Dart class that uses a strategy pattern to check the validity of an email

email_validation_strategy.dart

/// Written with “powerful” inspiration from the https://pub.dev/packages/email_validator.

/// An abstract strategy for validating objects of type [T].
///
/// Implementations of this class define specific validation rules
/// for a given type, allowing flexible and reusable validation logic.
abstract class ValidationStrategy<T, E extends Exception> {
  /// Validates the given [value] of type [T].
  ///
  /// If the validation fails, an exception of type [E] is thrown.
  void call(T value);
}

// Maximum overall email length (less than 255 characters)
const int kMaxEmailLength = 254;
// Maximum length for the local-part
const int kMaxLocalPartLength = 64;
// Maximum length for each domain component
const int kMaxDomainComponentLength = 64;
// Minimum required length for IPv4/IPv6 literal
const int kMinIpLiteralLength = 8;

/// Email validation strategy class.
/// Throws [EmailValidationException] if the email is invalid.
class EmailValidationStrategy implements ValidationStrategy<String, EmailValidationException> {
  EmailValidationStrategy({
    this.allowTopLevelDomains = false,
    this.allowInternational = true,
    this.debug = false,
  });

  /// Whether top-level domains are allowed.
  final bool allowTopLevelDomains;

  /// Whether international email addresses are allowed.
  final bool allowInternational;

  /// Whether the parser is in debug mode.
  final bool debug;

  @override
  void call(String email) {
    final parser = _EmailValidatorParser(email, allowTopLevelDomains, allowInternational, debug: debug);
    if (!parser.validate()) {
      // In debug mode, detailed error messages are available via parser.errorMessage
      throw EmailValidationException(parser.errorMessage ?? 'Invalid email address.');
    }
  }
}

/// Exception thrown when email validation fails.
class EmailValidationException implements Exception {
  EmailValidationException(this.message);

  /// The error message associated with the validation failure.
  final String message;

  @override
  String toString() {
    return 'EmailValidationException: $message';
  }
}

/// Enumeration for subdomain types: [none], [alphabetic], [numeric], [alphanumeric]
enum SubdomainType { none, alphabetic, numeric, alphanumeric }

/// Internal implementation of email validation using a parser.
/// Each validation step relies on state ([_position], [_domainType]) ensuring thread safety.
/// When debug mode is enabled, detailed failure reasons are recorded.
class _EmailValidatorParser {
  _EmailValidatorParser(this.email, this.allowTopLevelDomains, this.allowInternational, {this.debug = false});

  /// The email address being validated.
  final String email;

  /// Whether top-level domains are allowed.
  final bool allowTopLevelDomains;

  /// Whether international email addresses are allowed.
  final bool allowInternational;

  /// Whether the parser is in debug mode.
  final bool debug;

  /// The current position in the email address.
  int _position = 0;

  /// The type of the current domain.
  SubdomainType _domainType = SubdomainType.none;

  /// The error message, if any.
  String? _errorMessage;

  /// Helper function that sets an error message and returns false upon failure.
  bool _fail(String message) {
    if (debug && _errorMessage == null) {
      _errorMessage = message;
    }

    return false;
  }

  /// Provides the first failure reason when debug mode is active.
  String? get errorMessage => _errorMessage;

  /// Checks if the character [char] is a digit.
  bool _isDigit(String char) {
    final code = char.codeUnitAt(0);
    return code >= 48 && code <= 57;
  }

  /// Checks if the character [char] is an alphabetic letter.
  bool _isLetter(String char) {
    final code = char.codeUnitAt(0);
    return (code >= 65 && code <= 90) || (code >= 97 && code <= 122);
  }

  /// Checks if the character [char] is either a letter or a digit.
  bool _isLetterOrDigit(String char) {
    return _isLetter(char) || _isDigit(char);
  }

