Restrict IFRAME and OBJECT embeds certain domains
Allowing users to input , and doing validation on some subtypes of that, is certainly possible, but far from easy.

It's easy to think, "I can do that with a regular expression!". Read This SO question though, before you try, and save yourself a lot pain.

What you can do instead, is employ an XML parser and process the input from there, but that is far from trivial, and will require quite a bit of work.

What I recommend you should do instead, is create specific tags for what you want to allow. Something like [iframe:http://some.url.here], and then process these manually with an input filter. You would probably want to apply that filter in a text-format after you've run the regular "limited html" filter.

A filter is created by implementing hook_filter_info, and in your process callback you can do string substitution on the [iframe] tag, provided it references an approved domain.