it "should raise an error for string with illegal characters" do
  Test.expect_error(luck_check('6F43E8'))
end 

is obviously wrong - can't catch an exception that has already happened. that needs to be in a block. still strange that it doesn't complain when it doesn't raise when that's what it says it tests for. doesn't really matter though, unless this also happens for MRI 3.0.0

For some reason, I was getting an error message that my Ruby code was not throwing an error when I pushed "Test" although my code had this line: raise ArgumentError, "string with illegal characters" if str.empty? || str.match?(/\D/). By commenting out this line, the test passed. However, it wouldn't pass when I pushed "Attempt". So, I removed the # to uncomment the error line before pushing "Attempt", and the code was accepted.