title: Ruby regexp should exclude "125af" substrings
body: <p>I have the following regexp:</p>
  
  <pre><code>/^(\d{1,11})((?:(?:c?o|co?).*)?.*)$/i
  </code></pre>
  
  <p>that matches strings like:</p>
  
  <pre><code>125******* or 125co****** or 125CO*******
  </code></pre>
  
  <p>I would like to exclude substrings that begin with:</p>
  
  <pre><code>125af***** or 125AF****** or 125f****** or 125AF********
  </code></pre>
  
  <p>Thanks in advance</p>