Htmlstring = Regex.Replace(Htmlstring, @"(?is)<script\b[^>]*>(.*?)</script>", string.Empty, RegexOptions.IgnoreCase);

------解决方案--------------------
   string js = @"asdf
             <script>
             alert
             </script>
asdfads
<Script type=''>alert<script>asdfasdf
         ";
        js= Regex.Replace(js,@"\<\s*script(\s|.)*?script\s*\>","",RegexOptions.IgnoreCase);