Not best practice. This can be done with time O(n) and space O(n) using a dictionary.

This is very slow since it is using two for loops making it at worst O(n^2).

As a starter, try to use Markdown when asking a question as it is very difficult to read what you posted in its current form.

What is the reason for using \z and not $?

Does it catch any edge cases?

Have a look at the problem description again. It says that you will be given an array of numbers that you are supposed to convert to a phone number in the format that the example show you.

Kata.CreatePhoneNumber(new int[] {1, 2, 3, 4, 5, 6, 7, 8, 9, 0}) // => returns "(123) 456-7890" <----- **this!**

Also, remember that you do not need to convert numbers to strings as this is done implicitly (if you are using c#, which it looks like you do).

I experienced the same, and while the test case message is STILL(!) incorrect, the issue is not with the test case, but rather just an incorrect solution.

Interesting. My Console.WriteLine must be behaving strangely in the codewars IDE. Thanks for the help.