using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        string input = @"this is a string #PropertyName#

        new line asdasdas #PropertyTwo#

        ";

        string output = Test(input);

        Console.WriteLine(output);

    }

    ///"\{(.*?);(.*?);(.*?)\}"

    private static readonly Regex Pattern = new Regex("#(.*)#", RegexOptions.Compiled);

​

    public static string Test(string input)

    {

        return Pattern.Replace(input, match =>

        {

            string newMatch = match.Groups[1].Value.ToLowerInvariant();

            return newMatch;

        });

    }

}