Example of Locale validation.

locale.dart

abstract class Locale {
  String get languageCode;
  String get scriptCode;
  String get regionCode;
  const factory Locale(String languageCode,
    [String regionOrScriptCode, String regionCode]) = _Locale._select;
}


class _Locale implements Locale {
  final String _language;
  final String _region;
  final String _script;

  const _Locale._select(String language, [String regionOrScript, String region]) :
    this.validate(language,
         region == null ? null : regionOrScript,
         region == null ? regionOrScript : region);

  const _Locale.validate(this._language, this._script, this._region)
    : assert(_language != null),
      assert(_language.length >= 2),
      assert(_language.length <= 8),
      assert(_language.length != 4),
      assert((_script ?? "xxxx").length == 4),
      assert((_region ?? "xx").length >= 2),
      assert((_region ?? "xx").length <= 3);

  // Or whatever normalization to do in getters.
  String get languageCode => _language.toLowerCase();
  String get scriptCode => _script == null ? null : _capitalize(_script);
  String get regionCode => _region?.toUpperCase();

  String toString() {  
    var builder = new StringBuffer(languageCode);
    if (_script != null) {
      builder.write("_");
      builder.write(scriptCode);
    }
    if (_region != null) {
      builder.write("_"); 
      builder.write(regionCode);
    }
    return builder.toString();
  }
}

// ASCII only. (TODO: Check first if already valid).
String _capitalize(String word) => 
  word.substring(0, 1).toUpperCase() + word.substring(1).toLowerCase();


main() {
  print(const Locale("zh"));
  print(const Locale("zh", "TW"));
  print(const Locale("zh", "Hant", "TW"));
  print(const Locale("ZH", "HANT", "tw"));
  try {
    Locale("zhzh");
  } catch (e) { 
    print(e); 
  }
}