Filter by Single capital letter

Can someone point me, how to filter for single standalone capital letter from tags?

For example find all files containing (In this case, capital N having space before and after a letter): Hook N Sling , but exclude those which have Hook'n'Sling or or any variations Hook N' Sling, Hook 'N' Sling, etc, etc... :rolleyes:

%field% HAS " n "
OR (for any single letter in the middle of 2 spaces but this includes hyphens and other alphanumerics)
%field% MATCHES " . "
%field% MATCHES " [a-z] "