iso_regex = re.compile(
  '([0-9]{4})' #year
  '-?'
  '([0-9]{1,2})' #month
  '(?:'
    '-?'
    '('
      '[0-9]{1,2}' #day?
    ')'
    '(?:'
      '[T ]'
      '([0-9]{1,2})' #hour??
      '(?:'
        ':?'
        '([0-9]{1,2})' #minute???
        '(?:'
          ':?'
          '(?:'
            '([0-9]{1,2})' #second????
            '(?:'
              '\\.?([0-9]+)' #ms?????
            ')?'
          ')'
        ')?'
        '(?:'
          '(Z)|([+-][0-9]{1,2})' #
          ':?'
          '([0-9]{1,2})' #TZ???
        ')'
      ')?'
    ')?'
  ')?'
)

def regex_parse_datetime(s):
  match = iso_regex.match(s)
  if not match:
    return
  groups = list(itertools.takewhile(lambda e: e is not None, match.groups()))
  args = [int(g) for g in groups[:7]]
  # No tzinfo
  return datetime.datetime(*args)