Looks like you may need to consider illegal leading and trailing characters.

That's right, it's supported by most modern compilers like gcc and clang but not vc++ for example.

Oh yes, thanks.

There's a test case where the argument is an int.

I think the op is assuming the tests don't go to 10000.

Compare the first and second run times of a passing solution:

In [271]: %time Primes.first(10000)[-5:]
CPU times: user 692 ms, sys: 639 µs, total: 693 ms
Wall time: 693 ms
Out[271]: [104707, 104711, 104717, 104723, 104729]

In [272]: %time Primes.first(10000)[-5:]
CPU times: user 493 µs, sys: 0 ns, total: 493 µs
Wall time: 497 µs
Out[272]: [104707, 104711, 104717, 104723, 104729]

Yes, with memoization.