title: sed: -e expression #1, char 27: invalid reference \1 on `s' command's RHS
body: <pre><code>echo $variable = {"name":"AAA","username":"BBB"}
  </code></pre>
  
  <p>I would like to filter only <code>BBB</code> from above output. I have used this sed command:</p>
  
  <pre><code>sed 's/."username":"(["])".*/\1/')
  </code></pre>
  
  <p>I am getting</p>
  
  <blockquote>
    <p>sed: -e expression #1, char 27: invalid reference \1 on `s' command's RHS</p>
  </blockquote>
  
  <p>How can I fix this?</p>