All of the regexes listed above seem to have problems in one or more situations.

use strict;
use warnings;

while (<DATA>) {
    chomp;
    my ($last) = m|/([^/]+)/[^/]*$|;
    print "$last\n";
}

__DATA__
/rootdir/sub1/sub2/sub3/sub4/
/rootdir/sub1/sub2/sub3/sub4/file
/rootdir/
/
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