private final static boolean _validBackRef(int index){
  return (index & 0xFF) < 0xFE;
}

private final static boolean _validBackRef ( int index ) { return ( index & 0xFF ) < 0xFE ; }
helper method used to ensure that we do not use back - reference values that would produce illegal byte sequences ( ones with byte 0xfe or 0xff ) . note that we do not try to avoid null byte ( 0x00 ) by default , although it would be technically possible as well .
