title: find complement of regular expression
body: <p>There's a question on my exercise sheet to find the complement of  two formulas</p>
  
  <p>(1) <code>(aa|bb)*</code></p>
  
  <p>and</p>
  
  <p>(2) <code>(a|b)(aa|bb)(a|b)</code>.</p>
  
  <p>complement of both is in my opinion <code>a* | b*</code>, meaning only <code>a</code>'s or only <code>b</code>'s?</p>