I'm confused how dired's 'mark containing' (e.g. dired-mark-files-containing-regexp)command works. I have a directory with a bunch of files I accidentally created and want to delete of the form 1-bad-filename.csv-really-bad.csv. I want to mark these files and then delete them. Using the regex-tool, I see that csv-really should be enough to identify them. But nothing gets marked when I do this:
0 matching files marked.
What am I missing?
Your question is not very clear to me. It sounds like you are expecting dired-mark-files-containing-regexp to match a regexp against a file name, instead of against the file contents.
If that is what you want, then use command dired-mark-files-regexp (bound to % m), not dired-mark-files-containing-regexp.