Mathematica, regex length 11, non-competing, crackedcracked
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}}