Idea: "puncture" some consecutive fragments and check whether the
b64decode result of a punctured string is a valid JSON or not.
It's a O(N^3) algorithm, but should be fast for a small N (less than 30
fragments in all test cases)
The re engine does not want to repeat an empty string, for fear that something like
(.*)*
could be matching the tokens ...
""
"" ""
"" "" ""
Of course, that's harmless with a question mark, although still somewhat strange.