title: Combining two strings by removing duplicate substrings
body: <p>I have two strings that I would like to combine, removing duplicate substrings. Note that every two consecutive numbers constitute a substring. Consider string str1 and str2:</p>
  
  <pre><code>str1 = "#100#123#100#678"
  str2 = "#100#678#100#56"
  </code></pre>
  
  <p>I would like to produce a combined string as:</p>
  
  <pre><code>comboStr = "#100#123#100#678#100#56" (i.e. I removed the duplicate #100#678)
  </code></pre>
  
  <p>What's the easiest way to do this? Is there a way I can achieve this using regular expressions? </p>