[string].split() method doesn't accept regex on split but just [char[]] or [string[]].
You can try like this:
$line -split ',(?=(?:[^"]|"[^"]*")*$)'
powershell -split accept regex for splitting text
Using .net you can do it like this:
[regex]::Split( $line , ',(?=(?:[^"]|"[^"]*")*$)' )