Regex to Remove HTML Tags

htmlremover.cs

@inherits PartialViewMacroPage
@using Umbraco.Cms.Web
@using Umbraco.Cms.Web.Macros
@using Umbraco.Framework
@using System.Text.RegularExpressions

@functions {
   String HtmlRemover(string html)
   {
      //Source: http://stackoverflow.com/questions/307013/how-do-i-filter-all-html-tags-except-a-certain-whitelist

      string acceptable = "p|a"; //Tags that are a-okay to display
      string stringPattern = @"</?(?(?=" + acceptable + @")notag|[a-zA-Z0-9]+)(?:\s[a-zA-Z0-9\-]+=?(?:(["",']?).*?\1?)?)*\s*/?>";
      string replacement = Regex.Replace(html, stringPattern, "");

      return replacement;
   }
}

If accept "i",
"iframe" didnt remove