Martina,

You would need to "protect" the function calls from the replacement of x. I would do a temporary replacement by removing x from the function calls and return it back after setting x to zero. For example:

In[9]:= exp2 = f[x, y, z] + x^2*g[x, y, z] + x*g[x, y, z]

Out[9]= f[x, y, z] + x g[x, y, z] + x^2 g[x, y, z]

In[6]:= newexp = 
 exp2 /. {f[x, a_, b_] -> fx[a, b], g[x, a_, b_] -> gx[a, b]}

Out[6]= fx[y, z] + x gx[y, z] + x^2 gx[y, z]

In[8]:= newexp /. x -> 0 /. {fx[a_, b_] -> f[x, a, b], 
  gx[a_, b_] -> g[x, a, b]}

Out[8]= f[x, y, z]

Regards,

Neil

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Hi Martina,

Do you mean something like this

ClearAll[x, y, z, f, g]

exp = f[x, y, z] + x*g[x, y, z]
exp /. Coefficient[exp, x] -> 0
(* f[x, y, z] *)

exp2 = f[x, y, z] + x^2*g[x, y, z]
exp2 /. Coefficient[exp2, x] -> 0
(* f[x, y, z] + x^2 g[x, y, z] *)