Your "specification" says: "... until it either reaches the end of the string on its right (NOT on its left), or until it reaches an N ..." so that is what I implemented.

As a general the better you specify a task the easier it becomes to implement. A good approach in this sort of case is to provide tests for each case you are interested in then the tests become part of the specification and also allow you to validate the behaviour of the code.

Also note that I already gave you a test framework so the appropriate way to check that case is to provide the start string and the expected string as I did for the other test cases. That allows us to see what you expected to happen and how that differs from what did happen. It also allows you to play with the substitution code and check that you haven't broken test cases that were previously working.

This technique of writing tests first then writing the code so they pass is a very powerful way of developing software and, as can be seen from this example, works even for trivial code.