> pigeon% perl -Dr -e '/X(?:.*..?.)+X/'
>
> rarest char X at 0
> anchored `X' at 0 floating `X' at 3..2147483647 (checking floating) minlen
4
>
>  Floating at 3.. is important
>
>
> Guy Decoux

Could you explain this output or/and send a link where Perl RE optimization
method is described?

BR,
Sergei