  /// Determines if the character [char] is allowed in the local-part atom.
  bool _isAtomChar(String char) {
    const atomCharacters = r"!#$%&'*+-/=?^_`{|}~";
    if (char.codeUnitAt(0) < 128) {
      return _isLetterOrDigit(char) || atomCharacters.contains(char);
    }

    return allowInternational;
  }

  /// Checks domain characters while updating [_domainType].
  bool _isDomainChar(String char) {
    if (char.codeUnitAt(0) < 128) {
      if (_isLetter(char) || char == '-') {
        _domainType = SubdomainType.alphabetic;
        return true;
      }

      if (_isDigit(char)) {
        _domainType = SubdomainType.numeric;
        return true;
      }

      return false;
    }

    if (allowInternational) {
      _domainType = SubdomainType.alphabetic;
      return true;
    }

    return false;
  }

  /// Checks the starting character of a domain.
  bool _isDomainStartChar(String char) {
    if (char.codeUnitAt(0) < 128) {
      if (_isLetter(char)) {
        _domainType = SubdomainType.alphabetic;
        return true;
      }

      if (_isDigit(char)) {
        _domainType = SubdomainType.numeric;
        return true;
      }

      _domainType = SubdomainType.none;

      return false;
    }

    if (allowInternational) {
      _domainType = SubdomainType.alphabetic;
      return true;
    }

    _domainType = SubdomainType.none;

    return false;
  }

  /// Parses the atom part of the local-part.
  bool _skipAtom() {
    final start = _position;
    while (_position < email.length && _isAtomChar(email[_position])) {
      _position++;
    }

    if (_position == start) {
      return _fail('Failed to parse local-part atom at position $_position');
    }

    if (_position > kMaxLocalPartLength) {
      return _fail('Local-part exceeds maximum length of $kMaxLocalPartLength characters');
    }

    return true;
  }

  /// Parses a single subdomain.
  bool _skipSubDomain() {
    final start = _position;
    if (_position >= email.length || !_isDomainStartChar(email[_position])) {
      return _fail('Failed domain start character validation at position $_position');
    }

    // Consume the starting domain character
    _position++;

    while (_position < email.length && _isDomainChar(email[_position])) {
      _position++;
    }

    // A single-character TLD is not valid.
    if (_position == email.length && (_position - start) == 1) {
      return _fail('Single-character domain is not valid at position $start');
    }

    if ((_position - start) >= kMaxDomainComponentLength) {
      return _fail('Domain component exceeds maximum length of $kMaxDomainComponentLength characters');
    }

    if (email[_position - 1] == '-') {
      return _fail('Domain cannot end with "-" at position ${_position - 1}');
    }

    return true;
  }

  /// Parses the entire domain (subdomains and top-level domain).
  bool _skipDomain() {
    if (!_skipSubDomain()) {
      return false;
    }

    if (_position < email.length && email[_position] == '.') {
      do {
        // Skip the dot ('.')
        _position++;

        if (_position >= email.length) {
          return _fail('Insufficient content after dot in domain parsing');
        }

        if (!_skipSubDomain()) {
          return false;
        }
      } while (_position < email.length && email[_position] == '.');
    } else if (!allowTopLevelDomains) {
      return _fail('Top-level domains are not allowed');
    }

    if (_domainType == SubdomainType.numeric) {
      return _fail('Numeric domains are not valid');
    }

    return true;
  }

  /// Parses the content within a quoted string.
  bool _skipQuoted() {
    var escaped = false;
    // Skip the starting double quote
    _position++;

    while (_position < email.length) {
      if (email[_position].codeUnitAt(0) >= 128 && !allowInternational) {
        return _fail('International characters are not allowed in quoted string at position $_position');
      }

      if (email[_position] == r'\') {
        escaped = !escaped;
      } else if (!escaped) {
        if (email[_position] == '"') {
          break;
        }
      } else {
        escaped = false;
      }

      _position++;
    }

    if (_position >= email.length || email[_position] != '"') {
      return _fail('Missing ending double quote in quoted string');
    }

    // Skip the ending double quote
    _position++;

    return true;
  }

  /// Parses an IPv4 literal.
  bool _skipIPv4Literal() {
    var groups = 0;
    while (_position < email.length && groups < 4) {
      final groupStart = _position;
      var value = 0;
      while (_position < email.length && _isDigit(email[_position])) {
        value = (value * 10) + (email[_position].codeUnitAt(0) - 48);
        _position++;
      }

      if (_position == groupStart || (_position - groupStart) > 3 || value > 255) {
        return _fail('Failed to parse IPv4 group starting at $groupStart');
      }

      groups++;

      if (groups < 4) {
        if (_position < email.length && email[_position] == '.') {
          // Skip the dot ('.')
          _position++;
        } else {
          return _fail('Missing IPv4 group delimiter (.) at position $_position');
        }
      }
    }

    if (groups != 4) {
      return _fail('IPv4 address does not have 4 groups');
    }

    return true;
  }

  /// Checks if the character [char] is a hexadecimal digit.
  bool _isHexDigit(String char) {
    final code = char.codeUnitAt(0);
    return (code >= 65 && code <= 70) || (code >= 97 && code <= 102) || (code >= 48 && code <= 57);
  }

