Here's a regex that will max out your CPU (just one)

failregex.cs

using System;
using System.Text.RegularExpressions;

namespace RegexFail
{
    class Program
    {
        static void Main(string[] args)
        {
            var urlPattern = new Regex(@"(?i)\b((?:[a-z][\w-]+:(?:/{1,3}|[a-z0-9%])|www\d{0,3}[.]|[a-z0-9.\-]+[.][a-z]{2,4}/)(?:[^\s()<>]+|\(([^\s()<>]+|(\([^\s()<>]+\)))*\))+(?:\(([^\s()<>]+|(\([^\s()<>]+\)))*\)|[^\s`!()\[\]{};:'"".,<>?«»“”‘’]))", RegexOptions.Compiled | RegexOptions.IgnoreCase);

            string message = "http://developer.android.com/reference/android/app/Activity.html#onCreateView(android.view.View,%20java.lang.String,%20android.content.Context,%20android.util.AttributeSet)";

            foreach (Match m in urlPattern.Matches(message))
            {
                Console.WriteLine(m.Value);
            }
        }
    }
}