A Perl regex that matches composite numbers (and doesn't match prime numbers)

Take a look at this Perl regular expression: perl -lne '(1x$_) =~ /^(11+?)\1+$/ && print "$_ is composite"' This regular expression matches only the composite numbers (and doesn't match prime numbers). Can you figure out how it works? I'll give the explanation below but try to figure it out yourself. Here is what happens when you run it: $ perl -lne '(1x$_) =~ /^(11+?)\1+$/ && print "$_ is composi

[at_yasu]

なるほど、わからん。

[tmatsuu]

素数にマッチする正規表現だと！？なにこれ簡単なようで分からん。

[gugod]

倒底是 Perl 難還是數學難喔... #perl #math