title: user regex in $nin mongo
body: <p>I can't make work the following regex in the aggregation. I read lots of example but it's not working. What I want is to exclude words that has 3 or less characters(example: 1 a2 aaa a-). However they still appear.</p>
  
  <p>This is my regex (I think it's correct) <code>\b\w?\w?\w?\b</code>.</p>
  
  <p>And here is the aggregation:</p>
  
  <pre><code>Meteor.publish("topics", function () {
    ReactiveAggregate(this, Articles, [
      {
        $project: {
                topic: { $split: ["$title", " "] },
  
                article: "$$ROOT"
        }
      },
      {
        $unwind: {
                path: "$topic"
        }
      },
      {
        $match: {
                topic: { $nin: ["sports", "\b\w?\w?\w?\b"] }
        }
      },
      {
        $group: {
                _id: "$topic",
                count: { $sum: 1 },
                articles: { $push: "$article" }
        }
      }
    ], { clientCollection: "clientTopic" });
  });
  </code></pre>
  
  <p>Example: </p>
  
  <pre><code>    /* Article 1 */
  {
      "_id" : "SB4mKAxaBijQXnS73",
      "title" : "Messi signs new contract with Barcelona"
  }
  /* Article 2 */
  {
       "_id" : "rhqioBkePzGCrRFLp",
       "title" : "Messi has 30 years old"
   }
  /* Article 3 */
  {
        "_id" : "X6LochRZw32op39W8",
        "title" : "President of Argentina visits Messi"
  }
  </code></pre>
  
  <p>I want to get a collection like:</p>
  
  <pre><code>Messi ==&gt; {_id: "Messi", articles: [Article 1, Article 2, Article 3], count: 3 }
  signs ==&gt; {_id: "signs", articles: [Article 1], count: 1 }
  contract ==&gt; {_id: "contract", articles: [Article 1], count: 1 }
  with ==&gt; {_id: "with", articles: [Article 1], count: 1 }
  Barcelona  ==&gt; {_id: "Barcelona", articles: [Article 1], count: 1 }
  
  years  ==&gt; {_id: "years", articles: [Article 2], count: 1 }
  
  President  ==&gt; {_id: "President", articles: [Article 3], count: 1 }
  Argentina  ==&gt; {_id: "Argentina", articles: [Article 3], count: 1 }
  visits     ==&gt; {_id: "visits", articles: [Article 3], count: 1 }
  </code></pre>
  
  <p>As you can see, the words <code>new</code>, <code>has</code>, <code>30</code>, <code>old</code>, <code>of</code> should not appear because their lengths are 3 chars or less.</p>
  
  <p>The articles with the word <code>sports</code> in theirs title are not contain in the collection, however the one that match the regex are contain.</p>