title: Match alternating digits and characters
body: <p>I have read the Google python regex tutorial about regular expression and tried to test one of the patterns I need.</p>
  
  <ul>
  <li>String must be 10 characters long.</li>
  <li>1,3,5,7,9 chars must be digits (1 - 5) and should not be repeated</li>
  <li>other symbols are letters (a, b or c) and the can be repeated</li>
  </ul>
  
  <p>For example:</p>
  
  <pre><code>1a2b4a3c5b # is valid
  
  1a5c4b4a3b # is not valid because of two 4s
  </code></pre>
  
  <p>So far I've tried:</p>
  
  <pre><code>pattern = r'([1-5])[abc]([1-5^\1])[abc]([1-5^\1\2])[abc]([1-5\1\2\3])[abc]([1-5\1\2\3\4])[abc]'
  </code></pre>
  
  <p>but it failed...</p>