using System;
using System.Text.RegularExpressions;

public class Test
{
    const string Str = "30-Nov-2012 30-Nov-2012 United Kingdom, 31-Oct-2012 31-Oct-2012 United Arab Emirates, 29-Oct-2012 31-Oct-2012 India";
    static readonly Regex r1 = new Regex("(\\d{4})\\s");
	static readonly Regex r2 = new Regex("(?<=\\d{4})\\s");
    static void TestRegex(Regex r, string s) {
        DateTime start = DateTime.Now;
        for (var i = 0 ; i != 10000 ; i++) {
            r.Replace(Str, s);
        }
        Console.WriteLine(DateTime.Now-start);
    }
    public static void Main()
	{
		TestRegex(r1, "$1@");
    	TestRegex(r2, "@");
    	TestRegex(r1, "$1@");
    	TestRegex(r2, "@");
    	TestRegex(r1, "$1@");
    	TestRegex(r2, "@");
    	TestRegex(r1, "$1@");
    	TestRegex(r2, "@");
	}
}