Regex Dutch postcode (postal code)

I've tried the following regular expressions separately, but entering 1000AB into the string $Selectievraag/PostcodeTmAntwoord and having the entry checked in an exclusive split in an On Change microflow results in a false each time. Anyone knows why? isMatch($SelectieVraag/PostcodeTmAntwoord,'/^[1-9][0-9]{3} (?!sa|sd|ss)[a-z]{2}$/i') isMatch($SelectieVraag/PostcodeTmAntwoord,'/^[1-9][0-9]{3} ?(?!sa|sd|ss)[a-z]{2}$/i') isMatch($SelectieVraag/PostcodeTmAntwoord,'^[1-9][0-9]{3} (?!sa|sd|ss)[a-z]{2}$i') isMatch($SelectieVraag/PostcodeTmAntwoord,'^[1-9][0-9]{3} ?(?!sa|sd|ss)[a-z]{2}$i')