ERJ 170/175 Captain reported they failed to properly set the parking brake upon arrival at the gate and the aircraft rolled until the ramp crew notified the Captain. The Captain immediately set the parking brake and no injuries or damage was reported.

Narrative

After the flight into ZZZ; I pulled the airplane into the gate at X. The Marshaller waived us in and gave us the signal to stop rolling forward and push the brakes. I stopped right where said and commenced my parking flow. I looked to see if the APU was running and to see if the engines had had their 2 minute shut down. They did; so I would have bet my bottom dollar that I set the parking brake first; and then commenced with the rest of the parking flow. After the parking flow; I unlocked the flight deck door and greeted the Flight Attendant (FA). There was no activity from the jet bridge for a beat and then I caught the ramper waiving at me from the ground. He gave me the headphone" signal because he wanted to tell me something. I put the headphones on and listened to what he had to say. He said over the headphones that the parking brake is off and the airplane rolled "like 2 feet." I remember him saying that. I looked down and sure enough the parking brake was released. I confirmed with him the parking brake wasn't set. The ground crew told me to set the parking brake so they could tow us back up to the stop line so the jet bridge could be moved up to the airplane. After I set the parking brake I made an announcement to the passengers that the aircraft had rolled out of position and then asked if they would sit back down for their safety. I believe I turned the fasten seatbelt sign back on. Once the ground crew was ready to pull the aircraft up; I released the brake. The aircraft was pulled back into position and I made sure to set the parking brake. Afterwards the jetbridge was connected to the plane and the plane was properly deplaned afterwards. I can't believe this has happened to me. Not once in my (number) years as a Captain; have I not set the parking brake at the gate. The procedures are already spot on. The checklist has us check that the parking brake is set on parking. The ramp crew has an indicator for when the brakes are set/not set. There is no need for any change in procedure. There is already a double safety net for this situation."