Looks like a broken regular expression parser. ) should not be treated as a special character in an extended regular expression unless it occurs outside a bracket expression in conjunction with a preceeding ( that is also outside a bracket expression.

If this fails ...

Code:

awk 'BEGIN {s ~ /)/}'

but this succeeds ...

Code:

awk 'BEGIN {s ~ /\)/}'

... then I'm almost certainly correct.

\) is technically an undefined sequence, but that workaround has been known to work with some buggy implementations.

Regards,
Alister