Regular expression help

I know this is not a regex forum. But I always find bright people in here so I figured I would give it a try

OK, brand new at regular expressions. We have a application that groups files and sorts them based on name. One of the options in this application is to sort based on a selection of part of the name defined by a regular expression

My file names that I need to sort on are this

Example 16p_Multiple_1.pdf

The part I need to sort on would be the “_1.pdf.” Now of course this could go to any number, depending on how many pages are submitted. I would appreciate any help. Thank You.

It’s not regular expression, it’s sorting data. I use shell command to sort a lot because it can sort many complex data.

set theFiles to {"p16_filename_1.txt", "p14_filename_2.txt", "p14_filename_1.txt", "p01_filename_5.txt"}

set AppleScript's text item delimiters to string id 10
set sortedFiles to every paragraph of (do shell script "echo " & quoted form of (theFiles as string) & " | sort -f -k 1.5")
set AppleScript's text item delimiters to ""
return sortedFiles