Renaming files using regex

I have a number of files in a folder on a Linux machine with the following names: 11, 12, 13, 14, 15, 21, 22, 23, 24, 25, 31, 32, 33, 34, 35 I would like to use regex in order to rename with the .inp extension I tried mv * *.inp mv: target '*.inp' is not a directory which provided an error. I also tried using the regex instead of the *. So, I understand that mv is used to move files around. I also got the idea that perhaps I could use ./*.inp to force mv to write in the same folder but it failed. So, apart from not understading correctly how mv works, how would I proceed to have have this done with mv?