def iterate_tag(text_chunk, old_string, tagname):
  tag_re = r'(?:<[^>]*>)*'

  full_regex = '(' + tag_re + ''.join(
    itertools.chain(*zip(
      map(re.escape, old_string),
      itertools.cycle([tag_re])))) + ')'
  curr_tag = '{tagname}'.format(tagname=re.escape(tagname))
  return re.sub(full_regex,
              '<START:' + curr_tag + '>' + r'1' + '<END:' + curr_tag + '>', text_chunk)