Ok, it seems I'm finishing the core miniBB release, and got back to this question. I have updated the Registration script on our forums, so you may now safely test it here for mentioned bugs.
On our forums, usernames are set:
- to have a maximum length of 15 chars
- to contain only alphanumerical chars and '_'
- to not consist of the digits only
Regarding your suggestions, you should not be able to register usernames which:
- have repeatedly multiplied char, like in this topic's title (any kind of char, not just '_')
- start with not a Unicode char, incl. with '_', or any other specific char.
I have made the function universal, so it's possible to override in bb_plugins.php, specifying your very own set of rules and conditions applied to usernames. That's how it will work in the new release.
You have time to test it before I make it released :)