[dimitrisvb]

Originally Posted by ReCom

You would need to use regular expression (regex) for that:

1. Go to AdminCP > vBulletin Options > User Registration Options.

2. Look for setting "Username Regular Expression".

3. Now let say you want to limit the usernames to those containing a, b, c, d or e only. You would need to write in the setting:

^[abcde]$

4. So to allow greek characters only you need to replace 'abcde' with a string of all greek characters. Quite a long string I'd imagine

I have tried it but it doesn't seem to work!
I have entered all greek characters (upper and lower case, with accents) within this notation: ^[ ]$
in the "Username Regular Expression" field. Then I tried to create a user but neither greek nor latin characters would be accepted by the system! When I emptied the field, registration would work again!

EDIT:
you can add plugin that hooks to register_addmember_process:

1. Check if $vbulletin->GPC['username'] is valid (greek characters only) using any PHP algorithm you want.
2. If it's invalid, append array $userdata->errors with your error message.

I'll see if can make anything out of these notes!
thanks!