Actually I didn't think this one through, $regsubex can't be made to return the negative error value if there is one as that could be a valid result: $regsubex(@10,@,-). I don't even know where I got that $regsubex since the report isn't mentioning it.
$regsubex does return -10 if you provide a variable output, which is good, if there's no output, $regsubex returns the original input, which is probably consistent. As long as it's possible to get the error code and tell the difference between no match and an error via the output variable, I don't think anything should be changed.
