using System;

using System.Text.RegularExpressions;

using System.Globalization;

​

namespace Rextester

{

    public class Program

    {

        public static void Main(string[] args)

        {

            NumberFormatInfo nfi1 = new CultureInfo("en-US").NumberFormat;

            nfi1.NumberGroupSeparator = ",";

            nfi1.NumberNegativePattern = 2;

            nfi1.NumberGroupSizes = new int[] { 3 };

            string pattern = @"(,)\s*";

            var input = "0.12345678901234511e15";                       

            decimal result = Convert.ToDecimal(Regex.Replace(input, pattern, "$1"), nfi1);

            Console.WriteLine(result);

        }

    }

}