Just as a suggestion, maybe this tip can be included in the man pages. Something on the lines of:

Use folder:"/<regex>/" in case the regex contains special characters interpretable by Xapian. Double escape as necesary. For instance:

notmuch search 'folder:"/^.*/(Junk|Spam)$/"'

Javier Garcia <javiertury@gmail.com> writes:

Hi,

I'm configuring notmuch in a multi-account directory. I want it to tag
spam and deleted mails using a post hook. Using regexes with parentheses
fails, or perhaps I misunderstood the documentation. The following one
liner replicates the problem I face.

$ notmuch search 'folder:/^.*/(Junk|Spam)$/'
notmuch search: A Xapian exception occurred
A Xapian exception occurred parsing query: unmatched regex delimiter in
'/^.*/(Junk|Spam'
Query string was: folder:/^.*/(Junk|Spam)$/

Try

 notmuch search 'folder:"/^.*/(Junk|Spam)$/"'