perl -lne '(1x$_) =~ /^1?$|^(11+?)\1+$/ || print "$_ is prime"'

Can you figure out how it works? I give an explanation below, but try to figure it out yourself. Here is what happens when you run it:

$ perl -lne '(1x$_) =~ /^1?$|^(11+?)\1+$/ || print "$_ is prime"'
2 is prime
3 is prime
5 is prime
7 is prime
11 is prime

Here is how it works.

First, the number is converted in its unary representation by (1x$_). For example, the number 5 gets converted into 1x5, which is 11111 (1 repeated 5 times.)

Next, the unary string gets tested against the regular expression. If it matches, the number is a composite, otherwise it's a prime.

The regular expression works this way. It consists of two parts ^1?$ and ^(11+?)\1+$.

The first part matches number 1 and the empty string. Clearly, the empty string and number 1 are not prime numbers, therefore this regular expression matches, which indicates that they are not prime numbers.

The second part determines if two or more 1s repeatedly make up the whole number. If two or more 1s repeatedly make up the whole number, the regex matches, which means that the number is composite. Otherwise it's a prime.

Let's look at the second regex part on numbers 5 and 4.

The number 5 in unary representation is 11111. The (11+?) matches the first two ones 11. The back-reference \1 becomes 11 and the whole regex now becomes ^11(11)+$. It can't match five ones, therefore it fails. But since it used +?, it backtracks and matches the first three ones 111. The back-reference becomes 111 and the whole regex becomes ^111(111)+$. It doesn't match again. This repeats for 1111 and 11111, which also don't match, therefore the whole regex doesn't match and the number is a prime.

The number 4 in unary representation is 1111. The (11+?) matches the first two ones 11. The back-reference \1 becomes 11 and the regex becomes ^11(11)+$. It matches the original string, therefore the number is not a prime.

PS. I didn't invent this regular expression, it was invented in 1998 by Abigail.

Don't take this regular expression too seriously, it's actually neither a regular expression (as defined in automata theory), nor a way to check if a number is a prime. It's just an awesome thing that Perl can do. See this cool article called The Prime That Wasn't by Andrei Zmievski for a discussion about how this regex fails for larger numbers because of backtracking.

Also if you wish to learn more about Perl one-liners, check out my Perl One-Liners Explained article series and download the perl1line.txt file.


Gaal Yahas Permalink
December 25, 2011, 21:30

I believe credit for this is due to Abigail, who posted it (spelled a little differently) over a decade ago.

December 26, 2011, 14:08

That is correct!

Jonas Permalink
December 25, 2011, 22:18


December 26, 2011, 14:10

Good find.

December 26, 2011, 01:47

I just wrote a coprimality test with a regular expression tonight, based on the same technique.

December 26, 2011, 14:08

Very cool!

December 26, 2011, 07:09

This perl program gives a segmentation fault on x86 Linux (RHEL 5) when served an input of 1234321

December 26, 2011, 14:08

Yup, doesn't work for large numbers. There is so much backtracking going on for large numbers that Perl runs out of memory.

epiphany47 Permalink
December 26, 2011, 08:47

This isn't a "regular expression", it's a "regexp" =] - see comments on HN:

This script uses backreferences which give regexp's more "computing power" than regular expressions actually have, but remove the guaranteed runtime of an O(n) regular expression. For more, read the excellent article at

You can actually prove that "regular expressions" don't have the computing power to enumerate the prime numbers. For more, read the wiki article,

On an unrelated note, I've been a longtime fan of your NodeJS articles, and look forward to more of your writing. =]

December 26, 2011, 14:10

I have studied computer science intensively and you're right that this regular expression isn't a real regular expression as in automata theory.

I'm publishing the next nodejs post in a few days!

newestbie Permalink
December 27, 2011, 09:16

my $string = "-" x 1000;
$string =~ s/(?!(-{2,})\1{1,}$)(?=(-{2,}))/print length($2), "\n"/eg;

January 12, 2012, 12:05

Links to other explanations and to a usenet post by Abigail that dates it back to 1997 can be found at my blog.
Abigail's looked like this
perl -wle 'print "Prime" if (1 x shift) !~ /^1?$|^(11+?)\1+$/'

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