SQL & PL/SQL :: Enforce Comma Separated Column Values?

I have a requirement wherein I need to enforce certain column to have only comma as a delimiter, where multiple values exists.

Example:

Create table:

create table test_oz ( slot number, server_name varchar2(50), used_by varchar2(50),
constraint test_oz_pk primary key (slot, server_name) );

Insert Test Data:

insert into test_oz values ( 1,'SRV1','SAMMY' );
insert into test_oz values ( 2,'SRV1','SAMMY,TOM' );
commit;

Data:

SQL> select * from test_oz;
SLOT SERVER_NAM USED_BY
---------- ---------- ----------
1 SRV1 SAMMY
2 SRV1 SAMMY,TOM

From above, the USED_BY column data need to be only comma separated. Is there a way to enforce that?

When user tries to insert data using any other delimiter, it should fail.

I was trying to see if a CHECK CONSTRAINT could be of use, but could not find it to work.