using System;
using System.Text.RegularExpressions;

public class Test
{
	public static void Main()
	{
		string myString = "words words words FIRSTPHRASE these words I want SECONDPHRASE but not these words";
		string theWordsIWant = Regex.Replace(myString, @"^.*?FIRSTPHRASE\s*(.*?)\s*SECONDPHRASE.*$", "$1");
		Console.WriteLine(theWordsIWant);
	}
}