Checks to see if the PDB file has been deprecated and, if so, what the new ID is.

def _get_replacement_pdb_id(self):
        '''Checks to see if the PDB file has been deprecated and, if so, what the new ID is.'''
        deprecation_lines = self.parsed_lines['OBSLTE']
        date_regex = re.compile('(\d+)-(\w{3})-(\d+)')
        if deprecation_lines:
            assert(len(deprecation_lines) == 1)
            tokens = deprecation_lines[0].split()[1:]
            if tokens[1].upper() in obsolete_pdb_ids_with_no_replacement_entries:
                assert(len(tokens) == 2)
            else:
                assert(len(tokens) == 3)
            if self.pdb_id:
                mtchs = date_regex.match(tokens[0])
                assert(mtchs)
                _day = int(mtchs.group(1))
                _month = mtchs.group(2)
                _year = int(mtchs.group(3)) # only two characters?
                assert(tokens[1] == self.pdb_id)
                self.deprecation_date = (_day, _month, _year) # no need to do anything fancier unless this is ever needed
                self.deprecated = True
                if len(tokens) == 3:
                    assert(len(tokens[2]) == 4)
                    self.replacement_pdb_id = tokens[2]