Weird c# Regex behaviour

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Text.RegularExpressions;

using TestSupportService.ServiceReference;

namespace TestSupportService
{
    class Program
    {
        static void Main(string[] args)
        {

            String example = "I have four child of seven years each, [seven] years ago I had no child, because I was fourteen";
            Regex rexWillDo = new Regex(@"\bfour\b|\[\bseven\b\]");
            Regex rexWontDo = new Regex(@"\bfour\b|\b\[seven\]\b");

            Console.WriteLine("Now you see it!");
            MatchCollection matches = rexWillDo.Matches(example);
            foreach (Match match in matches)
            {
                Console.WriteLine(match.Value);
            }

            Console.WriteLine("\nAnd now you don't!");
            matches = rexWontDo.Matches(example);
            foreach (Match match in matches)
            {
                Console.WriteLine(match.Value);
            }
            Console.ReadLine();

        }
    }
}