title: Grep with complex regular expressions
body: <p>How to grep for a complex regex in bash?</p>
  
  <p>I'm trying to grep a file <code>backend.conf</code> for:</p>
  
  <pre><code>cat backend.conf | grep "server (?:[0-9]{1,3}\.){3}[0-9]{1,3}"
  </code></pre>
  
  <p>But I am not seeing any matches, even though the contents of <code>backend.conf</code> has:</p>
  
  <pre><code>server 172.31.21.45 max_fails=3 fail_timeout=30s;
  </code></pre>
  
  <p>Which should be matching.</p>