MacOS – Use of regular expressions in macOS search fields

Does anyone know where I can find the docs describing Regex parsing capabilities of macOS search fields?

What I mean, I'd like to read about the regex syntax rules that are allowed and parsed by the search field. For example, what I didn't know until today that, in macOS Preview (or any other application with a standard search field, guessing) we can use input like this:

"\s" AND "an" AND "\s"

This works and hints that search fields interpret more regex then we know, and this is what I want to read/find out about. The example above searches for "an" with one trailing and leading whitespace character \s within a opened document.

I'd like to know more about those grouping rules and regex characters wrapping/escaping so it can be correctly parsed. Also does this format of input apply to systemwide search fields of any application?

I can't find any docs describing these features and was found only this on developer.apple.com library instead.

Below you can see an example of default search behaviour for a "an" word within a PDF document, it shows all "an" occurences in the results, regardless if you put a space before the "an" word, so your input is " an "

When we put "\s" AND "an" AND "\s" into the search field we get the more desired search result

This makes a huge difference for searches containing common words!

In short: I'd like to know more about the functionality you see above. Where can I find any (official) documentation about it?

The answers provided (so far) are not answering my question, thank you for them though – aggregating more related information is good.