using System;
using System.Text.RegularExpressions;
public class Test
{
	public static void Main()
	{
		 string text = "UserFirstName";
		 int SkipIndex = 5;
		 string pattern = $@"(?!^)(?<!^.{{{SkipIndex-1}}}).";
		 Console.WriteLine(Regex.Replace(text, pattern, "*"));
		 
	}
}