On Mongo 2.4.4:

> db.y.insert(

{a:'foo'}

)
> db.y.findOne({a: {$regex: /O/i}})

{ "_id" : ObjectId("51f1ba8aa9b3808265d87c80"), "a" : "foo" }

On master:

> db.y.insert(

{a: 'foo'}

)
> db.y.findOne({a: {$regex: /O/i}})
null

This only happens if you use `$regex` and you put the options directly on the BSON regex. `

{a: /O/i}

` and `{$regex: 'O', $options: 'i'}` have the same behavior as in 2.4.

This is presumably because in `expression_parser.cpp` in `MatchExpressionParser::_parseRegexDocument`, if `$regex` points to a `RegEx`, we only look at its `regex()` and not at its `regexFlags()` too.