Please, what is the rule regarding characters in usernames?
Strict Latin alpha? Alphanumeric? Accented characters? Etc.
Which punctuation is allowed? Underscore(_)? Period(.)? hyphen(-)? Etc.
I am trying to understand the rule, for programming, and for user documentation. (Experimenting by creating many usernames to discover the exceptions, would not be a reliable way to discover the rule, in my opinion.)