title: C# equivalent of C++ std::string find_first_not_of and find_last_not_of
body: <p><code>IndexOf</code>, <code>IndexOfAny</code> and <code>LastIndexOf</code>, <code>LastIndexOfAny</code> dont seem to do these (or maybe they do). I'm looking for the equialent of std::string's <code>find_first_not_of</code> and <code>find_last_not_of</code>. I'm thinking about creating an extension class but I'm not sure if C# already provides this functionality.</p>