using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.Threading.Tasks;

using Newtonsoft.Json;

using System.Text.RegularExpressions;

​

namespace ConsoleApp16

{

    public class Program

    {

        public static void Main(string[] args)

        {

           var reg = new Regex("(?<=STRING ).*?(?= END)");

           var matched = reg.Matches("This is STRING a222 END, and this is STRING b2838 END, STRING a123 END.");

​

           foreach(var m in matched)

           {

               Console.WriteLine(m.ToString());

           }

        }

    }

}