This requires a positive lookahead to include the uppercase separators:
using System;
using System.Text.RegularExpressions;
class Program
{
static void Main(string[] args)
{
string r = @"(?=[A-Z])";
foreach (string s in Regex.Split ("oneTwoThree", r))
Console.Write (s + " "); // one Two Three
}
}
The output:
one Two Three