Regex does not see group

I have the following regular expression (C++ string):

Python (\\d+(\\.\\d+){0,3})

For this string: "Python 3" it has match, but lastCapturedIndex is 1. Thus I can see the whole match only. I can't access the group with the version.

But if the string will be for example: "Python 3.5" then it see the group. lastCapturedIndex is not 1 anymore.