Projection by a substring match

Let's start from an example: suppose that each document in a collection contains a UserAgent string as a field. Androids have something like "xxxxxx Android xxxx", iOS have something like "xxxxxxx iPad/iPhone xxxxxx".

I would like to count those documents, grouped by platform (Android, iPad, iPhone). To get the platform, all I need is to count them by a substring match. For example, /Android/ vs /iPad/ vs /iPhone/ (each counted separately).

Any solution with aggregation framework will make it for me. I can clarify/whirpool on any request with pleasure.