Using substring-after in XPath node

Dear all,

I have a quite basic question with xpath.

With this (very simplified file) xml file :

<?xml version="1.0" encoding="UTF-8"?>

(PPN)229067271

I look for a xpath expression to extract only the number after (PPN).

something like /dns:subfield[substring-after(text(),‘(PPN)’)] but i don’t know the exact syntax to do this kind of calculation.

Best regards,

Joël