title: Ruby Regex to Match Grayscale Color
body: <p>I want a ruby regex to match a hex greyscale color. </p>
  
  <p>So it would match </p>
  
  <pre><code>#000000
  #ababab
  #ffffff
  </code></pre>
  
  <p>but NOT </p>
  
  <pre><code>#ccddcc
  #afafa0
  </code></pre>
  
  <p>etc.</p>