title: How can I use a calculated value in a RegEx replace operation in C#?
body: <p>I'm looking for a way to use the length of a match group in the replace expression with the  c# regex.replace function. </p>
  
  <p>That is, what can I replace ??? with in the following example to get the desired output shown below?</p>
  
  <p><strong>Example:</strong> </p>
  
  <pre><code>val = Regex.Replace("xxx", @"(?&lt;exes&gt;x{1,6})", "${exes} - ???");
  </code></pre>
  
  <p><strong>Desired output</strong> </p>
  
  <pre><code>X - 3
  </code></pre>
  
  <p><strong>Note:</strong> This is an extremely contrived/simplified example to demonstrate the question. I realize for this example a regular expression is not the ideal way of doing this. Just trust me that the real world application of the answer is part of a more complex problem that does necessitate the use of a RegEx replace here.</p>