There is an expression

`(Sqrt[\[Pi]] (a b^2 d^3 + a (b^2 d^3 + 2 a (b + b d^3))) r[1, 1] r[1, 
   4])/(2 c^2 (b + b d^3)^(3/2)) - (
 Sqrt[\[Pi]] (a b^2 d^4 + a (b^2 d^4 + 2 c (b d + b d^3))) r[5, 2] r[
   1, 4])/(2 a^2 (b d + b c^3)^(3/2))`

where `a`, `b`, `c`, `d`, `r[1, 1]`, `r[1, 4]`, `r[5, 2]`, `r[1, 4]` are variables.

I would like to write this expression as a function `f` where `a`, `b`, `c`, `d`, `r[1, 1]`, `r[1, 4]`, `r[5, 2]`, `r[1, 4]` are variables. It can be indicated that `a`, `b`, `c` and `d` are variables as follows `f[a_,b_,c_,d_]:= the expression`, but how can I specify that `r[1, 1]`, `r[1, 4]`, `r[5, 2]` and `r[1, 4]` are also variables `f[a_,b_,c_,d_,...]:= the expression`?