<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def regroup(self, group_by=None):
 """Regroup items."""

        if not group_by:
            group_by = self.group_by

        groups = self.groups
        self.groups = {}

        for g in groups:
            for item in groups[g]:
                self.add(item, group_by)