using System;
using System.Text.RegularExpressions;
public class Test
{
	public static void Main()
	{
		string pattern = @"(?<={{)\w+(?=}})";
        string input = @"{{aaa}}";
        
        foreach (Match m in Regex.Matches(input, pattern))
        {
            Console.WriteLine("'{0}' found at index {1}.", m.Value, m.Index);
        }
	}
}