It's homework. Look at his other questions.
But he doesn't really need a regular expression for this, if his actual input is as limited as he says.
var s1 = "q,r,t";
var s2 = s1.replace(",","/"); // only replaces first comma
var s3 = s2.replace(",","/"); // then the second one
Now, Jon: Figure out how to use that information to make sure you have replaced *ALL* commas, no matter how many there are.