title: How do I make a regex NOT match when I first find something unwanted
body: <p>I want to use a regex to find a particular string in my sample, but I want the regex to fail if I first find another string.  Let me give an example:</p>
  
  <p>Match <code>find_me</code> only if we do not first encounter <code>stop_here</code>.  (I don't care if <code>stop_here</code> occurs later in the sample.)</p>
  
  <p>So, this should match:</p>
  
  <p><code>blah blah find_me blah stop_here</code></p>
  
  <p>But this shouldn't:</p>
  
  <p><code>blah blah stop_here blah find_me</code></p>
  
  <p>(I'm using the .NET regex engine)</p>