  /// Parses an IPv6 literal.
  bool _skipIPv6Literal() {
    var compact = false;
    var colons = 0;
    while (_position < email.length) {
      final segmentStart = _position;
      while (_position < email.length && _isHexDigit(email[_position])) {
        _position++;
      }

      if (_position >= email.length) {
        break;
      }

      // Handle mixed IPv6/IPv4: if enough hex digits are followed by '.', switch to IPv4 parsing
      if (_position > segmentStart && colons > 2 && email[_position] == '.') {
        _position = segmentStart;
        if (!_skipIPv4Literal()) {
          return _fail('Failed to parse mixed IPv4 within IPv6');
        }

        return compact ? colons < 6 : colons == 6;
      }

      final count = _position - segmentStart;
      if (count > 4) {
        return _fail('IPv6 segment exceeds 4 characters at $segmentStart');
      }

      if (email[_position] != ':') {
        break;
      }

      final colonStart = _position;
      while (_position < email.length && email[_position] == ':') {
        _position++;
      }

      final colonCount = _position - colonStart;
      if (colonCount > 2) {
        return _fail('Too many consecutive colons (:) starting at $colonStart');
      }

      if (colonCount == 2) {
        if (compact) {
          return _fail('Multiple uses of compression (::) at $colonStart');
        }

        compact = true;
        colons += 2;
      } else {
        colons++;
      }
    }

    if (colons < 2) {
      return _fail('Insufficient number of colons in IPv6 address');
    }

    return compact ? colons < 7 : colons == 7;
  }

  /// Performs full email validation.
  bool validate() {
    // Check if the email is empty or exceeds the maximum length.
    if (email.isEmpty || email.length >= kMaxEmailLength) {
      return _fail('Email is empty or exceeds $kMaxEmailLength characters');
    }

    // Parse the local-part: if it starts with a double quote, treat it as a quoted string.
    if (email[_position] == '"') {
      if (!_skipQuoted() || _position >= email.length) {
        return _fail('Failed to parse quoted string in local-part');
      }
    } else {
      if (!_skipAtom()) {
        return _fail('Failed to parse local-part atom');
      }

      while (_position < email.length && email[_position] == '.') {
        // Skip the dot ('.')
        _position++;

        if (_position >= email.length || !_skipAtom()) {
          return _fail('Failed to parse local-part segment after dot');
        }
      }
    }

    // Validate the '@' delimiter and local-part length.
    if (_position + 1 >= email.length || _position > kMaxLocalPartLength || email[_position] != '@') {
      return _fail('Failed to validate the local-part and domain delimiter (@) at position $_position');
    }

    // Skip the '@' character.
    _position++;

    if (_position >= email.length) {
      return _fail('Insufficient characters for domain parsing');
    }

    if (email[_position] != '[') {
      // Parse a standard domain.
      if (!_skipDomain()) {
        return false;
      }

      return _position == email.length;
    }

    // Parse the address literal ([...])
    // Skip the '[' character.
    _position++;

    if (_position + kMinIpLiteralLength >= email.length) {
      return _fail('IP literal length is insufficient');
    }

    final literal = email.substring(_position - 1).toLowerCase();

    if (literal.contains('ipv6:')) {
      _position += 'ipv6:'.length;
      if (!_skipIPv6Literal()) {
        return false;
      }
    } else {
      if (!_skipIPv4Literal()) {
        return false;
      }
    }

    if (_position >= email.length || email[_position] != ']') {
      return _fail("Missing closing bracket (']') for IP literal");
    }

    // Skip the closing bracket.
    _position++;

    return _position == email.length;
  }
}

Thank you for your warm response, @fredeil

Could you link the blog post here? :) @AndrewDongminYoo