title: Regular expression with quantity dependence
body: <p>Is it possible to create regular expression that will describe words like:</p>
  
  <pre><code>aabc
  aaaabcbc
  aaaaaabcbcbc
  </code></pre>
  
  <p>?</p>
  
  <p>Words are created like each occurrence of (<code>bc</code>) on the right is connected with occurrence (<code>aa</code>) on the left.</p>
  
  <p>Words below are not valid:</p>
  
  <pre><code>aa
  bc
  aaabc
  aaaabc
  aabcbc 
  </code></pre>