Gmail filters don't enable regex checking
Google Apps Script is surprisingly simple
const regex = new RegExp(/Next Appearance: \d{2}\/\d{2}\/\d{4}/)
const threads = GmailApp.getInboxThreads()
threads.forEach(thread => {
thread.getMessages().forEach(email => {
const body = email.getBody()
if(!regex.test(body)) {
email.forward('forwarding-email@gmail.com')
}
})
thread.moveToArchive()
})
To my knowledge, there is no "on email received" event you can tap into in Apps Script, so I: