class PhoneNumber {
  static final RegExp _nonNumeric = RegExp(r'[^0-9]+');
  static final RegExp _letters = RegExp('[a-z]');
  static final RegExp _punctuations = RegExp('[@:!?\$%^&*_`~,;]');

  String clean(String input) {
    _checkForLetters(input);
    _checkForPunctuations(input);
    String cleaned = _removeNonNumeric(input);
    _validateLength(cleaned);
    _validateAreaCode(cleaned);
    _validateExchangeCode(cleaned);
    return (cleaned.length == 11)
        ? cleaned = cleaned.replaceFirst('1', '')
        : cleaned;
  }

  void _checkForLetters(String input) {
    if (input.toLowerCase().contains(_letters))
      throw FormatException('letters not permitted');
  }

  void _checkForPunctuations(String input) {
    if (input.toLowerCase().contains(_punctuations))
      throw FormatException('punctuations not permitted');
  }

  String _removeNonNumeric(String input) {
    return input.replaceAll(_nonNumeric, '');
  }

  void _validateLength(String number) {
    if (number.length < 10)
      throw FormatException('must not be fewer than 10 digits');
    if (number.length > 11)
      throw FormatException('must not be greater than 11 digits');
    if (number.length == 11 && number[0] != '1')
      throw FormatException('11 digits must start with 1');
  }

  void _validateAreaCode(String number) {
    if (number.length == 10 && number[0] == '0' ||
        number.length == 11 && number[1] == '0')
      throw FormatException('area code cannot start with zero');

    if (number.length == 10 && number[0] == '1' ||
        number.length == 11 && number[1] == '1')
      throw FormatException('area code cannot start with one');
  }

  void _validateExchangeCode(String number) {
    if (number.length == 10 && number[3] == '0' ||
        number.length == 11 && number[4] == '0')
      throw FormatException('exchange code cannot start with zero');

    if (number.length == 10 && number[3] == '1' ||
        number.length == 11 && number[4] == '1')
      throw FormatException('exchange code cannot start with one');
  }
}