using System;

using System.Text.RegularExpressions;       

​

public class Program

{

    public static void Main()

    {

        // Your string

        var formatedString = "<newNameSpace.HeresMyString>100000000000</newNameSpace.HeresMyString>";

        formatedString = Regex.Replace( formatedString , "<.+HeresMyString>[0-14]+([0-14]{8})</.+HeresMyString>" , "<b.HeresMyString>****************$1</b.HeresMyString>" , RegexOptions.IgnoreCase );

​

        Console.WriteLine(formatedString);

    }

}