Regex 101 Exercise S6 - Change the extension on a file

Given a filename including path, change the extension to .out.

Input string example: 

C:\utility\Processor.cs

Notes:

  1. The best answer to this is really to use System.IO.Path.ChangeExtension(), but that wouldn't be much of a Regex exercise, now would it?
  2. It's not as simple as it looks