I haven't been able to provide an example that can be run easily, but you can observe this by sending a regex query which will scan a lot of records, and then tailing the slow query log.

Basically, this:

BSON::Regex.new('a', 0)

will send /a/ as expected, but this:

BSON::Regex.new('a', 1)

will send /a/ilmsux.

1 is equivalent to Regexp::IGNORECASE, so I would expect this to send /a/i.

I'm not ready to upgrade to v2.0 of the driver so I haven't tested with that yet.