When investigating #36 I learned that the standard iMessage Preview UA is:

Mozilla/5.0 (Macintosh; Intel Mac OS X 10_11_1) AppleWebKit/601.2.4 (KHTML, like Gecko) Version/9.0.1 Safari/601.2.4 facebookexternalhit/1.1 Facebot Twitterbot/1.0

Which will overlap with both facebookexternalhit and twitter bot, both are already in the list.

Proposed Solution

Refactor the patterns key in the json to be an object containing include and exclude. You could then define iMessage Crawler like this:

{
  "id": "imessage-preview",
  "patterns": {
    "include": ["facebookexternalhit", "Twitterbot"],
    "exclude": []
  }
}

And you could define Twitter Bot like this:

{
  "id": "imessage-preview",
  "patterns": {
    "include": ["Twitterbot"],
    "exclude": ["facebookexternalhit"]
  }
}

The instances key should be updated similarly so that we can test that the excluding patterns work correctly in validate.js