Input validation in the keydown event

If you're checking a printable key, which is exactly what you seem to be doing, you should use the keypress event instead, since that's the only place you're going to be able to get reliable information about the character the keypress represents. You can't detect numeric keypresses reliably in the keydown event. Also, it's a bad idea to suppress arrow keys and delete/backspace keys. What do you gain from doing that?

There's also some errors: in Firefox, you'll need to get the Event object from the parameter passed into the event handler function, and if you're using a DOM0 event handler function rather than addEventListener() or attachEvent(), you should use return false; to suppress default behaviour. Here's my recommended code:

var input = document.getElementById("your_input_id");

input.onkeypress = function(evt) {
    evt = evt || window.event;
    var charCode = evt.which || evt.keyCode;
    var charStr = String.fromCharCode(charCode);
    if (/\d/.test(charStr)) {
        return false;
    }
};