title: Java RegEx back slash Number
body: <p>What does it mean to have a <code>\number</code> in a regex in java. </p>
  
  <p>Let's say I have something like <code>\1</code> or <code>\2</code>. What does this mean and how is it used?
  An example would be really helpful.</p>
  
  <p>Thanks</p>