Let say I have any expression

ex= f[h[r[x y]]] g[x z]

and I want to replace the x with w, but only for the x within the head f. Is there any simple way to do that?

Another, if I want to substitute an expression, say x, but only if it it is inside a pattern, say complicateExpression x, how can I do it? Of course, I can use the rule

complicateExpression x -> complicateExpression w

But this often means copy and paste complicateExpression

  • 2
    $\begingroup$ For the replacement of x inside f you could perhaps use a rule like {f[args__]:>f@@({args}/.{x->w})} $\endgroup$
    – Hausdorff
    Jul 9 '20 at 9:48
ex /. a_f :> (a /. x -> w)
 f[h[r[w y]]] g[x z]
ex2 = f[h[r[complicatedexpression  x ]]] g[x z];

ex2 /. ( p : complicatedexpression) x :> p w
f[h[r[complicatedexpression w]]] g[x z]
  • $\begingroup$ What if f is Inactive[f]? How to make that work? $\endgroup$
    – ablmf
    Jul 9 '20 at 12:19
  • 1
    $\begingroup$ @ablmf, something like ex4 = Inactivate[f[h[r[x y]]] g[x z], f]; ex4 /. a : (Inactive[f]@__) :> (a /. x -> w) or ex3 = Inactive[f][h[r[x y]]] g[x z]; ex3 /. a : (Inactive[f]@__) :> (a /. x -> w)? $\endgroup$
    – kglr
    Jul 9 '20 at 12:35

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