IMO "scan" should not be using an interpretation of the string (i.e. internal rep). 
It should only be using the string value. 
It also should be strict with the format type, 
so %d should accept only { 0 1 2 3 4 5 6 7 8 9 + - }. 
Nothing else.
Only if the # flag is used in the %d format specifier should the 0d prefix be accepted.

The C standard library definition should be used here to inform the implementation of [scan].