nixCraft 🐧
A Perl programmer is asked to write a script to find all the prime numbers. They finish in 5 minutes. "How did you do it so fast?" asks their coworker. "I just used the is_prime function," they reply. "But Perl doesn't have an is_prime function!" yells back the coworker. The Perl programmer laughs. "I know. I wrote it in three lines."
Feb 12, 2025 at 2:41pmWeb
Show threadMichal ŠpondrFeb 12, 2025
@nixCraft And moreover, as a regular expression.
Show threaddieschFeb 12, 2025

@michal @nixCraft
sub is_prime {
return !((1x$_[0]) =~ /^.?$|^(..+?)\1+$/);
}

(from illya.sh/the-codeumentary-blog…)

Demystifying The Regular Expression That Checks If A Number Is Prime

Ever wondered how the ^.?$|^(..+?)\1+$ regular expression can tell you if a number is not prime? In this blog post you'll learn how and why it works. Code examples in Java, JavaScript, Python and Perl are provided.

The Codeumentary
Show threadpeter honeymanFeb 12, 2025
@nixCraft i don’t really get the joke — it’s not hard to write a three-line is_prime function
Show threadreinierlFeb 12, 2025
@peterhoneyman @nixCraft I'm not a mathematician, but I am under the impression that "find all the prime numbers", implying that this has to happen happen within finite time and space, is an impossible requirement. Not sure if that's the point of the joke or just sloppiness.
Show threadpeter honeymanFeb 12, 2025

@reinierl @nixCraft

🤔

thanks

Show threadKailas IdavanmuzhiFeb 12, 2025
@nixCraft we do not have a practical algorithm for
factoring extremely large numbers
Show threadFubaroqueFeb 12, 2025
@nixCraft All prime numbers? I think I spotted a bug in that Python script. 🥴 🥳