using System;

using System.Text.RegularExpressions;

​

namespace Demo

{

    public static class RegexForYear

    {

        public static void Main()

        {

            var text =

                "This is a string that has a 2 digit year - 15 4 digit year 2015 and from date 01/01/2015 to date 02/03/2016";

​

            var regex = new Regex("\\b(?<prefix>\\d{2}/\\d{2}/)?(?<year>\\d{2}|\\d{4})\\b");

            var result = regex.Replace(text, match => string.Format("{0}{1}", match.Groups["prefix"].Value, int.Parse(match.Groups["year"].Value) + 1));

            Console.WriteLine(result);

        }

    }

}