In the manual, you can see that most of the time there are no spaces in anchors. This page is a good example:

http://docs.mongodb.org/manual/reference/limits

At most levels there are no spaces in the anchors:

http://docs.mongodb.org/manual/reference/limits/#sharding-operational-restrictions

But when you get to the lowest level there are:

http://docs.mongodb.org/manual/reference/limits/#Sharding Existing Collection Data Size