dart Phone number validation with regex

phone.dart

class Phone {
  final String _areacode;
  final String _specialDigit;
  final String _localPhone;

  Phone._(this._areacode, this._specialDigit, this._localPhone);

  static Phone? create(String? candidate) {
    final candidateFormatted = candidate!.replaceAll(RegExp('[^0-9]'), '');
    //https://regex101.com/r/P85Luy/1
    final regExp = RegExp(r'^(55)?(0?\d{2})(9)?(\d{8})$');
    final tokens = regExp.allMatches(candidateFormatted);

    if (tokens.isEmpty) return null;

    tokenAt(int index) => tokens.elementAt(0).group(index) ?? '';
    return Phone._(tokenAt(2), tokenAt(3), tokenAt(4));
  }

  String get formatted {
    return '$_areacode$_specialDigit$_localPhone';
  }

  bool get isCellPhone {
    return _specialDigit.isNotEmpty;
  }
}

phone_test.dart

import 'package:flutter_test/flutter_test.dart';
import 'phone.dart';

void main() {
  group('Phone number tests', () {
    test('valid phone number -> should be a Phone instance', () {
      const value = '011 9 5559 7125';
      final phone = Phone.create(value);
      expect(phone, isNotNull);
    });

    test('mobile phone without area code -> should be invalid', () {
      const value = '9 5559 7125';
      final phone = Phone.create(value);
      expect(phone, isNull);
    });

    test('number without country code -> should return a valid full mobile number', () {
      const value = '011 9 5559 7125';
      final phone = Phone.create(value);
      expect(phone!.formatted, '011955597125');
    });

    test('number without country code -> when receive a custom country code -> should return a valid full mobile number', () {
      const value = '011 9 5559 7125';
      final phone = Phone.create(value);
      expect(phone!.formatted, '011955597125');
    });

    test('number with country code -> should return a valid full mobile number', () {
      const value = '+55 011 9 5559 7125';
      final phone = Phone.create(value);
      expect(phone!.formatted, '011955597125');
    });

    test('number without special digit -> should be a phone', () {
      const value = '011 5559 7125';
      final phone = Phone.create(value);
      expect(phone!.isCellPhone, isFalse);
    });

    test('number with special digit -> should be a mobile phone', () {
      const value = '011 9 5559 7125';
      final phone = Phone.create(value);
      expect(phone!.isCellPhone, isTrue);
    });

    test('number without special digit -> when formatted -> should be a full phone number', () {
      const value = '011 5559 7125';
      final phone = Phone.create(value);
      expect(phone!.formatted, '01155597125');
    });
  });
}