Can you explain what the '?:' in the beginning and the '\Z' in the end do?

if you want to solve this kata with re module,
remember that in regex
you should be really carefull with '^' and '-',
especiall when it comes to the case of Square brackets('[]'),
'^' CANNOT be at the HEAD of a regex, BUT it can be '^' or in other part of a regex,
as well as
'-' CANNOT be in the MIDDLE of a regex, BUT it can be at the HEAD,
which is the most vital part you should have known.