string end = string.Empty;
string inputHttp = @"https:\/\/pp.userapi.com\/c841226\/v841226735\/72c15\/CrqCyt7DhCA.jpg";
var result = inputHttp.Split('\\');
foreach (var tempFor in result)
{
end += tempFor;
}
var htmlString = ...
var withoutTags = Regex.Replace(htmlString, @"<.*?>", ""); // удаляем теги
var withoutSpaces = Regex.Replace(withoutTags, @"\s+", " "); // удаляем пробелы