Help needed with simple rewrite rule

[RoastedPeanut]

Ok, so, I've been banging my head against the wall for an hour or so, and I feel an idiot..:

I've just moved a vbulletin 4 forum to nginx, and for some arcane reason, 
these URLs work: http://mysite.com/forumdisplay.php?f=25
but these do not
http://mysite.com/forumdisplay.php?25-Forum-Name
(the second URL is a built-in "friendly url" function in vb4)

So, since I couldn't find anything in the logs, I decided I'm going to simply rewrite those non-functioning URLs to the functioning ones..

In short, I want to rewrite this:

http://mysite.com/forumdisplay.php?25-Forum-Name

to this:

http://mysite.com/forumdisplay.php?f=25

My attempt is this:
rewrite ^/forumdisplay\.php\?([0-9]+) /forumdisplay.php?f=$1;

Apparently, no matter how I escape it, inserting the "?" after forumdisplay.php causes the regex to not match at all.

Could any of you show me why it isn't working, please?

Thanks

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,76061,76061#msg-76061