https://github.com/Elgg/community

RvR thank you very much for the info..

Thanks but about username only contain the a to z , A to Z, 0 to 9 ???

http://learn.elgg.org/en/2.0/guides/hooks-list.html#user-hooks

Register a function to the 'registeruser:validate:username', 'all' plugin hook. Within the function check $params['username'] if the username contains characters (or whole specific usernames) you won't allow. Return true/false depending on the username is valid or not according to your requirements.

You can also take a look at the function validate_username() in vendor/elgg/elgg/engine/lib/users.php to see how Elgg validates a username