C/ctype.h/isalnum

    
 
//Declaration:  int isalnum(int ch); 
//Return:       returns nonzero if its argument is either a letter of the alphabet or a digit.   
                returns zero if the character is not alphanumeric.  
  
           
  #include <ctype.h>
  #include <stdio.h>
  int main(void)
  {
    char ch;
    printf(". to exit:");
    for(;;) {
      ch = getc(stdin);
      if(ch == ".")
         break;
      if(isalnum(ch))
         printf("%c is alphanumeric\n", ch);
    }
    return 0;
  }
         
/*
2 is alphanumeric
1 is alphanumeric
1
1 is alphanumeric
2
2 is alphanumeric
12
1 is alphanumeric
2 is alphanumeric
1
1 is alphanumeric
1
1 is alphanumeric
1
1 is alphanumeric
!
~
.
*/