title: Insert space after every one or two characters in string
body: <p>I need to write code to correct user input. The input is a mathematical expression in infix form. 
  In order for my calculations to be correct, i need each operand spaced away from the operator. </p>
  
  <p>Can you please help me write a method that will provide the following expected results. </p>
  
  <p>Expected input: 13*21-5+33
  Expected Output: 13 * 21 - 5 + 33</p>
  
  <p>Research I have done: I could only find out how to add spaces between every character in the string and that will not work for calculating expressions with double digit values. Ie: 1 3 * 2 1 - 5 + 3 3. <a href="https://stackoverflow.com/questions/4469984/how-to-insert-space-after-every-character-of-an-existing-string-in-java">How to insert Space after every Character of an existing String in Java?</a></p>
  
  <p>Thanks </p>