title: Insert separators into a string in regular intervals
body: <p>I have the following string in php:</p>
  
  <pre><code>$string = 'FEDCBA9876543210';
  </code></pre>
  
  <p>The string can be have 2 or more (I mean <strong>more</strong>) hexadecimal characters
  I wanted to group string by 2 like :</p>
  
  <pre><code>$output_string = 'FE:DC:BA:98:76:54:32:10';
  </code></pre>
  
  <p>I wanted to use regex for that, I think I saw a way to do like "recursive regex" but I can't remember it.</p>
  
  <p>Any help appreciated :)</p>