using System;
using System.Text.RegularExpressions;

namespace ConsoleApp29
{
    class Program
    {
        static void Main(string[] args)
        {
            string html = "<a href=\"http://localhost\" />移除html標簽<a>";
            Console.WriteLine(html);
            Console.WriteLine(RemoveHtml(html));

            Console.Read();
        }

        /// <summary>
        /// Remove HTML tags
        /// </summary>
        /// <param name="input">input</param>
        /// <returns></returns>
        static string RemoveHtml(string input)
        {
            if (string.IsNullOrEmpty(input)) { return ""; }

            // 1
            input = Regex.Replace(input, @"&(quot|#34);", "\"", RegexOptions.IgnoreCase);
            input = Regex.Replace(input, @"&(amp|#38);", "&", RegexOptions.IgnoreCase);
            input = Regex.Replace(input, @"&(lt|#60);", "<", RegexOptions.IgnoreCase);
            input = Regex.Replace(input, @"&(gt|#62);", ">", RegexOptions.IgnoreCase);
            input = Regex.Replace(input, @"&(nbsp|#160);", " ", RegexOptions.IgnoreCase);
            input = Regex.Replace(input, @"&(iexcl|#161);", "\xa1", RegexOptions.IgnoreCase);
            input = Regex.Replace(input, @"&(cent|#162);", "\xa2", RegexOptions.IgnoreCase);
            input = Regex.Replace(input, @"&(pound|#163);", "\xa3", RegexOptions.IgnoreCase);
            input = Regex.Replace(input, @"&(copy|#169);", "\xa9", RegexOptions.IgnoreCase);
            input = Regex.Replace(input, @"&#(\d+);", "", RegexOptions.IgnoreCase);

            // 2
            input = Regex.Replace(input, @"<script[^>]*?>.*?</script>", "", RegexOptions.IgnoreCase);
            input = Regex.Replace(input, @"<(.[^>]*)>", "", RegexOptions.IgnoreCase);
            input = Regex.Replace(input, @"([\r\n])[\s]+", "", RegexOptions.IgnoreCase);
            input = Regex.Replace(input, @"-->", "", RegexOptions.IgnoreCase);
            input = Regex.Replace(input, @"<!--.*", "", RegexOptions.IgnoreCase);

            return input;
        }
    }
}