Help with Import Page.

Good afternoon;
I hope you can guide me
I have an import page where I need to get a value from the name of the file being uploaded and then add it to the database.
The format of the file name is:
XXX_XXXXXXXXX_XXXXXXXX_XXXXXX_33.csv
And I need to get in this example 33
Any suggestions on how to do it?
(Google Translate)
Many thanks