The user will be told the format (RTFM?). All other formats will be invalid. In terms of the formats decyphered, I'm not trying to cover everything in the world, just this country.
I will have to look into an try the strtol() functions. I'm not seeing the difference yet. The cplusplus.com reference shows that if no valid conversion is performed, then 0 is returned. The examples provided, do not cover bad input as I listed above. I will do some testing to check and hope you're right.