title: Convert timestamp to datetime with sed
body: <p>I have a log file, and each line in it is like:</p>
  
  <pre><code>timestamp=1356431101, entity=xxx, event: xxxxxx
  </code></pre>
  
  <p>Now I want to use sed to replace timestamp wth human-readable datetime:</p>
  
  <pre><code>timestamp=2012-12-24 10:00:00, entity=xxx, event:xxxxx
  </code></pre>
  
  <p>My command is:</p>
  
  <pre><code>sed "s/^timestamp=\([0-9]\{10\}\),/timestamp=\`date +%D --date=@\1\`,/"
  </code></pre>
  
  <p>But the problem is that the <code>\1</code> can not be substituted by the 10-digits timestamp, instead, it is always treated as digit <code>1</code>. Can anyone tell me how to tackle this problem? Thank you in advance!</p>