title: Keep only lines in String which contain numbers
body: <p>I have tried couple of regular expressions but have not been able to come up with one that works correctly. I have string with lines and I want to keep the lines which contains numbers.</p>
  
  <pre><code>Current String
  -----------------
  
  Dog
  
  Cat
  
  Cat 1
  
  Dog 22
  </code></pre>
  
  <p>Once processed the expected result is:</p>
  
  <pre><code>Filtered String
  -----------------
  Cat 1
  
  Dog 22
  </code></pre>