If you need the usernames for the open internet, just set up a simple rule (e.g. /^\w{2,12}$/) and let the user choose what he wants.

But if you need usernames for a company, I'd use a rather strict rule. What this role looks like depends on the system, e.g.

• length between 8 and 20 chars
• first 4 chars are the department
• next chars are the surname in international spelling [A-Za-z], with a maximum of 12 chars
• first char of the givenname in international spelling

If this is not unique, add more letters of the givenname, and if still not unique, add something like [a-z]

So my username for a company could look like 1234fabianim or 1234fabianima or 1234fabianimartinx (if there are a lot of persons with the name martin fabiani in the same department)

The advantage of this rule is that these usernames are rather easy to remember