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.

its a shame c++ regex doesnt have lookbehinds

When you know your test function is going to fail, you can write to error out to find out exactly what the author wrote.

i had an issue with static variables in lambdas for this kata.
in VC++ and MinGW locally it doesn't behave as globally static, where here it does.
e.g.
std::for_each({static bool something = true})// locally, something is set to true for each test case. on codewars, it is set once.

"always passes the basic tests and fails the advanced & random tests."
remember: ascii vs literal numbers