Regim is described as 'makes some processing steps that are unique to astronomical images a bit easier. These steps are calibration (dark and flat), deblooming, registration, combination, color calibration, gradient removal and some more'. There are three alternatives to Regim for Windows, Linux and Mac. The best alternative is DeepSkyStacker, which is free. Other great apps like Regim are Siril (Free, Open Source) and IRIS (Free Personal).
DeepSkyStacker is a freeware for astrophotographers that simplifies all the pre-processing steps of deep sky pictures. Registering / Stacking / Simple post-stacking processes to quickly view the final result.