* a[bz]+z against “abcbzcdze” returns “abcbzcdz“

it does not return abcdzcdz at all. The expression actually means something like this:
Matches a and any b or z that comes after it, one or more times, then z at the end.

So, when it matches against “abcbzcdze”, it finds a, then find b, then but it cannot find z as required. So, it doesn’t return anything!

a better expression should be: a[a-z]+?z or a.+?z if you want to return a[a-z]+?z, or a[a-z]+z or a.+z if you want to return abcbzcdz