what is correct format of bundle identifier in iOS?

The Bundle ID can only contain alphanumeric characters, hyphens, and periods. It can't end on a period.

This regexp should be correct: .[\w\.-]+[\w-]

All the formats which you have mentioned are correct and can be used as bundle identifier. But the standard format followed is com.company.appname (reverse domain name).

The First type is the correct one.It is a standard to use the reverse domain name as the bundle identifier along with app name. For example com.google.gmail,com.fb.messanger