one line code to determine prime number

code:
perl -e 'print "prime number\n" unless ("1" x shift)=~/^(11+)\1+$/;'

example:
$ perl -e 'print "prime number\n" unless ("1" x shift)=~/^(11+)\1+$/;' 18
$
$ perl -e 'print "prime number\n" unless ("1" x shift)=~/^(11+)\1+$/;' 17
prime number