Extracting one of a dual occurring string from a variable

I am extracting text from PDFs into a variable called “OCR”. i would like to lookup and extract a 15 digit number from “OCR” into another variable “extractedOCR”. The PDFs used will always contain 2 sets of 15 digit numbers. 1 of these numbers will always be static, while the other changes. I need to capture the one that changes.

For Example:
The PDFs are invoices containing a 15 digit tax number of the Supplier and the 15 digit tax number of the client. The client number will always be there in each invoice, thus should be ignored. What i am interested in capturing is the Supplier tax number, which will be different in each PDF.

Is the pos of that no is fixed?

Unfortunately no