def is_arxiv(obj):
    """Return ``True`` if ``obj`` contains an arXiv identifier.

    The ``idutils`` library's ``is_arxiv`` function has been
    modified here to work with two regular expressions instead
    of three and adding a check for valid arxiv categories only"""
    arxiv_test = obj.split()
    if not arxiv_test:
        return False

    matched_arxiv = (RE_ARXIV_PRE_2007_CLASS.match(arxiv_test[0]) or
                     RE_ARXIV_POST_2007_CLASS.match(arxiv_test[0]))

    if not matched_arxiv:
        return False

    if not matched_arxiv.group('category'):
        return True

    valid_arxiv_categories_lower = [category.lower() for category in valid_arxiv_categories()]
    category = matched_arxiv.group('category').lower()
    return (category in valid_arxiv_categories_lower or
            category.replace('-', '.') in valid_arxiv_categories_lower)