my $regex = qr/\[(\d{1,20})(??{  " {" . (20 - length $1) . "}"  })\]/;

while (<DATA>) {
  print /$regex/ ? "yes\n" : "no\n";
}

__DATA__
[12345678901234567890]
[123                 ]
[234223423      ]
[23409234329c        ]