title: Using RegEx to Insert Character before Matches
body: <p>I know I can use RegEx to replace all occurrences of 'a', 'b', or 'c' with a blackslash character in a string like this:</p>
  
  <pre><code>string result = Regex.Replace(input, "[abc]", "\\");
  </code></pre>
  
  <p>But how can I replace each occurrence with a backslash followed by the character that matched?</p>