import re


def truncate_html_words(s, num, end_text='...'):
    """Truncates HTML to a certain number of words.

    (not counting tags and comments). Closes opened tags if they were correctly
    closed in the given html. Takes an optional argument of what should be used
    to notify that the string has been truncated, defaulting to ellipsis (...).

    Newlines in the HTML are preserved. (From the django framework).
    """
    length = int(num)
    if length <= 0:
        return ''
    html4_singlets = ('br', 'col', 'link', 'base', 'img', 'param', 'area',
                      'hr', 'input')

    # Set up regular expressions
    re_words = re.compile(r'&.*?;|<.*?>|(\w[\w-]*)', re.U)
    re_tag = re.compile(r'<(/)?([^ ]+?)(?: (/)| .*?)?>')
    # Count non-HTML words and keep note of open tags
    pos = 0
    end_text_pos = 0
    words = 0
    open_tags = []
    while words <= length:
        m = re_words.search(s, pos)
        if not m:
            # Checked through whole string
            break
        pos = m.end(0)
        if m.group(1):
            # It's an actual non-HTML word
            words += 1
            if words == length:
                end_text_pos = pos
            continue
        # Check for tag
        tag = re_tag.match(m.group(0))
        if not tag or end_text_pos:
            # Don't worry about non tags or tags after our truncate point
            continue
        closing_tag, tagname, self_closing = tag.groups()
        tagname = tagname.lower()  # Element names are always case-insensitive
        if self_closing or tagname in html4_singlets:
            pass
        elif closing_tag:
            # Check for match in open tags list
            try:
                i = open_tags.index(tagname)
            except ValueError:
                pass
            else:
                # SGML: An end tag closes, back to the matching start tag,
                # all unclosed intervening start tags with omitted end tags
                open_tags = open_tags[i + 1:]
        else:
            # Add it to the start of the open tags list
            open_tags.insert(0, tagname)
    if words <= length:
        # Don't try to close tags if we don't need to truncate
        return s
    out = s[:end_text_pos]
    if end_text:
        out += ' ' + end_text
    # Close any tags still open
    for tag in open_tags:
        out += '</%s>' % tag
    # Return string
    return out