"Why the (?: ) around the space character? It runs just fine as below?"

It does work well and is less complex to explain. I use the (?: ) to show the practice of not capturing matches into memory which wont be used in the replacement. In this specific example memory isn't going to be a problem because we're only dealing with a single string. But I personally like using it even as a way to mark what I want and don't want to work with in the replacement.