hehe i know that it is not a realible fix, especially that is compulsory in all cases
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Hopefully MyBB 1.2 will come with an automated wrap for usernames containing ], so that users will not have to enter them manually. But for now there is no way but to do that.
Making it an automated process isn't a little code that you change, it will need somehow a total modification for the the quote function.
Regards