From IceBB Wiki

The username is a name that you identify yourself with and is chosen when registering. You use it when logging in and it is what other people will know you by. This can only be changed by an administrator and it is likely that they will refuse to change it, so choose carefully when. A username has to be unique.

A username may contain the following characters:

• a-z
• A-Z
• 0-9
• _ - .