regex.py 文件源码

def down(self, visited: FrozenSet[Any]=None) -> Set[Any]:
        """ Returns the set of reachable nodes by going down on this node """
        if visited is None:
            visited = frozenset()

        if self in visited:
            return {self} if self.symbol not in OPERATORS else set()

        visited |= {self}
        if self.symbol == '|':
            return self.left.down(visited) | self.right.down(visited)
        elif self.symbol == '.':
            return self.left.down(visited)
        elif self.symbol == '*' or self.symbol == '?':
            return self.left.down(visited) | self.right.up(visited)
        elif self.symbol == EPSILON:
            return self.right.up(visited)
        return {self}