Dear Stata experts,

For my master thesis, I'm figuring out how to analyse in order to measure environmental innovation patents.
I'm using IPC classes from the NBER dataset. But I struggle to prepare my data so if anyone can help me would be a blessing!

I want to keep a certain amount of different IPC classes which I use as environmental patents. But this means I need to get rid of all the other observations(patent types /IPC classes).
So I tried to do this with drop if, egen, bysort egen, but it all didn't work sadly. The tricky part is the IPC classes are string variables. I tried to use destring, but due to non-numeric observations (B01C example of a class) i can't use Destring or generate. I tried to fix this with force but this only gerenates dots, so I'm kinda lost how to proceed.

I need to sort and keep only a few specified IPC classes so I can merge that prepared data set with other data sets.

Thanks in advance and sorry if this is kind of a beginner/easy question.