Return a normalized arXiv identifier from ``obj``.

def normalize_arxiv(obj):
    """Return a normalized arXiv identifier from ``obj``."""
    obj = obj.split()[0]

    matched_arxiv_pre = RE_ARXIV_PRE_2007_CLASS.match(obj)
    if matched_arxiv_pre:
        return ('/'.join(matched_arxiv_pre.group("extraidentifier", "identifier"))).lower()

    matched_arxiv_post = RE_ARXIV_POST_2007_CLASS.match(obj)
    if matched_arxiv_post:
        return matched_arxiv_post.group("identifier")

    return None