using System;

using System.Linq;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        string option1 = "nijdansd8z2hebdzxunasicuas dhasu8du jas 15.23 diuh09jh2u98djq 0909";

​

var doublevalue = Regex.Split(option1, @"[^0-9\.]+").Where(c => c.Contains(".") && c.Trim() != "").FirstOrDefault();

        Console.WriteLine(doublevalue);

    }

}