Simple string handling

Doesnt seem to be so simple

Trying to take the string value of a button passed to a function and add “_mc” on the end! It just keeps returning undefined? Any ideas?

function openBox(){
var newName = this.split("_level0.");
trace(newName);
var newName2 = newName.split(“Btn”);
trace(newName2);
var newName3 = newName2+"_mc";
//_root[newString]._visible = true;
trace(newName3);
}

It’s not even working when I switch to substr