IMO...
This si a good time to use a regex.
The input is from a 'trusted' source (not user input), is short and relatively constant.
A regex will produce clear and maintainable code.
A regex can be a configuation item, allowing easy change without a rebuild.
BTW if performance is an issue, 'for' is faster than 'foreach' and implicit comparisions (>='0' and <= '9') are faster than string functions (IsDigit)