from numpy import bool_, nonzero, ones

  def primes_upto(limit):
      is_prime = ones(limit, dtype=bool_)
      for n in xrange(2, int(limit**0.5) + 1): 
          if is_prime[n]:
             is_prime[n*n::n] = 0
      return nonzero(is_prime)[0][2:]

  /^1?$|^(11+?)\1+$/

See my answer http://news.ycombinator.com/item?id=479514

cryptic is anything that can stay in the head 'ram' of a normal programmer. stuff that typically needs to be written down to make sense.

i went through the above code with pencil and paper and it made sense. now i can look at the code and it makes sense. the person who originally wrote the code had to go through those steps too (not necessarily on paper, but loading the process into head ram). same with regex.

no big deal. i know about slices, and if i didn't that's what mentors are for---oh, that symbol? search for python slices! or a python book. at least python doesn't has less than a handful nonsearchable crypticness.

the comment linking to the eratosthenes sieving explanation was helpful.

1. Mark all integers that greater than 1 as primes. 2. Take the smallest prime that is not already considered and cross out all its multiples. 3. Repeat 2nd step for the next prime.

The above Python code that returns all primes number less than a given limit uses two optimization:

1. Repeat 2nd step upto sqrt(limit), not upto limit. 2. Start crossing out at the square of the prime, not at twice of the prime.

The only “intuitive” interface is the nipple. After that, it’s all learned. http://news.ycombinator.com/item?id=409288 (It might be not true literally but the quote is useful as a general idea that people have different backgrounds; an “intuitive” thing for one person is a cryptic for another. Intuitiveness changes with experience.)