using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        Regex startOfSequenceRegex = new Regex("^Herr"); //^ denotes the beginning of a string in Regular Expressions

        string origin = "Herr Herrman von Herremacher";

        var newText = startOfSequenceRegex.Replace(origin, "Herrn",1);

        Console.WriteLine(newText);

    }

}