What are the restrictions on FAF usernames?

[Rotoscope]

Are spaces not allowed? Special characters? I was trying to change my username but ended up getting "username not valid" not matter what I tried. Out frustration I tried "testwhynotworking". Which worked. So I guess I'm that for the next 30 days . Maybe it's a better name for a total noob anyway. But what are the rules? The username change page doesn't tell you.

I was going for "Entropic Uncertainty" ("Entropic_Uncertainty", "EntropicUncertainty", "entropicuncertainty" etc. It's an important principle on quantum mechanics and I figured it fit the whole quantum theme the game has.)

[speed2]

I think its 16 characters max