using System;

using HtmlAgilityPack;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        var html = @"text

        <p>some text and again some texts

        http://link.out/  normal text</p>

        <a href=""http://link.out"">link</a>";

        var doc = new HtmlDocument();

        doc.LoadHtml(html);

        Regex r = new Regex(@"(https?://[^\s]+)");

        var textNodes = doc.DocumentNode.SelectNodes("//text()");

        foreach (var textNode in textNodes) {

            var text = textNode.GetDirectInnerText();

            var withLinks = r.Replace(text, "<a href=\"$1\">$1</a>");

            textNode.InnerHtml = withLinks;

        }

        Console.WriteLine(doc.DocumentNode.InnerHtml);

    }

}