/* Usage 
string test1 = StripTags("<p>George</p><b>W</b><i>Bush</i>", new string[]{"i","b"});
string test2 = StripTags("<p>George <img src='someimage.png' onmouseover='someFunction()'>W <i>Bush</i></p>", new string[]{"p"});
string test3 = StripTags("<a href='http://www.dijksterhuis.org'>Martijn <b>Dijksterhuis</b></a>", new string[]{"a"});

*/
 
 
 
 
 
using System;
using System.Text.RegularExpressions;
 
namespace StripHTML
{
    class MainClass
    {
 
        private static string ReplaceFirst(string haystack, string needle, string replacement)
        {
               int pos = haystack.IndexOf(needle);
            if (pos < 0) return haystack;
            return haystack.Substring(0,pos) + replacement + haystack.Substring(pos+needle.Length);
        }
 
        private static string ReplaceAll(string haystack, string needle, string replacement)
        {
             int pos;
             // Avoid a possible infinite loop
             if (needle == replacement) return haystack;
              while((pos = haystack.IndexOf(needle))>0)
                       haystack = haystack.Substring(0,pos) + replacement + haystack.Substring(pos+needle.Length);
                        return haystack;
        }        
 
        public static string StripTags(string Input, string[] AllowedTags)
        {
            Regex StripHTMLExp = new Regex(@"(<\/?[^>]+>)");
            string Output = Input;
 
            foreach(Match Tag in StripHTMLExp.Matches(Input))
            {
                string HTMLTag = Tag.Value.ToLower();
                bool IsAllowed = false;
 
                foreach(string AllowedTag in AllowedTags)
                {
                    int offset = -1;
 
                    // Determine if it is an allowed tag
                    // "<tag>" , "<tag " and "</tag"
                    if (offset!=0) offset = HTMLTag.IndexOf('<'+AllowedTag+'>');
                    if (offset!=0) offset = HTMLTag.IndexOf('<'+AllowedTag+' ');
                    if (offset!=0) offset = HTMLTag.IndexOf("</"+AllowedTag);
 
                    // If it matched any of the above the tag is allowed
                    if (offset==0)
                    {
                         IsAllowed = true;
                        break;
                    }
                }
 
                // Remove tags that are not allowed
                if (!IsAllowed) Output = ReplaceFirst(Output,Tag.Value,"");
            }
 
            return Output;
        }
 
        public static string StripTagsAndAttributes(string Input, string[] AllowedTags)
        {
            /* Remove all unwanted tags first */
            string Output = StripTags(Input,AllowedTags);
 
            /* Lambda functions */
            MatchEvaluator HrefMatch = m => m.Groups[1].Value + "href..;,;.." + m.Groups[2].Value;
            MatchEvaluator ClassMatch = m => m.Groups[1].Value + "class..;,;.." + m.Groups[2].Value;
            MatchEvaluator UnsafeMatch = m => m.Groups[1].Value + m.Groups[4].Value;
 
            /* Allow the "href" attribute */
            Output = new Regex("(<a.*)href=(.*>)").Replace(Output,HrefMatch);
 
            /* Allow the "class" attribute */
            Output = new Regex("(<a.*)class=(.*>)").Replace(Output,ClassMatch);
 
            /* Remove unsafe attributes in any of the remaining tags */
            Output = new Regex(@"(<.*) .*=(\'|\""|\w)[\w|.|(|)]*(\'|\""|\w)(.*>)").Replace(Output,UnsafeMatch);
 
            /* Return the allowed tags to their proper form */
            Output = ReplaceAll(Output,"..;,;..", "=");
 
            return Output;
        }
 
        public static void Main(string[] args)
        {
            string test1 = StripTags("<p>George</p><b>W</b><i>Bush</i>", new string[]{"i","b"});
            string test2 = StripTags("<p>George <img src='someimage.png' onmouseover='someFunction()'>W <i>Bush</i></p>", new string[]{"p"});
            string test3 = StripTags("<a href='http://www.dijksterhuis.org'>Martijn <b>Dijksterhuis</b></a>", new string[]{"a"});
 
            Console.WriteLine(test1);
            Console.WriteLine(test2);
            Console.WriteLine(test3);
 
            string test4 = "<a class=\"classof69\" onClick='crosssite.boom()' href='http://www.dijksterhuis.org'>Martijn Dijksterhuis</a>";
            Console.WriteLine(StripTagsAndAttributes(test4, new string[]{"a"}));
        }
    }
}