This formula will result in "am" or "pm".
=REGEXEXTRACT(a1,"\d+:\d\d (am|pm)")
How do I make it show everything and not just the captured group? I want it to behave like normal regex, which would show the whole match.
You can change the capture group into a non-capturing group:
=REGEXEXTRACT(a1,"\d+:\d\d (?:am|pm)")