+For [specific
+fields](https://notmuchmail.org/doc/latest/man7/notmuch-search-terms.html#search-prefixes)
+it is possible to use regex searches (although these are often
+noticeably slower than native Xapian queries). The limited set of
+fields is a quirk of implementation which requires a Xapian "value
+slot" in the database schema. Adding regex support for more fields to
+would require of adding more value slots to the schema. The
+performance impact of that needs to be experimentally evaluated, and
+assuming it is not too bad, some database upgrade code would need to
+be written.
+
+## How do I search for folders or paths with spaces?
+
+The spaces in the names must be escaped. For example if you use bash or zsh,
+you can search for messages with tag `foo` in folder `INBOX/folder with spaces`
+with this query:
+
+ $ notmuch search tag:foo 'folder:"INBOX/folder with spaces"'
+