Hi
Not sure what you are asking for here. You have the user select a file from a dialog which has that filter set for it - then the filename returned will have the extension and in turn, it is simple to get the extension from that filename.
Where, in that process do you want to parse the filter? What reason to bother to parse it (presumably you set up the filter in your code in the first place)?
The filter, being a simple string can be manipulated quite easily as would any sting.