Allow only numbers in text field

gistfile1.txt

$(document).ready(function () {
    $("#field_id").keydown(function (e) {
        if (e.shiftKey) e.preventDefault();
        else {
            var nKeyCode = e.keyCode;
            //Ignore Backspace and Tab keys
            if (nKeyCode == 8 || nKeyCode == 9) return;
            if (nKeyCode < 95) {
                if (nKeyCode < 48 || nKeyCode > 57) e.preventDefault();
            } else {
                if (nKeyCode < 96 || nKeyCode > 105) e.preventDefault();
            }
        }
    });
});