You sure can. But before I get into it, here's a site that can help with your regex expression extractions:
https://regex101.com/
It would be better if you supplied the whole string in the field containing "Message accepted for delivery", as well as your search, as I can better answer this question with those provided, but this rex should do the trick:
yoursearch | rex field=fieldContainingYourMessage "(?<Message1>Message accepted for delivery)"
What the search above will do will provide you with a new field called Message1 and the content/values will be "Message accepted for delivery"
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