The only case the details says will not be tested is an empty string.
The input should consist of only binary didgets and those didgets should only represent numbers which are multiples of three.
anything else your regex should fail to match.

Your solution contains an off-by-one error which causes some trash to appear at the end of your string.

The error is 100% reproducible, so you should be perfectly able to find it running your solution in your local IDE and stepping through it with a debugger. Did you try?

It's difficult to say witout seeing your code, but I guess that either your strings contain some garbage at the end, or they are not terminated properly, or you write outside of some buffer and destroy the assertion message.