title: How to subtract known substring from larger string
body: <p>I have been trying to deconstruct a string in the following way:</p>
  
  <p><strong>Full String</strong></p>
  
  <pre><code>str = 'Hello world I am Bob'
  </code></pre>
  
  <p><strong>Substring 1</strong></p>
  
  <pre><code>sub_str1 = 'Hello world'
  </code></pre>
  
  <p><strong>Substring 2</strong></p>
  
  <pre><code>sub_str2 = 'world I'
  </code></pre>
  
  <p><strong>Desired Outcome:</strong></p>
  
  <ol>
  <li><p>Using substring 1:</p>
  
  <p>Apply some regular expression to get:</p>
  
  <pre><code>answer = ' I am Bob'
  </code></pre></li>
  <li><p>Using substring 2:</p>
  
  <pre><code>answer = 'Hello  I am Bob'
  </code></pre></li>
  </ol>
  
  <p>I have tried a number of different approaches with regular expression, but I have just started using them and am not at all proficient.</p>