Linux username invalid and valid characters

Hey, I'm wondering if in redhat Linux ES there are any invalid char's in usernames that you create. I have tried searching previous posts on here, and the linux website, coudln't find anything about invalid chars in usernames. Just wondering if any of you know about any limmitations on what you can use in a username..?

I met with some other distribution this problem (e.g. only letters and no numbers) but if you want to find out what are the invalid character in your distrbution and why, try to mail a RedHad forum or to RedHat support itself.