Why doesn't this work in KM?
RegEx101.com Match Sentence
([A-Z].+?(?=(?:\. {1,2}[A-Z])|[\.?!]$)[\.?!])
KM match stops at first period:
Here is a sentence with abbrev.

MACRO: RegEx to Match Sentence
~~~ VER: 1.0 2019-02-16 ~~~
DOWNLOAD:
RegEx to Match Sentence.kmmacros (5.4 KB)
Note: This Macro was uploaded in a DISABLED state. You must enable before it can be triggered.

Your regex depends on case but you've set the action to ignore case.
1 Like
Bingo! 
I can't believe I made such a rookie mistake. 
The RegEx and macro now work perfectly.
I have made some minor updates to the RegEx:
RegEx101.com Extract Sentence from Paragraph
([A-Z].+?(?=(?:[\.?!]\h+[A-Z])|[\.?!]$)[\.?!])
Thanks again @mrpasini.
Here's my finished Macro for Pages based on this:
MACRO: Pages -- Select Sentence [Example]
Happy I was able to help!