Bulk file name changer on Mac

Hi All,

First time posting on here so bear with me if I'm not explaining what I need very well.


I have a folder of files (Jpg) and I need to change the names of the files.
The problems are, I have multiple images starting with the same number, for example
2.1.jpg, 2.2.jpg, 3.1jpg, 3.2.jpg, 3.3jpg and I need to change everything starting with a 2 to something else say 100 and the everything starting with 3 needs to start with 101 and so on.

I have seen you can change a list of file names to another list of names in excel but I cannot solve the .1, .2, .3 problem

Any and all help greatly appreciated