String userEmailValidator(String value) {
    Pattern emailPattern =
        r'^(([^<>()[\]\\.,;:\s@\"]+(\.[^<>()[\]\\.,;:\s@\"]+)*)|(\".+\"))@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\])|(([a-zA-Z\-0-9]+\.)+[a-zA-Z]{2,}))$';
    RegExp regex = new RegExp(emailPattern);
    if (value.isEmpty) {
      return 'This field is required.';
    } else {
      if (!regex.hasMatch(value)) {
        return 'Please enter a valid email address.';
      } else {
        return null;
      }
    }
  }