Batch insert values to Subject and Keywords of pdf file description from the File Name

My task is to insert metadata in batch of pdf files: Title, Author, Subject and Keywords.

While Title and Author are the same for all files, I created Action - Add File Description, and processed batch quite easily and efficiently.

I can't do the same for Subject and Keywords, since these values are unique to each document. Both pieces of information are part of the File Name, for example:

PDF File Name is A_B_C.pdf 

A needs to be in the Subject line

B needs to be in Keywords line 

Is there a way to automate the process for batch of files, and insert desired information into each file description? I'll appreciate any helpful comments. Thank you.