replacement - Replacing of variables but not subscripts

It's an extremely brief question. I want to replace all variables in an expression to another ones, say $x \rightarrow y$. But the command

x + Subscript[a, x] /. x -> y

replaces also the subscript. How can I do replacements while leaving the Subscript object unchanged?

Answer

When you want to do replacements with a pattern that shouldn't be changed, I like to use multiple replacement rules (as in this answer):

ReplaceAll[
    x + Subscript[a, x],

    {
        s_Subscript :> s, (* pattern to avoid *)
        x -> y
    }
]

y + Subscript[a, x]