## Mathematica, regex length 11, [non-competing](https://codegolf.meta.stackexchange.com/q/11726/8478), [cracked](http://codegolf.stackexchange.com/a/112863/8478)

PCRE flavour:

 ^[^]@]{49}$

The correct solution will be a function which takes an integer and returns the output as a nested list like:

 {{1, 2, 3, 4}, {8, 7, 6, 5}, {9, 10, 11, 12}, {16, 15, 14, 13}}