Regular Expression Single Occurence

I want to have a regular expression that checks if the letters A-F and a-f occur only once. How do I do this? I want to check for there only being one letter in the user's input, and that letter is either from A-F or a-f.

Search for a single occurrence of a letter in the range, surrounded only by characters outside the range.

^[^A-Fa-f]*[A-Fa-f][^A-Fa-f]*$