How to stop exchange transport rule from adding duplicate disclaimers?

We needed to add a warning to all emails sent outside the organization. I went in to the M365 Exchange Admin center, under mail flow > rules, adding a rule that appends a disclaimer to every message sent outside the organization. Great, check that box for a job well done.

In practice, if you have a long email chain with an external contact you will find these disclaimers piling up at the end. The duplicate disclaimers can be quite overwhelming and much larger than the actual content of the messages themselves. Seems wasteful.

I need some way of avoiding duplicate disclaimers on a message that already has a disclaimer.

FROM THE LINK IN THE COMMENTS:

For example, to add the exception that prevents multiple disclaimers being added in an email conversation, select Add exception and then select The subject or body > Subject or body matches these text patterns, and then specify the words or phrases in your disclaimer.

I guess I am looking for a better description of how that is done. The exception language says "matches these text patterns" and the documentation says that this "pattern" is a Regular Expression. So I need a RegEx that matches my disclaimer without accidentally matching other strings of words.

You could create a transport rule to exclude the email body has the disclaimers, the rule look like below:

A quick add-on this thread, we prepend [EXT] to emails received outside of the organization. To prevent it from repeating, I added that same exception, except I noticed that it was repeating the [EXT] each time. To fix this, I had to add a space after the exception to separate the [EXT] from the subject. Just a thought!