Twitter card auto generator

TwitterCard.cs

using System;
using System.Text.RegularExpressions;
using System.Web;
using System.Web.UI.HtmlControls;
using BlogEngine.Core;
using BlogEngine.Core.Web.Controls;
using BlogEngine.Core.Web.Extensions;

namespace BlogEngine.NET.Custom.Extensions
{
     [Extension("Twitter card auto generator", "1.0.0", "Adrian Cini")]
    public class TwitterCard
    {
         public TwitterCard()
        {
            if (!ExtensionManager.ExtensionEnabled("TwitterCard"))
                return;

            Post.Serving += new EventHandler<ServingEventArgs>(Post_Serving);
        }

         /*Sample of Twitter card
          * <meta name="twitter:card" content="summary" />
            <meta name="twitter:site" content="@flickr" />
            <meta name="twitter:title" content="Small Island Developing States Photo Submission" />
            <meta name="twitter:description" content="View the album on Flickr." />
            <meta name="twitter:image" content="https://farm6.staticflickr.com/5510/14338202952_93595258ff_z.jpg" />
          */
         public void Post_Serving(object sender, ServingEventArgs e)
         {
             //during the loading of a full post
             if (e.Location == ServingLocation.SinglePost)
             {
                 //On page seo for posts only
                 var post = sender as BlogEngine.Core.Post;
                 if (post != null)
                 {
                     HttpContext context = HttpContext.Current;
                     if (context != null)
                     {
                         System.Web.UI.Page page = context.CurrentHandler as System.Web.UI.Page;
                         if (page != null)
                         {
                             //Add twitter card schema to the page html header 

                             HtmlMeta TwitterCardSummary = new HtmlMeta();
                             TwitterCardSummary.Attributes.Add("name", "twitter:card");
                             TwitterCardSummary.Attributes.Add("content", post.Description);
                             page.Header.Controls.Add(TwitterCardSummary);

                             HtmlMeta TwitterCardSite = new HtmlMeta();
                             TwitterCardSite.Attributes.Add("name", "twitter:site");
                             TwitterCardSite.Attributes.Add("content", post.AbsoluteLink.ToString());
                             page.Header.Controls.Add(TwitterCardSite);

                             HtmlMeta TwitterCardTitle = new HtmlMeta();
                             TwitterCardTitle.Attributes.Add("name", "twitter:title");
                             TwitterCardTitle.Attributes.Add("content", post.Title);
                             page.Header.Controls.Add(TwitterCardTitle);

                              HtmlMeta TwitterCardDescription = new HtmlMeta();
                             TwitterCardDescription.Attributes.Add("name", "twitter:description");
                             TwitterCardDescription.Attributes.Add("content", post.Description);
                             page.Header.Controls.Add(TwitterCardDescription);

                              HtmlMeta TwitterCardImage = new HtmlMeta();
                             TwitterCardImage.Attributes.Add("name", "twitter:image");
                             TwitterCardImage.Attributes.Add("content", GetFirstImageInArticle(post));
                             page.Header.Controls.Add(TwitterCardImage);



                         }
                     }
                 }
             }
         }

           private string GetFirstImageInArticle(BlogEngine.Core.Post thisPost)
        {
            var htmlString = thisPost.Content;
            //<\s*img [^\>]*src\s*=\s*(["\'])(.*?)\1
            string pattern = @"<\s*img [^\>]*src\s*=\s*([";
              pattern +=  "\"\\'])(.*?)\\1" ;
               
            Regex rgx = new Regex(pattern, RegexOptions.IgnoreCase);
            MatchCollection matches = rgx.Matches(htmlString);
            if (matches.Count > 0)
            {
                var firstImage = matches[0].ToString();
                string[] tmp = firstImage.Split('"');
                string relPath = tmp[1].Trim();
                string absPath = HttpContext.Current.Server.MapPath(relPath);
                return absPath;
            }
            return "";


        }
    }
}