Use of regular expressions in macOS search fields
I don't know if you discovered this already but you just need to click on the magnifying glass in the search field, it has a little down-pointing caret which is the clue, see this screenshot:



After clicking choose Insert Pattern and you get a range of options:

This is basically user-friendly regex, i.e. you get tokens you can easily arrange into quite powerful search patterns.



The example in the screenshot is TextEdit however this option doesn't appear to be available in Preview.