title: sed to insert colon in a mac address
body: <p>I have a list of mac addresses in this format:</p>
  
  <pre><code>412010000018
  412010000026
  412010000034
  </code></pre>
  
  <p>I want this output:</p>
  
  <pre><code>41:20:10:00:00:18
  41:20:10:00:00:26
  41:20:10:00:00:34
  </code></pre>
  
  <p>I tried this, but did not work:</p>
  
  <pre><code>sed 's/([0-9]{2})([0-9]{2})([0-9]{2})([0-9]{2})/\1:\2:\3:\4/g' mac_list
  </code></pre>
  
  <p>How should I do it?</p>