Suppose I have an expression of the form $$exp=a\,b\,c\,d\,+\,e\,f\,g\,h$$ where all the letters represent real numbers, and I want to replace $$a\,c$$ in the first term by $$w\,x$$ and $$f\,h$$ in the second term by $$y\,z$$, so Mathematica will actually evaluate $$w\,x\,b\,d\,+\,e\,g\,y\,z$$. How do I do that?