using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        Console.WriteLine("-----");

        //string html = "<p>This is a text with a link to https://example.com and another URL, but this one already has an <a href=\"https://existing-link.com\">href</a>.</p>";

        //html += "<p>This is a text with a link to http://google.com and another URL, but this one already has an <a href=\"http://google.com\">href</a>.</p>";

        //string html = "<p class=\"MsoNormal\"><a href=\"http://203.150.142.10/siam-synergy-test/docs/Portal.aspx\" moz-do-not-send=\"true\" class=\"moz-txt-link-freetext\">http://203.150.142.10/siam-synergy-test/docs/Portal.aspx</a><o:p></o:p></p>";

        string html = "<p class=\"MsoNormal\"><a href=\"http://www.google.com\" moz-do-not-send=\"true\">www.yahoo.com</a><o:p></o:p></p>";

        // Define a regular expression to find URLs not already in links

        string pattern = @"(?<!href="")\b(?:https?://|ftp://)\S+\b";

        //string pattern = @"(?<![""'])\b(?!(?:https?|ftp):\/\/)(?:https?|ftp):\/\/[-a-zA-Z0-9+&@#\/%?=~_|!:,.;]*[-a-zA-Z0-9+&@#\/%=~_|]";

​

        // Replace URLs with links

        string result = Regex.Replace(html, pattern, match =>

        {

            string url = match.Value;

            int index = url.IndexOf('<'); // Find the position of '<'

​

            if (index != -1)

            {

                url = url.Substring(0, index);

            }

            Console.WriteLine(url);

            Console.WriteLine("-----");

            return "<a href=\"" + url + "\">" + url + "</a>";

        });

​

        Console.WriteLine(result);

    }

}