It appears that busybox awk does not recognize \] in a character class,
interpreting the backslash literally and taking the ] as the end of the
character class:
Busybox does accept []] as meaning "] is part of the character class",
but this syntax is not portable to heirloom (and presumably also Solaris
10) awk, which only works with [\]]. GNU awk accepts either form.
So instead, let's avoid this problematic construct entirely by writing
the test in a different way.