using oracle functions in constraints

Hullo Oracle Gurus,
This is should be a fairly simple question for experienced Oracle users. So *do* help me! :)
In a table there is a char/varchar field. It can accept both upper and lower case letters. *But* I need to specify a case-insensitive unique constraint, ie if 'Oracle' is entered in one tuple of the table , 'ORACLE' should not be accepted in another tuple. How do I go about it. I think it is a very basic requirement of a database. I would like to define it in the table itself instead of checking in a trigger/ procedure. Imp : the field *should* be allowed to have both cases, so converting to all upper at the application/client end does not help. Maybe an elegant solution is there in Oracle, I just haven't found it. I tried - create table x (x char(15), constraint x_unique unique (upper(x)) ); but it did not work.
A million TIA,
Vani

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