How to force input to only allow Alpha Letters?

Short ONELINER:

<input onkeydown="return /[a-z]/i.test(event.key)" >

For all unicode letters try this regexp: /\p{L}/u (but ... this) - and here is working example :)

The property event.key gave me an undefined value. Instead, I used event.keyCode:

function alphaOnly(event) {
  var key = event.keyCode;
  return ((key >= 65 && key <= 90) || key == 8);
};

Note that the value of 8 is for the backspace key.