Here's the actual federal Rule concerning carry-on instruments:
https://www.transportation.gov/brief...travel-musical
Keep in mind that regional jets do not have large overhead bins, and any overhead space is first-come, first-served. It pays to get priority boarding when you book your flight. Also, gate-checking is a less onerous option than checking a guitar and having it go through the entire airport bag handling system.
Still, because of the Rule, flying with a guitar is easier now. Have a copy of the above webpage on your phone when flying, but be nice to your crew!