title: What does "\2" mean at the end of a regular expression
body: <p>I have the following assignment:</p>
  
  <p>Words of a song are in a file called stairway.txt. Which of the following lines will be printed out after this command:
  </p>
  
  <pre class="lang-none prettyprint-override"><code>grep -E '(^.{4})(.{2}).*[ ]\2' stairway.txt
  
  (a) Yes, there are two paths you can go by but in the long run
  
  (b) Its just a spring clean for the May queen.
  
  (c) Don't be alarmed now.
  
  (d) If there's a bustle in your hedgerow.
  
  (e) Theres still time to change the road you're on.
  </code></pre>
  
  <p>I don't understand what does the <code>\2</code> at the end mean?</p>