Extracts the first image Url from a html string

gistfile1.cs

/// <summary>Regular expression to get image urls from a HTML string</summary>
private const string STR_IMGTAG_SRC_EXP = @"<img\s+[^>]*\bsrc\s*\=\s*[\x27\x22](?<Url>[^\x27\x22]*)[\x27\x22]";

/// <summary>
/// Extracts the first image Url from a html string
/// </summary>
/// <param name="htmlString">A string containing html code</param>
/// <returns>a collection with the image Urls contained in htmlString parameter</returns>
/// <remarks>This method uses regular expressions,so using System.Text.RegularExpressions; 
/// must be addeed</remarks>
public static List<string> ExtractImageUrisFromHtml(string htmlString)
{

    var rgx = new Regex(STR_IMGTAG_SRC_EXP,
                        RegexOptions.IgnoreCase | RegexOptions.Multiline);

    var lista = new List<string>();
    var matches = rgx.Matches(htmlString);

    foreach (Match match in matches)
    {
        var url = match.Groups["Url"].Value;
        if (!string.IsNullOrWhiteSpace(url))
        {
            lista.Add(
            match.Groups["Url"].Value);
        }
    }
    return lista;
}

/// <summary>Default Uri</summary>
private const string TEMPURI = "http://tempuri.org";

/// <summary>
/// Extracts the first image Url from a html string
/// </summary>
/// <param name="htmlString">A string containing html code</param>
/// <returns>a string with the Url or first image in the htmlString parameter</returns>
/// <remarks>This method uses regular expressions,so using System.Text.RegularExpressions; must be addeed</remarks>
public static string ExtractFirstHtmlImage(string htmlString)
{
    string respuesta = TEMPURI;
    try
    {
        var rgx = new Regex(
            STR_IMGTAG_SRC_EXP,
                        RegexOptions.IgnoreCase | RegexOptions.Multiline);

        var match = rgx.Match(htmlString);

        respuesta = match.Groups["Url"].Value;

        if (respuesta == "")
            respuesta = TEMPURI;
    }
    catch { respuesta = TEMPURI; }

    return respuesta;
}