Trying to make an Attachment reminder using a RegEx, but the filter doesn't work So basically, in Mail, I'm trying to set it up so that if the word "attach", or "attachment" with any casing is detected and there isn't an attachment on the e-mail, it prompts me to add an attachment to the e-mail. I have Mail Act-On as a plugin to help me add more filters onto my Mail as it comes in and out.

I've made a RegEx in the javascript form for it to find out whenever I type the word in. I've tested it in a website and it comes back as true for the cases of words I've put in, but in Mail.app it doesn't work.

Does anyone have any ideas?
Mail rules are only applied to new messages in the inbox, not outgoing messages. I don't believe it is possible to have such functionality in Mail natively; you would need to create or install a plugin to do this. After some searching, it looks like there is a Mail plugin which does what you're looking for called Send Companion (free, with other paid features).

If you're running an older version of macOS (Tiger through Snow Leopard), Attachment Scanner (free) looks like it would do the trick.