For each subject string in the Series, extract groups from the
    first match of regular expression pat. This function is called from
    str_extract(expand=True), and always returns a DataFrame.

def _str_extract_frame(arr, pat, flags=0):
    """
    For each subject string in the Series, extract groups from the
    first match of regular expression pat. This function is called from
    str_extract(expand=True), and always returns a DataFrame.

    """
    from pandas import DataFrame

    regex = re.compile(pat, flags=flags)
    groups_or_na = _groups_or_na_fun(regex)
    names = dict(zip(regex.groupindex.values(), regex.groupindex.keys()))
    columns = [names.get(1 + i, i) for i in range(regex.groups)]

    if len(arr) == 0:
        return DataFrame(columns=columns, dtype=object)
    try:
        result_index = arr.index
    except AttributeError:
        result_index = None
    return DataFrame(
        [groups_or_na(val) for val in arr],
        columns=columns,
        index=result_index,
        dtype=object)