using System;

using System.Linq;

using System.Collections.Generic;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        var lno = "[StringLength(100, ErrorMessage = \"The {0} must be at least {2} characters long.\", MinimumLength = 6)]";

        var pattern = @"The \{0\} must be at least \{2\} characters long\.";

        string _name = Regex.Replace(lno, pattern, "TestString", RegexOptions.None);

        Console.WriteLine(_name);

    }

}