def _before_underscore(s):
    """
    :type s: str
    :rtype: str

    >>> _before_underscore('LC80880750762013254ASA00_IDF.xml')
    'LC80880750762013254ASA00'
    >>> _before_underscore('LC80880750762013254ASA00_MD5.txt')
    'LC80880750762013254ASA00'
    >>> _before_underscore('383.000.2013137232105971.ASA')
    >>> _before_underscore('_MD5.txt')
    ''
    """
    if '_' not in s:
        return None

    return s.split('_')[0]