On amazon product pages, the ASIN is in the URL, and probably a lot easier to match, and less likely to change over time. For instance:

https://www.amazon.com/Road-Serfdom-Documents-SERFDOM-Paperback/dp/B00QPVMNNA/ref=sr_1_4?keywords=the+road+the+serfdom&qid=1658186256&sr=8-4

Minus a bunch of noise, is just:

https://www.amazon.com/dp/B00QPVMNNA

And "B00QPVMNNA" is the ASIN. A regex like /dp/([^/?]+) should match, but you'll probably have to escape the forward slashes (/), I dunno I have not uses regexes in a Shortcut yet.