Can I use upstreaming based on a URI regex?

Hello,

I have an issue with our nginx conf.

How can I upstream to different machine clusters based on a URI.

I have tried the following within the server block,

if ($uri ~ ^/.*oo=oo*/) {upstream //bla}
if ($uri ~ ^/.*/) {upstream //blabla} (all the rest)

what am i doing wrong?

Can I use if for upstreaming?

Thanks,

Meir