using System;

using System.Text.RegularExpressions;           

public class Program

{

    public static void Main()

    {

        var tests = new[] {

            new { Input="abcdef", Search="abc", Replacement="xyz", Expected="xyzdef" },

            new { Input="ABCdef", Search="abc", Replacement="xyz", Expected="xyzdef" },

            new { Input="A*BCdef", Search="a*bc", Replacement="xyz", Expected="xyzdef" },

            new { Input="abcdef", Search="abc", Replacement="x*yz", Expected="x*yzdef" },       

            new { Input="abcdef", Search="abc", Replacement="$", Expected="$zdef" },

        };

        foreach(var test in tests){

            var result = ReplaceCaseInsensitive(test.Input, test.Search, test.Replacement);

            Console.WriteLine(

                "Success: {0}, Actual: {1}, {2}",

                result == test.Expected,

                result,

                test

            );

        }

    }

    private static string ReplaceCaseInsensitive(string input, string search, string replacement){

        string result = Regex.Replace(

            input,

            Regex.Escape(search), 

            replacement.Replace("$","$$"), 

            RegexOptions.IgnoreCase

        );

        return result;

    }

}