Question

How do replace Every UpperCase Letter with Underscore and the Letter in C#?
note: unless the character is already proceeded by a underscore.

UPDATE:
For example,
MikeJones
would be turned into
Mike_Jones

But Mike_Jones
would not be turned into
Mike__Jones

Is Regex the best approach? Where do I start with this one?

Answer 1

Regex sounds best:

string input = "Test_StringForYou";
string replaced = Regex.Replace(input, @"(?<!_)([A-Z])", "_$1");
Console.WriteLine(replaced);

Output: _Test_String_For_You

Be sure to add a using System.Text.RegularExpressions;