HtmlUrlReplacer

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

class HtmlUrlReplacer
{
    static void Main()
    {
        string input = "<p>Please visit <a  target = \"_blank\" href  =  \"   http://academy.telerik. com  \"  >our site<  /  a  > to choose a training course. Also visit <a href=\"www.devbg.org\">our forum</a> to discuss the courses.</p>";
        string result = Regex.Replace(input, @"<a[\s][^>]*\bhref\b[\s]*=[\s]*(""|')[\s]*(([^""'])+?)[\s]*\1[\s]*[^>]*[\s]*>([^<]*)<[\s]*/[\s]*a[\s]*>", "[URL=$2]$4[/URL]");

        Console.WriteLine(result);
    }
}