Edit in JSFiddle

function check(target) {
  target.setCustomValidity("");
  if (!target.validity.valid) {
    target.setCustomValidity("自訂訊息");
  }
}

<form>
  <input type="text" pattern=".{5}" oninvalid="check(this);" oninput="check(this)" required/>

  <input type="submit" />
</form>