validate method
Validates the given input and returns an error message if validation fails.
Returns null
if the input is valid.
Implementation
@override
String? validate(String input) {
final emailValid = RegExp(
r"^[a-zA-Z0-9.a-zA-Z0-9.!#$%&'*+-/=?^_`{|}~]+@[a-zA-Z0-9]+\.[a-zA-Z]+",
).hasMatch(input);
return emailValid ? null : message;
}