Not normalise, just set the rules you want to apply. Internationalised domain names suffer this issue, so cribbing their fix probably would work for you - pick

Not normalise, just set the rules you want to apply. Internationalised domain names suffer this issue, so cribbing their fix probably would work for you - pick a script, restrict allowable characters to it. You probably never want to allow unrestricted use of any character set for a username, even ASCII - otherwise I could take the username 'bjt2n3904 '.