# [dc][1], Regex length 12

 ^[^# !]{59}$

This regular expression is simple enough that I don't think the flavor of regex matters -- it should work across the board. (Note the space after the # in the regex.)

I've tested all four flavors at regex101.com (PCRE/PHP, Javascript, Python, and Golang), as well as the Ruby version at rubular.com. The dc program matches the regex in all five regex versions.

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The dc program takes its input on stdin and puts its output on stdout.

Sample output for input 4 (there's a trailing space at the end of each line):

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


 [1]: https://www.gnu.org/software/bc/manual/dc-1.05/html_mono/dc.html