Basically, you have a hammer, and you are trying to use it to screw boards together: it might work in some way, but it's not as easy and reliable as using the right tool - a screwdriver!
Regular Expressions are text matching tools, they are great at spotting substrings and have a little limited counting boult in - but what you are doing is moving into the world of grammars and syntaxes - an while you may be able to cobble something together in a Regex that will work for a single example gettin it to work for all possible inputs is really not feasible as they very quickly become unreadable and unmaintainable.
Instead you should be looking to use a Parser:
Parsing in C#: all the tools and libraries you can use[
^] will outline the process and Google will help you fill it in.