Hi Brandon:
Former Sierra admin here, I used the following RegEx snippets for the same purpose:
Fewer than 14 digits: (?e)^.{1,13}$
Greater than 14 digits: (?e).{15}
I got those from Richard Jackson’s Playing with Matches sessions. His latest one on the forum is from 2021. The second post at that link gives his handout and presentation (where he talks about min / max functions and uses patron barcode length as an example).