using System;
using System.Text.RegularExpressions;
using System.Linq;
public class Test
{
	public static void Main()
	{
		var source = "@fish and fries and @Mary's beer are @good";
        var res = Regex.Matches(source, @"(?<!\S)@([a-zA-Z0-9-_]+)")
        	.Cast<Match>()
        	.Select(m=>m.Groups[1].Value)
        	.ToList();
        Console.WriteLine(string.Join("; ", res));
	}
}