> While I can't speak for the original motivation from many moons ago, .Trim() still doesn't trim \u200c. It's useful in most cases, but not the complete strip we need here.

This would have probably been my train of thought (assuming that I consider regex to be a valid solution):

Trim() would have been the correct solution, were it not for that behavior. Substring is therefore the correct solution. Problem is, IndexOf only accepts a char array (not a set of some form, i.e. HashSet). You'd need to write the <Last>IndexOfNonWhitespace methods yourself. Use a regex and make sure that it doesn't backtrace, because it's expressive and regex "is designed to solve this type of problem." The real problem/solution here isn't substring, it's finding where to substring.

I consider regex too dangerous to use in any circumstance, but I can certainly see why someone would find it attractive at first.

[1]: https://www.reddit.com/r/programming/comments/4tt6ce/stack_e...