using System;

using System.Linq;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        var text = "Border Low P(sub)a(/sub) and (sub)b(/sub)";

​

        Regex regex = new Regex(@"(?<=\(sub\)).*?(?=\(\/sub\))");

​

        var v = regex

          .Matches(text)

          .Cast<Match>()

          .Select(match => match.Value)

          .ToArray();

        Console.WriteLine(string.Join(", ", v));

    }

}