validate method

Implementation

@override
String? validate(String? data) {
  if (data != null) {
    if (maxLength != null && data.length > maxLength!) {
      return 'Higher then permitted length';
    } else if (minLength != null && data.length < minLength!) {
      return 'Lower then permitted length';
    } else if (regExp != null && !regExp!.hasMatch(data)) {
      return regExpErrorString ?? 'Invalid input';
    }
  }
  return null;
}