title: str_replace and numbers
body: <p>The issue I'm having is I need to replace a few variables in strings with data, for example:</p>
  
  <pre><code>Blah blah S_10 blah S_1 blah blah S_5 blah blah S_15
  </code></pre>
  
  <p>So the problem is when I iterate through all of the strings, the <code>S_1</code> replacement undesirably can replace the first 3 characters of <code>S_10</code> and <code>S_15</code>, so the string will look like</p>
  
  <pre><code>Blah blah APPLE0 blah APPLE blah blah ORANGE blah blah APPLE5
  </code></pre>
  
  <p>What's the best way to approach this challenge?</p>