title: Check if number is made of both 0 and 1 using regex
body: <p>I have just started with regex. I am unable to make an expression to check if number contains only 1 and 0 and nothing else.
  In python</p>
  
  <pre><code>re.match("[01]", b)
  </code></pre>
  
  <p>This matches for either 0 or 1 I think.</p>
  
  <p>What will be the expression to check that the string contains all the elements of a given set and nothing else.</p>