how do i limit a textfield to specific alpha characters of one entry but in a textfield that needs to have a limit of two spaces in order to allow for a two-letter combination?

I want to force the user to limit the entries in a textfield to use only four single alphabet entries (E, C, W, or A), in addition to a two-letter alphabet entry "na". I have a textfield with a limit of two characters. Right now, it is possible for the user to incorrectly enter two of the same alpha entries. How do i restrict the user to entering just one entry, while at the same time allowing the user to enter the "na" entry?