using System;
using System.Linq;
using System.Text.RegularExpressions;
public class Test
{
	public static void Main()
	{
		var re = @"aa((?:(?!aa).)*?)kk"; 
		var str = "aa aa value kk 8718764 aa value1 kk kk kk 5178gkjh aathtkhkk"; 
 		var res = Regex.Matches(str, re)
 			.Cast<Match>()
 			.Select(p => p.Groups[1].Value)
 			.ToList();
		Console.WriteLine(string.Join("\n", res));
	}
}