On: The Greatest Regex Trick Ever (2014)

[high_byte]

it's nice. I'm way more dumbfounded by the prime thing though

[rprenger → high_byte]

Me too. I had to look it up. This page has pretty good breakdown:

https://itnext.io/a-wild-way-to-check-if-a-number-is-prime-u...

The main trick for me was you first have to convert the number to unary, which was done outside of the regex.