in lib/src/parser.dart [132:140]
bool _parseSoloOption() {
// Hand coded regexp: r'^-([a-zA-Z0-9])$'
// Length must be two, hyphen followed by any letter/digit.
if (_current.length != 2) return false;
if (!_current.startsWith('-')) return false;
var opt = _current[1];
if (!_isLetterOrDigit(opt.codeUnitAt(0))) return false;
return _handleSoloOption(opt);
}