Re: is it prime?

[brx]

Where is my hammer?

sub isPrime($) {
    $n=pop;
    1^(1x$n--)=~/^(1{2,$n})\1+$/;
}

With n>2

Just for fun as a contrib to https://github.com/fxn/math-with-regexps/blob/master/one-liners.sh (fxn).

Update, thanks to ambrus's reply: Re^2: is it prime?

sub isPrime($) {
   (1x pop)!~/^(11+)\1+$/
}