code: def normalize_arxiv(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
docs: Return a normalized arXiv identifier from ``obj``.