John the Baptist's showdown with the Pharisees and Sadducees in Matthew is pretty entertaining. He calls them a “brood of vipers”, says that stones could be better disciples than they are, and threatens that they will be chopped down like trees and then burned like chaff in a fire. Of course, John starts by exhorting them all to repent. This notion of repentance is what I'd like to briefly focus on.
Repentance isn't just feeling sorry for your sinfulness. That would be contrition, and John doesn't yell at the people to “be contrite!” To repent, in my opinion, is not about feeling, but rather about doing. We are not to feel something negative (regret, sorrow, etc.), but are to do something positive (pray, go to confession, reflect on yourself, do an act of charity, etc.). When we do something good to move us outside of our comfort zone and into a new positive reality of existence, then we are repenting.