The page explicitly says:
/[a-w&&[^c-g]z]/ # ([a-w] AND ([^c-g] OR z))
# This is equivalent to:
/[abh-w]/
"z" is not included in the left "AND" term, so it can't be matched.
See: "All things that are both apples, and also either apples or pears, at the same time" does not include pears. Only apples are both apples and (apples or pears). Likewise, a
is in both a-w
and [^c-g]z
, so it matches; z
is not in the left side, so "AND" is not satisfied, thus the whole expression fails.