How to check if a char belongs to a group of chars, C solution.

Hi,
I want to know how can I check if a char belongs to a group of chars.
for example:
char a;
if (  (a=='1') || (a=='2') || (a=='3') || (A=='4')  || (a=='5') || (a=='6')||(a=='7') || (A=='8') )
 {; }

Is there a way to write it in a short way by defining a group of chars and checking if char a belongs to this group?
The solution has to be in C.
Thanks!