title: Regular expression theory
body: <p>I have a little problem with <a href="http://en.wikipedia.org/wiki/Regular_expression#Formal_language_theory" rel="nofollow">RE theory</a>.</p>
  
  <p>Given an alphabet {0, 1}, I have to create a regular expression that matches all string that does NOT contain the substring 111.</p>
  
  <p>I'm not able to get the point, also for simplier substring like 00. </p>
  
  <p>Edit: The solution must contains only the three standard operation: concatenation, alternation, kleene star, as you can see in the wiki link</p>
  
  <p>Thank you.</p>