code: static void validateShortOption (@Nonnull final String sOpt)
    {
      // handle the single character opt
      if (sOpt.length () == 1)
      {
        final char ch = sOpt.charAt (0);
  
        if (!_isValidShortOpt (ch))
          throw new IllegalArgumentException ("Illegal option name '" + ch + "'");
      }
      else
      {
        // handle the multi character opt
        for (final char ch : sOpt.toCharArray ())
          if (!_isValidChar (ch))
          {
            throw new IllegalArgumentException ("The option '" +
                                                sOpt +
                                                "' contains the illegal " +
                                                "character '" +
                                                ch +
                                                "'");
          }
      }
    }
docs: Validates whether <code>opt</code> is a permissible Option shortOpt. The
  rules that specify if the <code>opt</code> is valid are:
  <ul>
  <li>a single character <code>opt</code> that is either ' '(special case),
  '?', '@' or a letter</li>
  <li>a multi character <code>opt</code> that only contains letters.</li>
  </ul>
  <p>
  In case {@code opt} is {@code null} no further validation is performed.
  
  @param sOpt
  The option string to validate, may not be <code>null</code>.
  @throws IllegalArgumentException
  if the Option is not valid.