From this article,
/^1?$|^(11+?)\1+$/ checks whether a number(its value in unary) is prime or not.
Using this, perl -l -e '(1 x $_) !~ /^1?$|^(11+?)\1+$/ && print while ++$_;' returns a list of prime numbers.
I do not have enough experience with Perl, but what I understand is that the regular expression will be true for a number that is not prime. So, if we print all numbers that do not produce a true with this expression, we have a list of prime numbers. Thats what the perl query is trying to do.
About the regex part,
^1?$ part is for counting 1 as not prime
^(11+?)\1+$ is for matching not prime numbers starting from 4.
What I do not understand is why is the ? in the regex needed at all.
According to me /^1$|^(11+)\1+$/ should be just fine and actually
perl -l -e '(1 x $_) !~ /^1$|^(11+)\1+$/ && print while ++$_;'
gives me the same set of prime numbers.
Is there any flaw in my understanding of the regular expression? Why are the ?s needed?
Isn't ? supposed to match zero or one occurrence of the expression preceding it?