HTML Strip Tags with Attributes

Here is a more robust way to strip tags from HTML

using System;
using System.Text.RegularExpressions;

namespace  WR
{
  public class HtmlStripper
{

        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;
}


}