int isalnum(int chr);

check if a character Alphanumeric

#include <stdio.h> /* including standard library */
//#include <windows.h> /* uncomment this for Windows */
 
#include <ctype.h>
 
int main ()
{
  int i;
  char string[11]="Test -!&%1";
  for (i=0; i <= 11;i++)
    {
     if ( isalnum(string[i]) != 0 )
      {
       printf("Character %i is alphanumeric\n",i);
      }
    }
  return 0;
}

  user@host:~$ ./isalnum 
  Character 0 is alphanumeric
  Character 1 is alphanumeric
  Character 2 is alphanumeric
  Character 3 is alphanumeric
  Character 9 is alphanumeric