$regex within $project mongodb aggregation

问题

I have a users collection which has a location field with type String. I want to pass a score field along the document which shows whether the document's location is similar to the text "Austin". For instance the recorded location is Austin, Texas, I want it to be matched with Austin. I thought it would be possible to use $regex for this.

I wrote this aggregation:

$project: {
  score: {
    $cond: 
      if: {
        $regex: {'$location': /Austin/}
      },
      then: 1,
      else: 0
    }
  },
  location: 1,
  firstName: 1,
  lastName: 1
}

But what I get is :

{
    "name": "MongoError",
    "errmsg": "exception: invalid operator '$regex'",
    "code": 15999,
    "ok": 0
}

来源：https://stackoverflow.com/questions/29866336/regex-within-project-mongodb-aggregation