def remove_prefix_including_first_slash_or_backslash(input_str):
    """
    >>> remove_prefix_including_first_slash_or_backslash('test')
    'test'
    >>> remove_prefix_including_first_slash_or_backslash('test/')
    ''
    >>> remove_prefix_including_first_slash_or_backslash('/test/')
    'test/'

    >>> remove_prefix_including_first_slash_or_backslash('test\\\\')
    ''
    >>> remove_prefix_including_first_slash_or_backslash('test/test2/test3')
    'test2/test3'
    >>> remove_prefix_including_first_slash_or_backslash('test\\test3\\test4')
    'test\\test3\\test4'

    """
    result = input_str
    if '\\' in input_str:
        result = input_str.split('\\', 1)[1]
    elif '/' in input_str:
        result = input_str.split('/', 1)[1]
    return result
