title: update mongo db documents with regex
body: <p>I need to find all the documents in mongodb that have keywords that start with number 1-9, then add a '+' in front of the keyword, I can easily find the documents but cannot figure out how to update them.</p>
  
  <p>I tried this one, but it doesn't work</p>
  
  <pre><code>db.placements.update({program_id:{$in:[113,107]},
                       keyword:{$regex:'^[0-9]', $options:'i'}}, 
                       {keyword:"+"+$keyword})
  </code></pre>
  
  <p>It cannot recognize $keyword, I also tried '.keyword', 'keyword', none of them works. Is there any way to reference the document itself like Java does, using 'this', so I can do something like </p>
  
  <pre><code>this.keyword: "+" + this.keyword
  </code></pre>