title: Regular expression for equal number of 0 and 1
body: <p>How to find regular expression with equal number of 1 and 0.
  I am also interested in how you think such solution ? </p>
  
  <p>example:
  should match : 1100, 00100111 , 01 .
  shouldn't match: 110 , 0, 11001.</p>
  
  <p>I need regular expression which gives set of all such string .
  If length of string in set given by regular expression in <code>2n</code> then number of <code>0s</code> should be equal to number <code>1s = n</code>.</p>