title: mongodb Replace a portion of a string
body: <p>I have a field</p>
  
  <pre><code>"data" : {
          "user" : "derp",
          "id" : "xHOSTNAME_xderp"
  </code></pre>
  
  <p>I want to replace all docs with xHOSTNAME_* to yHOSTNAME_.
  Any idea how to search and replace. I've seen some other posts similarly related but none seemed to work.</p>