code: public static char[] getAllowedFunctionCharacters() {
          char[] chars = new char[53];
          int count = 0;
          for (int i = 65; i < 91; i++) {
              chars[count++] = (char) i;
          }
          for (int i = 97; i < 123; i++) {
              chars[count++] = (char) i;
          }
          chars[count] = '_';
          return chars;
      }
docs: Get the set of characters which are allowed for use in Function names.
  
  @return the set of characters allowed
  @deprecated since 0.4.5 All unicode letters are allowed to be used in function names since 0.4.3. This API
  Function can be safely ignored. Checks for function name validity can be done using Character.isLetter() et al.