Refers to server ticket SERVER-14595 and this page:

http://docs.mongodb.org/manual/reference/operator/query/in/#use-the-in-operator-with-a-regular-expression

The documentation makes it sound like either the {$regex: /pattern/} syntax or the raw /pattern/ syntax for regular expressions is allowed inside an $in. However, to my knowledge only the raw /pattern/ syntax has ever been allowed. Versions of 2.6 through 2.6.3 will reject $regex inside an $in explicitly. The docs should state explicitly that only the /pattern/ syntax is currently allowed.