title: Firebase auth.uid not working in regex
body: <p>I want to allow read access to users only whose uid is somewhere in the node name.</p>
  
  <p>I have the following rule in Firebase:</p>
  
  <pre><code>{
    "rules": {
      "messages" : {
        "$chat" :{
          ".read": "$chat.matches(/(auth.uid)/)"
        }
      }
    }
  }
  </code></pre>
  
  <p>However, this does not seem to work. When <code>$chat</code> is <code>user45-user99</code> for example and the <code>auth.uid</code> is <code>user45-user99</code> or it is just simply <code>user45</code> it fails.</p>
  
  <p>What am I doing wrong?</p>