Flag all auto-save files (files whose names start and end with #) for deletion (Section 16.5).
Flag all backup files (files whose names end with ~) for deletion (Section 16.3.1).
Flag for deletion all files with certain kinds of names, names that suggest you could easily create the files again.
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 whose names match the regular expression regexp.
The #, ~, &, and . commands flag many files for deletion, based on their file names. These commands are useful precisely because they do not themselves delete any files; you can remove the deletion flags from any flagged files that you really wish 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, and the .orig and .rej files produced by patch.
# (dired-flag-auto-save-files) flags for deletion all files whose names look like auto-save files (Section 16.5)--that is, files whose names begin and end with #. ~ (dired-flag-backup-files) flags for deletion all files whose names say they are backup files (Section 16.3.1)--that is, whose names end in ~.
. (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 dired-kept-versions (not kept-new-versions; that applies only when saving) specifies the number of newest versions of each file to keep, and kept-old-versions specifies the number of oldest versions to keep.
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.
The % d command 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 subdirectories by hiding them (Section 30.13).