title: Regular expressions for strings not containing specific substring
body: <p>What could be the regular expression for - <strong><em>All words that do not have the substring <code>baa</code></em></strong> for alphabet set =<code>{a,b}</code>?</p>
  
  <p>Is it:</p>
  
  <pre><code>a* ((aa) * b *)?
  </code></pre>
  
  <p>Can a string of length 2 be acceptable for the above condition to hold?</p>