title: Regular expression to avoid exceeding character repeatance on a string
body: <p>I have the alphabet {a,b} and I want to give a regular expression which satisfies the set of strings L in which the symbol <code>a</code> appears at most twice</p>
  
  <p>This is what I have:</p>
  
  <blockquote>
    <p>(b*|a)b*(b*|a|ε)
    where ε is the empty string</p>
  </blockquote>
  
  <p>Am I correct or what am I missing?</p>