Hello,
I am looking for a proxy server that can do this.
If I set my firefox settings to that proxy, and go to google, then it should "proxy" to google normally. Just a normal proxy.
But I want there to be rules. I want to redirect anything that matches http://google.com/... to http://12.34.56.78/...
I was thinking about writing one but to be honest there has to be something to do this already built.
Fiddler should do what you want. If you type into the black box at the bottom "overridehost google.com 12.34.56.78" it should redirect any Google requests to the specified IP.
Not a proxy per se, but what about editing your local host file? If you are a Windows user just go to "C:\WINDOWS\system32\drivers\etc" and edit the "hosts" file accordingly. Eg. add a new line:
12.34.56.78 google.com
I know the use is limited, because you cannot make more complex redirects. However it might be sufficient for your problem. Good luck! (on linux/unix the file is often located in /etc/hosts).
If you want a better solution, it would be helpful to give us your OS.