How to restrict user to only using letters in typing their name in Processing?

I have only taken one coding class so I am still pretty new. The title is pretty much it. The user types their name at the beginning of my game using

 if (scr.showingTitleScreen()==true && keyCode!=ENTER) {
    name+=key;
  }

How do I make it so that they can only type letters and no other characters? Thanks!