title: Regular Expression Pattern \K Alternatives in C#
body: <p>I have a regex expression that I tested on <a href="http://gskinner.com/RegExr/" rel="noreferrer">http://gskinner.com/RegExr/</a> and it worked, but when I used it in my C# application it failed.</p>
  
  <p>My regex expression: <code>(?&lt;!\d)\d{6}\K\d+(?=\d{4}(?!\d))</code>
  Text: 4000751111115425
  Result: 111111</p>
  
  <p>What is wrong with my regex expression?</p>