$ perl -Dr -ce '/[^\B#]/'
Compiling REx `[^\B#]'
size 12 Got 100 bytes for offset annotations.
first at 1
   1: ANYOF[\0-"$-AC-\377{unicode_all}](12)
  12: END(0)
stclass `ANYOF[\0-"$-AC-\377{unicode_all}]' minlen 1 
Offsets: [12]
        1[6] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 7[0] 
Omitting $` $& $' support.

EXECUTING...

-e syntax OK
Freeing REx: `"[^\\B#]"'
$