using System;

using System.Text.RegularExpressions;

public static class Program

{

    public static void Main()

    {

        var str = "asdasda <row>hai</row>";

        Console.WriteLine(str.Between("<row>", "</row>"));

    }

    public static string Between(this string source, string left, string right)

        {

            return Regex.Match(

                    source,

                    string.Format("{0}(.*){1}", left, right))

                .Groups[1].Value;

        }

}