This is the regex that wasteof uses to check usernames:
^[a-z0-9_\\-]{1,20}$
for people who don’t understand regex:
matches any of a through z, 0 through 9, _ and -, and is at least 1 character but not more than 20
for people who don’t understand regex:
matches any of a through z, 0 through 9, _ and -, and is at least 1 character but not more than 20