for that you can’t use Regex. For that you will have to use standard bolcks.
I guess y ou can request @Pseudonium to include something for such a case.
I would rather suggest a different approach. Easier and something ANKI Manual and users suggest as well. Read here in the manual.
Basically, have limited decks. Maybe just one or two. And you can create custom decks using tags. And tagging is now really easy thanks to @Pseudonium. any tag around your card is now identified as an ANKI tag plus a global files tag or a folder tag option. I have moved on to this method as it was getting weary to add deck names on all my notes (some very small (atomic?). I made out tags that I would have used as decks and used them to create custom studies.