How to find position of substring in text field and in JSON text object

Hi guys,

I have a problem with formula in pipeline.
I have to extract a substring at the end of a text field (e.serial_number) and in the JSON object (b.section_pos.fields.pos_wp_number)
Format (example): CZXXX1000POSY  or  CZXXX1000POSYY
Number of characters between CZ and POS is not constant.

I need to get Y or YY characters (numbers) after the substring POS.

I tried to find response in previous community cases, but no one fit ...
Thanks in advance!

BR
Alex