How do I filter all HTML tags except a certain whitelist?

Attributes are the major problem with using regexes to try to work with HTML. Consider the sheer number of potential attributes, and the fact that most of them are optional, and also the fact that they can appear in any order, and the fact that ">" is a legal character in quoted attribute values. When you start trying to take all of that into account, the regex you'd need to deal with it all will quickly become unmanageable.

What I would do instead is use an event-based HTML parser, or one that gives you a DOM tree that you can walk through.

This is a good working example on html tag filtering:

Sanitize HTML

Here's a function I wrote for this task:

static string SanitizeHtml(string html)
{
    string acceptable = "script|link|title";
    string stringPattern = @"</?(?(?=" + acceptable + @")notag|[a-zA-Z0-9]+)(?:\s[a-zA-Z0-9\-]+=?(?:(["",']?).*?\1?)?)*\s*/?>";
    return Regex.Replace(html, stringPattern, "sausage");
}

Edit: For some reason I posted a correction to my previous answer as a separate answer, so I am consolidating them here.

I will explain the regex a bit, because it is a little long.

The first part matches an open bracket and 0 or 1 slashes (in case it's a close tag).

Next you see an if-then construct with a look ahead. (?(?=SomeTag)then|else) I am checking to see if the next part of the string is one of the acceptable tags. You can see that I concatenate the regex string with the acceptable variable, which is the acceptable tag names seperated by a verticle bar so that any of the terms will match. If it is a match, you can see I put in the word "notag" because no tag would match that and if it is acceptable I want to leave it alone. Otherwise I move on to the else part, where i match any tag name [a-z,A-Z,0-9]+

Next, I want to match 0 or more attributes, which I assume are in the form attribute="value". so now I group this part representing an attribute but I use the ?: to prevent this group from being captured for speed: (?:\s[a-z,A-Z,0-9,-]+=?(?:(["",']?).?\1?))

Here I begin with the whitespace character that would be between the tag and attribute names, then match an attribute name: [a-z,A-Z,0-9,-]+

next I match an equals sign, and then either quote. I group the quote so it will be captured, and I can do a backreference later \1 to match the same type of quote. In between these two quotes, you can see I use the period to match anything, however I use the lazy version *? instead of the greedy version * so that it will only match up to the next quote that would end this value.

next we put a * after closing the groups with parenthesis so that it will match multiple attirbute/value combinations (or none). Last we match some whitespace with \s, and 0 or 1 ending slashes in the tag for xml style self closing tags.

You can see I'm replacing the tags with sausage, because I'm hungry, but you could replace them with empty string too to just clear them out.