title: Python regular expression A followed by B followed by C
body: <p>I am new to <em>regular expressions</em>, I couldn't come up with a regex which can accept strings which are of the following form:</p>
  
  <p><code>A</code> followed by <code>B</code> followed by <code>C</code>.</p>
  
  <p>Examples:</p>
  
  <pre><code>ABC, AAAC, CCC, AABBCCCC   are accepted
  
  CCBBAAA, BCA, ABCABC       are rejected
  </code></pre>
  
  <p>Thanks in advance.</p>