using System;
using System.Text.RegularExpressions;

public class Example
{
   public static void Main()
   {
      string input = "1851 1999 1950 1905 2017";
      string pattern = @"(?<=19)\d{2}\b";

      foreach (Match match in Regex.Matches(input, pattern))
         Console.WriteLine(match.Value);
   }
}