When registering a user after "limit username chars" is set to "Alphanumeric and spacers" an error is reported when registering usernames similar to:-

PEACHES4U
peaches4u
[peachesu]
Peac[h]esu

The reported error is:-
"Username contains forbidden characters"

However:-
The - sign(minus/negative) works within a username and before and after the username.

The + sign(plus/positive) works within a username and before and after the username.

The _ sign(underscore) works within a username and before and after the username.