Sanguan regular script refers to Guange style, which is a type of font, not literally Sanguan.
Guange style was called Yuan style in the Song Dynasty, Taige style in the Ming Dynasty, and Guange style in the Qing Dynasty. It inherited the tradition of Tang Kai and further standardized the techniques. Its distinctive features are: square, smooth, black, and uniform in size. It is the official calligraphy style of the Song, Ming and Qing Dynasties.
The name Guange style has a derogatory connotation. People think it lacks the aura and vividness of calligraphy. Shen Kuo commented on it in "Mengxi Bi Tan": Sanguan regular script can not be said to be neither refined nor beautiful. If you strive for its best, there will be no single stroke until death and no stroke will be perfect until death (it has to be said that it is very delicate on the surface. Beautiful, but if you want to find an excellent place in it, you can't find it.) However, the standard writing in Guange style has a certain use value and can beautify it in study and work. Function, and Guange style writing is very difficult, and it is impossible to write without strong basic skills.