You can either use ismember to determine which positions contain the characters you're looking for, or you can use isstrprop. Then you can loop over the groups of digits. You can use find and diff to determine the positions where such a group starts. If you need to support decimal values as well things will get slightly more complicated.
I will not provide a complete working solution because this is homework. You can find guidelines for posting homework on this forum here.
You should also be aware that the string data type is something different from a char array.