using System;

using  System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        string resultString = Regex.Match("1984 test as ", @"\d+").Value;

        Console.WriteLine(resultString);

        resultString = Regex.Match("ass1984 test as ", @"\d+").Value;

        Console.WriteLine(resultString);

​

    }

}