Limit text entry to specific characters
I would like to create a text filed where only certain characters are allowed. I work in a genetics lab and when working with DNA sequences, it would be nice to make sure that someone can only enter the letters A, T, C or G into the field. I have tried value lists, but that did not help (maybe I am using this incorrectly). I'm sure this is simple, but I am just learning to use FM and it's related scripts.