title: Regex replacement with look behind and ignoring in c#
body: <p>I need to remove all "'s" from text, except word "let's". </p>
  
  <p>For example: "jerry's let's cake's" >> "jerry let's cake".</p>
  
  <pre><code>strText = Regex.Replace(strText, @"\b(?!let's)([a-z-]+)'s\b", @"$1");
  </code></pre>
  
  <p>- this works, but it takes long time on big texts.</p>
  
  <pre><code>strText = Regex.Replace(strText, @"\b(?!let's)(?&lt;=[a-z-]+)'s\b", "");
  </code></pre>
  
  <ul>
  <li>this one is not ignoring "let's"</li>
  </ul>
  
  <p>What am I doing wrong in the second statement?</p>