title: grep or sed for word containing string
body: <p>example file:</p>
  
  <pre><code>blahblah 123.a.site.com   some-junk
  yoyoyoyo 456.a.site.com   more-junk
  hihohiho 123.a.site.org   junk-in-the-trunk
  lalalala 456.a.site.org   monkey-junk
  </code></pre>
  
  <p>I want to grep out all those domains in the middle of each line, they all have a common part <code>a.site</code> with which I can grep for, but I can't work out how to do it without returning the whole line?</p>
  
  <p>Maybe sed or a regex is need here as a simple grep isn't enough?</p>