Regular expression to strip all HTML tags except sub & sup tags

I need to strip all html tags from a string except <sub></sub> & <sup></sup> tags.

Suppose if a string is "<p>Hello world&开发者_Python百科lt;/p> <span class="required">H<sub>2</sub>O</span>"

Output should be Hello world H<sub>2</sub>O

Rather than using regular expressions, you may find it easier to use something like Html Agility Pack which gives you a read/write DOM to play with.