I wrote the same code but for some reason my test kept insisting I always returned 0 xD I even tried just returning elements of the input but it still said 0 every time.

What difference does that make? the input isn't nullable and the method Replace works fine with an empty string.

you might have forgotten to write Replace with a capital R.

That shouldn't make a difference? Array.Length is a property with O(1) constant lookup time.