def add(self, item, group_by=None):
        """General purpose class to group items by certain criteria."""
        key = None

        if not group_by:
            group_by = self.group_by

        if group_by:
            # if group_by is a function, use it with item as argument
            if hasattr(group_by, '__call__'):
                key = group_by(item)

            # if the item has attribute of group_by as string, use that as key
            elif isinstance(group_by, str) and hasattr(item, group_by):
                key = getattr(item, group_by)

            else:
                key = None
                # try to match str(item) with regular expression
                if isinstance(group_by, str):
                    match = re.search(group_by, str(item))
                    if match:
                        if len(match.groups()) > 0:
                            key = match.group(1)
                        else:
                            key = match.group()

        self.groups.setdefault(key, list()).append(item)