So far so good, does exactly what I want which is still asking a desired username for the forum itself. I'm using a regex expression [a-zA-Z0-9_] to validate the username (which is the same requirements for a minecraft username).
However, people still somehow manage to register with their real names, despite of the regex. I'm talking stuff like Joe Smith or Oscar Johanssen. Obviously these names shouldn't be expected with the current regex. So how does this actually happen?