using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {   

        string[] strs = {

            "HelloWorld",

            "HelloManyPeopleOfTheWorld",

            "ID",

            "ContactID",

            "No.Five",

            "URL",

            "ImageURL",

            "FTPHost",

            "ClientFTP",

            "SMTPServer"

        };

        foreach (string str in strs)

        {

            Console.WriteLine("str: " + str);

            string a = ToSentenceCaseA(str);

            string b = ToSentenceCaseB(str);

            Console.WriteLine("a:   " + a);

            Console.WriteLine("b:   " + b);

            Console.WriteLine("\n\n");

        }

    }

    public static string ToSentenceCaseA(string str)

    {

        // If string is all uppercase with no space, return itself. 

        if (str.ToUpper().Replace(" ", "") == str)

        {

            return str;

        }

​

        return Regex.Replace(str, "(\\B[A-Z]+)", " $1".ToLower());

    }

    public static string ToSentenceCaseB(string str)

    {

        // If string is all uppercase with no space, return itself. 

        if (str.ToUpper().Replace(" ", "") == str)

        {

            return str;

        }

​

        return Regex.Replace(str, "(\\B[A-Z]+)", m => " " + m.ToString().ToLower());

    }

}