When building a large custom PHP CMS system for DigitalPeach, I ran into a very difficult issue: truncating text but maintaining HTML nested tags correctly. Specifically, we were looking to breaking up large articles composed using FCKEditor into separate pages after a certain character threshold. Once can easily see the problem:

<p>This is a test <strong>with some bold in here</strong>.</p>
                Now imaging having to truncate this text to 30 characters, and you end up with this:
                
                This is a test <strong>with
                

While this example isn't quite so severe and at worst would only make the rest of the text within the block-level element bold, clearly if we do a truncation that is blind to HTML some serious problems can arise. Furthermore, even if it doesn't have much practical significance, you are breaking XHTML.

I was lucky to stumble across an excellent article by Mike Burns who describes a Ruby method using REXML's pull parser that can accomplish this. His example extended the String class, so I modified it to work as a Rails helper all in one method:

def truncate_html(input, len = 30, extension = "...")
                  def attrs_to_s(attrs)
                    return '' if attrs.empty?
                    attrs.to_a.map { |attr| %{#{attr[0]}="#{attr[1]}"} }.join(' ')
                  end
                
                  p = REXML::Parsers::PullParser.new(input)
                    tags = []
                    new_len = len
                    results = ''
                    while p.has_next? && new_len > 0
                      p_e = p.pull
                      case p_e.event_type
                    when :start_element
                      tags.push p_e[0]
                      results << "<#{tags.last} #{attrs_to_s(p_e[1])}>"
                    when :end_element
                      results << "</#{tags.pop}>"
                    when :text
                      results << p_e[0].first(new_len)
                      new_len -= p_e[0].length
                    else
                      results << "<!-- #{p_e.inspect} -->"
                    end
                  end
                
                  tags.reverse.each do |tag|
                    results << "</#{tag}>"
                  end
                
                  results.to_s + (input.length > len ? extension : '')
                end
                

Note that the nested method above is a completely valid use of Ruby, though not widely used.

And now look at what it can do:

truncate_html("<p>Test <strong>bold</strong> done.</p>", 30)
                # => "<p>Test <strong>bold</strong></p>..."