title: Mongo $in query with case-insensitivity
body: <p>I'm using Mongoose.js to perform an $in query, like so:</p>
  
  <pre><code>userModel.find({
              'twitter_username': {
                  $in: friends
              }
          })
  </code></pre>
  
  <p><code>friends</code> is just an array of strings. However, I'm having some case issues, and wondering if I can use Mongo's $regex functionality to make this $in query case-insensitive?</p>