So the issue was the syntax of the query.

MongoDB supports either:

{ 'author' : { '$regex' : /^Oscar/ }}

-> without quotes!
or

{ 'author' : { '$regex' : '^Oscar' }}

-> with quotes but without the slashes!

See https://docs.mongodb.com/manual/reference/operator/query/regex/ .

Note that due to a bug in the MongoDB Java driver, the first syntax can't be supported right now: see https://jira.mongodb.org/browse/JAVA-2224 and https://github.com/mongodb/mongo-java-driver/pull/355.

I added a test with the correct syntax and a warning in the documentation.