rewrite problem

[sosogh]

HI list
I want to redirect
http://www.mysite.com/bbs/viewthread.php?tid=8123&extra=
to
http://www.mysite.com/forum.php?mod=viewthread&tid=8123
 
This is what I  place in the "server" block in the nginx.conf
rewrite  ^/bbs/viewthread.php?tid=([0-9]+).*$   /forum.php?mod=viewthread&tid=$1 last;
 
but when visit http://www.mysite.com/bbs/viewthread.php?tid=8123&extra=.
It shows 404 error.
 
Could anyone point me out what is wrong?
Thank you!
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