It is helpful to change
System.out.print(m.start() + m.group());
to
System.out.println(m.start() + ": " + m.group());
This way the output is much clearer:
0:
1:
2: 34
4:
5:
6:
You can see that it matched at 7 different positions: at position 2 it matched string "34" and at any other position it matched an empty string. Empty string matches at the end as well, which is why you see "6" at the end of your output.
Note that if you run your program like this:
java Regex2 "\d+" ab34ef
it will only output
2: 34
