Plugin does not work for me

Hello! Before I begin, I would like to thank you for spending your time to develop this neat plugin ??

I’ve installed this plugin and modified it so that it accepts the German “Umlaute” (??ü):

preg_replace (‘|[^a-z\p{Arabic}\p{Cyrillic}0-9 _.\-@??ü??ü?]|iu’, ”, $username);

Unfortunately, it does not seem to work at all. Whenever I try to register on the following domain:

https://ferienspass.dreigestalten.ch/kursverwaltung/?action=register

I still get an error message stating that my username contains characters that are not allowed.

It would be very much appreciated if you could look into this, as the acceptance of “Umlaute” in usernames is critical to the functionality of my site.

Thank you very much in advance! ??

The page I need help with: [log in to see the link]