dired-mark-files-regexp
dired-mark-files-regexp is an interactive compiled Lisp function in `dired.el
'.
(dired-mark-files-regexp REGEXP &optional MARKER-CHAR)
Mark all files matching REGEXP for use in later commands.
A prefix argument means to unmark them instead.
`.' and `..' are never marked.
REGEXP is an Emacs regexp, not a shell wildcard. Thus, use `\.o$' for
object files--just `.o' will mark more than you might think.