It looks much better now. It's really challenging but it's fun as well :)

As you can imagine it's my first kata. I formatted the text, hope it is more readable now :)

Thank you both for the feedback.
I have changed something, I have added some random tests and specified in the description what to do exactly. Then I removed the check on the letters and added the check only on the prefixes.
Hope now it a bit better, let me know guys :)

I assumed that every letter has to be considered a typo since I have never seen letters used as separators or prefixes. But as you said later in the comment, maybe it is better to keep only the encryption side and removing the cleaning part. The latter is not complete and it is another job which maybe could be another kata! :)

About the random test, I do not see how it could be useful in this case. In the validation test there are all the different cases which can possibly occur. A random test here would only change the numbers but not the position of the separators. The latter case would be much more interesting but in reality you will not see a number with a format for example like this 1-245-2-4-654-63 or like this 09837422/90. Do you agree?

Thank you for your suggestions :)

You won