def unscan(self):
self.pos_history.pop()<EOL>self._pos = self.pos_history[-<NUM_LIT:1>]<EOL>self.match_history.pop()<EOL>self._match = self.match_history[-<NUM_LIT:1>]<EOL>
Undo the last scan, resetting the position and match registers.

    >>> s = Scanner('test string')
    >>> s.pos
    0
    >>> s.skip(r'te')
    2
    >>> s.rest
    'st string'
    >>> s.unscan()
    >>> s.pos
    0
    >>> s.rest
    'test string'