Minecraft player usernames must follow certain restrictions when being set. They can contain following characters: abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789_

In order to prevent players with strange usernames from connecting (in offline mode server), mod checks username of a player.

If you want to check whether a certain username is valid, you can use regex, set in config (default value: ^[a-zA-Z0-9_]{3,16}$). Or just head to the fancy website debuggex.com