The #, ~, ., % &, and % d commands flag many files for deletion, based on their file names:

#

Flag all auto-save files (files whose names start and end with #) for deletion (Auto Save).

~

Flag all backup files (files whose names end with ~) for deletion (Backup).

. (Period)

Flag excess numeric backup files for deletion. The oldest and newest few backup files of any one file are exempt; the middle ones are flagged.

% &

Flag for deletion all files with certain kinds of names which suggest you could easily create those files again.

% d regexp RET

Flag for deletion all files whose names match the regular expression regexp.

# (dired-flag-auto-save-files) flags all files whose names look like auto-save files—that is, files whose names begin and end with #. Auto Save. ~ (dired-flag-backup-files) flags all files whose names say they are backup files—that is, files whose names end in ~. Backup. . (period, dired-clean-directory) flags just some of the backup files for deletion: all but the oldest few and newest few backups of any one file. Normally, the number of newest versions kept for each file is given by the variable dired-kept-versions (not kept-new-versions; that applies only when saving). The number of oldest versions to keep is given by the variable kept-old-versions. Period with a positive numeric argument, as in C-u 3 ., specifies the number of newest versions to keep, overriding dired-kept-versions. A negative numeric argument overrides kept-old-versions, using minus the value of the argument to specify the number of oldest versions of each file to keep. % & (dired-flag-garbage-files) flags files whose names match the regular expression specified by the variable dired-garbage-files-regexp. By default, this matches certain files produced by TeX, .bak files, and the .orig and .rej files produced by patch. % d flags all files whose names match a specified regular expression (dired-flag-files-regexp). Only the non-directory part of the file name is used in matching. You can use ^ and $ to anchor matches. You can exclude certain subdirectories from marking by hiding them while you use % d. Hiding Subdirectories.