* lisp/dired-x.el (dired-mark-unmarked-files): Add an argument which controls case folding for matching the regex (Bug#18716). (dired-omit-case-fold): New variable. Defaults to `t' on case-sensitive systems, `nil' otherwise. (dired-mark-omitted, dired-omit-expunge): Use dired-omit-case-fold. * doc/misc/dired-x.texi, etc/NEWS: Document dired-omit-case-fold.