using System;
using System.Text.RegularExpressions;
public class Test
{
	public static void Main()
	{
		var text = "Look at http://google.com some more text here possibly,\nLook at www.google.com some more text here possibly";
		text = text.Trim();
		text = Regex.Replace(text,
        @"((https?|ftp)://(?:www\.|(?!www))[^\s.]+\.\S{2,}|www\.\S+\.\S{2,})", m => 
        m.Groups[2].Success ? 
           string.Format("<a target='_blank' href='{0}'>{0}</a>", m.Groups[1].Value) :
           string.Format("<a target='_blank' href='http://{0}'>{0}</a>", m.Groups[1].Value));
        Console.WriteLine(text);
	}
}