According to the issue below, the MongoDB supports queries like the following:

db.docs.find( { "bar" :

} )

http://jira.mongodb.org/browse/SERVER-275

That feature has been around for a while, but it does not exist in the docs (Advanced Queries). It also does not have any unit tests wrapping it, so we should not doc it until we're protecting it via a test.

Expected Fix:
1. Add jstest for this behavior ( "x" : {$regex : /^a/} )
2. Update the Advanced Queries document to clarify this behavior.