Also need to make sure it’s actually a colon. : is a colon. ꞉ is not. Can you tell the difference? A regex compiler can.
Try running the line through What Unicode character is this ? (babelstone.co.uk), and see if it is actually a colon or not…
(This often happens with “quotation marks” as well… Macs (and forums, apparently!) in particular are fond of using ‘fancy quotes’ which arent actually the quotation mark symbol.)