I have an expression of the form:

```
expression = A1*x + A2*x^2 + ...

```

and I would like to replace `A1` by `F1[y]`, `A2` by `F2[y]`, etc. 

Regular expressions would seem to be the thing to use, but since the above is not a string, I'm not sure how to do this. Would like the solution to be something simple like

```
expression /. A* :> F*[y]

```

but of course this doesn't work.

How do I accomplish the above?