Replacing multiple elements with regexp

Hi,

I have a textedit object where the user can enter simple functions using x,y,z. Now I want to replace every x by in[0].p[0], every y by in[0].p[1] and every z by in[0].p[2].

So far I've only managed to replace 1 letter using: regexp x in[0].p[0]

Can someone help me with how to enter all three substitutions in the regexp object...I've been trying several tactics but I'm afraid my regexp knowledge is virtually non-existent.

grtz.
Brecht.