Restrict data entry in a cell to a capital X

Can someone help me write a formula that will restrict the entry into one cell to only a capital "X" in the Data Validation Custom window?

The restriction will be ONE character and UPPER case.

=exact

Also, can the formula be written (without VBA) that will change a lower case x to an UPPER case X?

thank you!

Follow-up question, what if I want to add an option for a lower case "x"

=EXACT(A1,"X") and add:

or"x"

Thank you again!