Hi F,
You can use a '-' and even a '&' but this last one will not show in BP. Also some country specific alphabetic characters seem to be allowed, in my case the Swedish language has three country specific characters that work fine. So some are ok, others are forbidden as Pablo pointed out, and some may be not forbidden but do not show in BP, which I think is a good reason to avoid them.
My advice: stick to the English alphabet and the '-' character, and you should be ok.
Happy coding!
---------------
Paul
Sweden
Happy coding!
Paul, Sweden
(By all means, do not mark this as the best answer!)