type".

Rather than creating completely new types, I would start with extending existing types, but this seems not possible. Is that really impossible or am I using the wrong approach?