title: How to remove all the values in a string except for the chosen ones
body: <p>So my code is 
  <code>value = "123456"</code>
  I want to remove everything except for 2 and 5. 
  the output will be <code>25</code>
  the program should work even the value is changed for example
  <code>value = "463312"</code>
  the output will be 2</p>
  
  <p>I tried to use <code>remove()</code> and <code>replace()</code> function. But it didn't work. 
  <strong>Doing it on python 3.6.2</strong></p>