Use REGEXEXTRACT() to extract only upper case letters from a sentence in Google sheets

I'm thinking this should be basic but having tried a number of things I'm nowhere nearer a solution:

I have a list of names and want to extract the initials of the names in the next column:

Name (have)	Initials (want)
John Wayne	JW
Cindy Crawford	CC
Björn Borg	BB
Alexandria Ocasio-Cortez	AOC
Björk	B
Mesut Özil	MÖ

Note that some of these have non-English letters and they may also include hyphens. Using REGEXMATCH() I've been able to extract the first initial but that's where it stops working for me. For example this should work according to regex101:

=REGEXEXTRACT(AH2, "\b[A-Z]+(?:\s+[A-Z]+)*") but only yields the first letter.