Filtering inputs with onKeyPress

I want to create an input which only allows letters a-z. I thought having the key press return true/false would prevent characters from being input. Or am I thinking of a JQuery library?
checkKey: function(inSender, inEvent) {
  var keyCode = inEvent.keyCode;
  // this should only allow a through z (I think)
  if (keyCode >= 65 && keyCode <= 90) {
        return true;
  } else {
        return false;
Here's a fiddle


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