def replace_ext(file_path, new_ext):
    """
    >>> replace_ext('one/two/three.four.doc', '.html')
    'one/two/three.four.html'
    >>> replace_ext('one/two/three.four.DOC', '.html')
    'one/two/three.four.html'
    >>> replace_ext('one/two/three.four.DOC', 'html')
    'one/two/three.four.html'
    """
    if not new_ext.startswith(os.extsep):
        new_ext = os.extsep + new_ext
    index = file_path.rfind(os.extsep)
    return file_path[:index] + new_ext