validate method

Implementation

@override
String? validate(String? value) {
  if (value == null || value.isEmpty) return null;

  RegExp regex;
  if (pattern != null) {
    regex = pattern!;
  } else if (country != null) {
    regex = switch (country!) {
      PhoneCountry.us =>
        RegExp(r'^\+?1?\s*\(?\d{3}\)?[\s.-]?\d{3}[\s.-]?\d{4}$'),
      PhoneCountry.uk => RegExp(r'^\+?44[\s.-]?\d{10}$'),
      PhoneCountry.india => RegExp(r'^\+?91[\s.-]?\d{10}$'),
      PhoneCountry.australia => RegExp(r'^\+?61[\s.-]?\d{9}$'),
      PhoneCountry.brazil => RegExp(r'^\+?55[\s.-]?\d{10,11}$'),
      PhoneCountry.germany => RegExp(r'^\+?49[\s.-]?\d{10,11}$'),
      PhoneCountry.france => RegExp(r'^\+?33[\s.-]?\d{9}$'),
      PhoneCountry.japan => RegExp(r'^\+?81[\s.-]?\d{10}$'),
      PhoneCountry.china => RegExp(r'^\+?86[\s.-]?\d{11}$'),
    };
  } else {
    // Generic international fallback
    regex = RegExp(r'^\+?[0-9\s\-().]{7,20}$');
  }

  if (!regex.hasMatch(value)) {
    return message;
  }
  return null;
}