The closest airports are Nürnberg and Munich.
The airport is located 30 km North of Munich, roughly 100 km South of Regensburg. The closest rail station (DB Deutsche Bahn) is Freising; a local bus connects all terminals to long term parking and the rail station (DB) in Freising. For a schedule of the bus, please see this link.
The ride on the local bus will take you 20-25 min. Trains to Regensburg run every sixty minutes, currently approximately 10 minutes after the hour. Travel time to Regensburg is about one hour. There is also a shuttle service to Regensburg which you should reserve in advance.
You can buy your ticket for the bus and the train from a DB ticket counter in the airport, follow the rail signs. Note that a DB ticket is valid on all trains running from Freising to Regensburg, including the Alex trains operated by Arriva. You cannot buy Alex-tickets at the DB ticket machines; buy a DB ticket instead. Also, you cannot buy the bus ticket at the ticket machine or online.
If you have bought DB tickets online before, then you might want to buy the ticket from Freising to Regensburg online. You can pay the driver on the bus for the local bus from the airport the Freising.
If you anticipate to have a long wait at the train station in Freising, then you might want to skip one or two of the shuttle buses and enjoy coffee/the shopping experience at the airport.
The train ride on a local commuter train from the airport to the main station takes you only 12 minutes. The travel time from the main station to Regensburg is approximately 1h both on local and on IC/ICE trains. Please check the schedule online.
The easiest way is to take a local bus. Both line 6 in the direction of “Klinikum” and line 11 to “Burgweinting” stop at “Universität”. During rush hour, there are the additional campus lines C6 and C1, which you can use as well. Cabs are available at both exits from the train station.
You can buy tickets for the buses from the machines at the bus stops and on the buses (pay the diver).
The university is in the south of the train station. The easiest way to get there is to use public transportation. To get to the bus stop from the platform, take the stairs to the walkway crossing the tracks and turn towards the shopping center Arcaden, not the ticket office. Before you enter the shopping center through the revolving doors, take a left and walk outside of the shopping center towards the bridge crossing the tracks. You will see the bus stop in front of you. Cabs are also available at both exits of the station.
Both bus lines (6/C6, 11/C1) stop at “Universität”. Cross the road on the marked crosswalk, take the stairs, and walk straight, passing the Mensa on your right and the administration/convenience stores on your left. Take the few steps uphill and you will see the math building slightly to your left.
You also can print a campus map.