ReplaceHTML.cs

using System;
using System.Text.RegularExpressions;

class ReplaceHTML
{
    static void Main()
    {
        string text = @"<p>Please visit <a 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>";

        foreach (Match match in Regex.Matches(text, "<a href=\"(.*?)\">(.*?)</a>"))
        {
            string link = match.Groups[1].Value;
            string word = match.Groups[2].Value;

            text = Regex.Replace(text, "<a href=\"" + link + "\">" + word + "</a>", "[URL=" + link + "]" + word + "[/URL]");
        }
        Console.WriteLine(text);
    }
}