You need to ecsape the [ characters as they introduce a character set, e.g.,
[a-z] or [0-9] or even [img] to match any of ‘i’, ‘m’, or ‘g’.
It have noticed previously that some regexp engines are clever enough
to work out that a mismatched [ does not need to be escaped, which is
likely why it appeared to work in FF. I would have thought though
that something that purports to be a ‘tester’ should enforce the rules