There’s a function gap in Scrivener’s project search. The current search only searches in a single metadata field. This manacles the useful power of a project’s metadata. To put that power to use RegEx is available but there is a learning overhead to use it.
Better to deploy a search model similar to that used in Finder search on a Mac where you can add multiple filters, one at a time, so that it is as simple or as sophisticated as you like. For example, on a Mac a Finder search can filter by Filename and / or filter by Last Accessed and / or filter by File Type etc etc.
For a large project such a user-friendly and powerful tool can be invaluable.
I might not be understanding what it is you are looking for, precisely, as you say regular expressions solves the problem of searching for multiple metadata fields, but I do not understand how it could possibly do that.
Well, as for how I would read that query, the answer for selecting multiple metadata fields is found in the user manual PDF, under §11.1.2, directly under the subheading, Search In, which goes over the top part of that options menu.
But again if I’m completely misunderstanding, if you could perhaps show how you are doing what you want with a regular expression, that might help with the comprehension.
Better to deploy a search model similar to that used in Finder search …
Yes, or rather more generally speaking, as it has been mentioned before here on the forum, project search is an area that badly needs modernisation, in nearly all facets of its use.
Amber is right, holding the Alt key and going back to search you can add an additional single element including metadata at a time for a more focused search versus the All option.