isLetter

int isLetter(int *c){
   static int upperLowerDiff = 'a' - 'A';

   //A-Z: make lower-case and return 1
   if (*c <= 'Z' && *c >= 'A'){
#ifndef CASE_SENSITIVE
      *c += upperLowerDiff;
#endif
      return 1;
   }

   //a-z, apostrophe: return 1
   if (*c >= 'a' && *c <= 'z' || *c == '\'')
      return 1;

   //non-letter: return 0
   return 0;
}