Wes Scantlin -- Caught Riding Dirty in Beverly Hills

Wes Scantlin learned the first rule about driving while (allegedly) holding ... don't have expired tags.

Beverly Hills cops tell us the Puddle of Mudd singer was pulled over for expired registration a week ago and things escalated when they realized there was a warrant out for his arrest.

Cops searched his car, and found a controlled substance ... whatever it was, it was enough for cops to hook him up and take him to jail.

Scantlin was booked on misdemeanor drug charges.