Logically, $1 can be anything that exists at the end of the string because (?:(\d+)|::)* matches a zero-length string. So whatever is in (\d+) is irrelevant.(this is wrong)

I thought the regex engine had been "fixed" to set inner captured matches to undef when outer matches were zero length, but I still wouldn't depend on it.(this doesn't apply here)

Update: The uncapturing parens along with the "*" do match multiple digits, but the inner parens match many times, and only the last thing is captured.