[elzr]

This is indeed beautiful. But the author limits itself to a low-level unpacking of the regex and running a few tests cases, all the while remaining mystified as to why this works.

A high-level description helps makes obvious what's happening:

  To check the primality of a number N with string matching,     
    repeat a string (say, "1") N times
    and declare N prime:
      UNLESS the repetitions match the string 0 or 1 times
      OR the repetitions can be EVENLY matched into groups of MORE than one string.

[city41]

I wouldn't say he remained mystified. His exploration of the 9 case basically explained how it worked, just the author chose to leave it a bit implied for the reader to figure out. I would agree for a blog post it's usually better to hammer the point home though.