Is there a way to look at past regionals to see what teams received yellow and red cards?
The FRC Events API can show you if a team was DQ’d in a match (Under the Match Results endpoint), but I don’t think it says anything specifically about red or yellow cards. I don’t know of any other way to get that sort of information. To use the API, you’ll have to register on Teamforge.
And a DQ can come without a red card (though no red card will show up without a DQ).