using System;

using System.Linq;

using System.Text.RegularExpressions;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        var text = "PartySiteEO_AddressLine1";

        var formatText = GetFormattedName(text);

        Console.WriteLine(formatText.ToUpper());

        var result = string.Concat(GetFormattedName2(text));

        Console.WriteLine(result.ToUpper());

    }

    public static string GetFormattedName(string originalText)

    {       

        var invalidCharsRgx = new Regex("[^_a-zA-Z0-9]");

        var whiteSpace = new Regex(@"(?<=\s)");

        var startsWithLowerCaseChar = new Regex("^[a-z]");

        var firstCharFollowedByUpperCasesOnly = new Regex("(?<=[A-Z])[A-Z0-9]+$");

        var upperCaseInside = new Regex("(?<=[A-Z])[A-Z]+?((?=[A-Z][a-z])|(?=[0-9]))");

        var putUndescoreBeforeUpperCase = new Regex(@"(?<!_|^)([A-Z])");

​

        // replace white spaces with undescore, then replace all invalid chars with empty string

        var text = invalidCharsRgx.Replace(whiteSpace.Replace(originalText, "_"), string.Empty)

        // split

         .Split(new char[] { '_' }, StringSplitOptions.RemoveEmptyEntries)

        // set first letter to uppercase

        .Select(w => startsWithLowerCaseChar.Replace(w, m => m.Value.ToUpper()))

        // replace second and all following upper case letters to lower if there is no next lower (ABC -> Abc)

        .Select(w => firstCharFollowedByUpperCasesOnly.Replace(w, m => m.Value.ToLower()))

        // lower second and next upper case letters except the last if it follows by any lower (ABcDEf -> AbcDef)

        .Select(w => upperCaseInside.Replace(w, m => m.Value.ToLower()));

​

        var resultText = string.Concat(text);

        resultText = putUndescoreBeforeUpperCase.Replace(resultText, "_$1");

        return resultText;

    }

    public static IEnumerable<char> GetFormattedName2(string formattedName)

        {

            bool isFirstCharacter = true;

            bool isPreviousUpper = false;

​

            foreach(char c in formattedName)

            {

                if(Char.IsUpper(c) && !isPreviousUpper)

                {

                    if(!isFirstCharacter && !isPreviousUpper) yield return '_'; 

                    isFirstCharacter = false;

                }

​

                yield return c;

                isPreviousUpper = Char.IsUpper(c) || c == '_';

            }

​

        }

}