PyMongo can error when retrieving regular expressions not supported by python

Since MongoDB's PCREs have an alternate syntax from Python's regular expressions, and since PyMongo tries to compile every regex-type BSON value it retrieves from MongoDB, PyMongo will throw an error retrieving some regular expressions.

It would be better to return either a new bson.Regex type that encapsulates the string from MongoDB, or to return a dictionary like

.

In either case, PyMongo shouldn't try to compile every regex-type BSON value as a Python regex.