I think there is a misunderstanding, as I only want to keep the blocks whose id has a match in the second file. If I have a block as:

object
type="user"
id="999999999999"
encryptedPassword=""
>
<checkListAttributes>
</checkListAttributes>
</object>

and 999999999999 is not in the second file, the whole block should be discarded. but after I run your code, it is still there. any idea?