Figure 10.15  Recursive Function to Extract Capital Letters from a String
/*
 *  Forms a string containing all the capital letters found in the input 
 *  parameter str.
 *  Pre:  caps has sufficient space to store all caps in str plus the null
 */
char *
find_caps(char       *caps,  /* output - string of all caps found in str	*/
          const char *str)   /* input  - string from which to extract caps	*/
{
      char restcaps[STRSIZ]; /* caps from reststr  */

      if (str[0] == '\0') 
            caps[0] = '\0';  /* no letters in str => no caps in str      	*/
      else 
            if (isupper(str[0]))
                  sprintf(caps, "%c%s", str[0], find_caps(restcaps, &str[1]));
            else
                  find_caps(caps, &str[1]);

      return (caps);
}