Python(Tkinter) - (if/else) extra characters?

So I'm doing my homework but I can't seem to figure out how to add "extra" characters to regular statement.

screenshot

What I mean is how can I make IF statement that MUST include letters and may also include "-" or/and space. I want to make fname only letters and "-" or/and space. (This is for a name field)

I'm sorry if this has been asked before but I'm not sure how to search for this exact problem.