using System;

using System.Text.RegularExpressions;

​

public class Solution

{

    public static void Main(String[] args)

    {

        string sample = "where dog is and cats are and bird is bigger than a mouse";

        MatchCollection matches = Regex.Matches(sample, @"(?<=(where|and)\s)(?<gr>.+?)(?=(and|$))");

        foreach (Match m in matches)

        {

            Console.WriteLine(m.Value.ToString());

        }

    }

}

​