var a = '';
function checkdom(v) {
  var newName = document.getElementById('textfield').value ;
  if (/[^A-Za-z0-9-]/.test(v.value)) {
    alert("A domain name can only contain upper or lowercase letters, numbers or a hyphen");
    v.value = a;
  }else{
    a = v.value;
  }
}