title: C# regexp negative lookahead
body: <p>i have a problem with replacing characters after specific character. For example i want to replace first  'aa' to '33' with this code. </p>
  
  <pre><code> string str = "dc1aaaafg";
   string pattern = @"a{2}(?!(1))";
   Regex rgx = new Regex(pattern);
   string result = rgx.Replace(str, "33");
  </code></pre>
  
  <p>but the result is 'dc13333fg'. It replaced the second group after '1'. I need to replace only first group like 'dc133aafg'. How can i achive this. I have a large string and it can be many replacing, this is just example.   </p>