Username can contain letters of the Latin alphabet (A-Z), numbers (0-9), sign underscores(-), hyphens (_) and dots (.). The username can be 1 to 60 characters long. Username, is part of the full xmpp address 'firstname.lastname@example.org'
Several dozen legacy usernames can contain any character. These usernames may not support the new control panel features, but the old features are still available in XMPP clients