Allow Only Letters In Text Box

HTML

<input type="text" id="field"> <input type="button" value="Validate" onClick="textfield()">

JS

function textfield(){
            var firstname = /^[a-zA-Z]*$/;
            var fvalue = document.getElementById("field").value;
            alert(fvalue);
            if(!firstname.test(fvalue)){
                alert("Invalid");
            }
        }

Example