word according to the `nnmail-split-fancy-syntax-table' syntax table.
You can use \".*\" in the regexps to match partial field names or words.
-FIELD and VALUE can also be lisp symbols, in that case they are expanded
+FIELD and VALUE can also be Lisp symbols, in that case they are expanded
as specified in `nnmail-split-abbrev-alist'.
GROUP can contain \\& and \\N which will substitute from matching