code: public static Validator<CharSequence> letterOrNumber(@NonNull final Context context,
                                                           @NonNull final Case caseSensitivity,
                                                           final boolean allowSpaces,
                                                           @NonNull final char... allowedCharacters) {
          return new LetterOrNumberValidator(context, R.string.default_error_message, caseSensitivity,
                  allowSpaces, allowedCharacters);
      }
docs: Creates and returns a validator, which allows to validate texts to ensure, that they only
  contain letters or numbers. It is possible to specify, whether only uppercase or lowercase
  letters should be accepted, or if they should be threatened case-insensitive. Letters are
  considered to be all alphabetical characters from A to B. Additionally, it is possible to
  specify whether spaces should be allowed and to add special characters, which should also be
  accepted. Empty texts are also accepted.
  
  @param context
  The context, which should be used to retrieve the error message, as an instance of
  the class {@link Context}. The context may not be null
  @param caseSensitivity
  The case senstivitiy, which should be used by the validator, as a value of the enum
  {@link Case}. The value may either be <code>UPPERCASE</code>, <code>LOWERCASE</code>
  or <code>CASE_INSENSITIVE</code>
  @param allowSpaces
  True, if spaces should be allowed, false otherwise
  @param allowedCharacters
  The allowed special characters as an array of the type <code>char</code>. The array
  may not be null
  @return The validator, which has been created, as an instance of the type {@link Validator}