Convert Rtf Tag To Html Tag

using vb.net, i have a richtextbox in my application. i would like to have the emphasis to have an html tags.for example /b this is a test /b0 would be <bold>this is a test</bold> and it's simple, but the most difficult part for me is when the words have 2 emphasis /b/i this is a test /b0 /i0 the output becomes <bold><italic>this is a test</bold></italic> this will have an error in parsing. then output should be <bold><italic>this is a test</italic></bold>.