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With optional regular expressions, on fields selected by the user.

The simple interface has a popup for fields, so you could do All or just one field at a time.
The complex interface lets you select any available field(s), the same way View Options (Apple-J) works, with an extra button for Select All. If Select All is checked, checking it again Deselects All.

You could also have variables whose value is that of the associated field for an entry. For example, the replacement pattern "www.amazon.com/dp/$(ASIN)" substitutes the value of the ASIN field into the text of the pattern. You might use different brackets "${ASIN}" to indicate that variable substititution and final replacement only occur if the ASIN field is non-empty.

Thank you for the feedback. We are thinking more of a pop-up field (like the smart collection rules) in order to keep the interface cleaner for a find and replace. However, your idea of using regular expressions might be something we can integrate as there some good regular expressions libraries available for Cocoa.