c# Truncate HTML safely for article summary

Question

Hi All,

Does anyone have a c# variation of this?

This is so I can take some html and display it without breaking as a summary lead in to an article?

http://stackoverflow.com/questions/1193500/php-truncate-html-ignoring-tags

Save me from reinventing the wheel!

Thank you very much

---------- edit ------------------

Sorry, new here, and your right, should have phrased the question better, heres a bit more info

I wish to take a html string and truncate it to a set number of words (or even char length) so I can then show the start of it as a summary (which then leads to the main article). I wish to preserve the html so I can show the links etc in preview.

The main issue I have to solve is the fact that we may well end up with unclosed html tags if we truncate in the middle of 1 or more tags!

The idea I have for solution is to

a) truncate the html to N words (words better but chars ok) first (be sure not to stop in the middle of a tag and truncate a require attribute)

b) work through the opened html tags in this truncated string (maybe stick them on stack as I go?)

c) then work through the closing tags and ensure they match the ones on stack as I pop them off?

d) if any open tags left on stack after this, then write them to end of truncated string and html should be good to go!!!!

-- edit 12112009

• Here is what I have bumbled together so far as a unittest file in VS2008, this 'may' help someone in future
• My hack attempts based on Jan code are at top for char version + word version (DISCLAIMER: this is dirty rough code!! on my part)
• I assume working with 'well-formed' HTML in all cases (but not necessarily a full document with a root node as per XML version)
• Abels XML version is at bottom, but not yet got round to fully getting tests to run on this yet (plus need to understand the code) ...
• I will update when I get chance to refine
• having trouble with posting code? is there no upload facility on stack?

Thanks for all comments :)

using System;
using System.Collections.Generic;
using System.Text.RegularExpressions;
using System.Xml;
using System.Xml.XPath;
using Microsoft.VisualStudio.TestTools.UnitTesting;

namespace PINET40TestProject
{
    [TestClass]
    public class UtilityUnitTest
    {
        public static string TruncateHTMLSafeishChar(string text, int charCount)
        {
            bool inTag = false;
            int cntr = 0;
            int cntrContent = 0;

            // loop through html, counting only viewable content
            foreach (Char c in text)
            {
                if (cntrContent == charCount) break;
                cntr++;
                if (c == '<')
                {
                    inTag = true;
                    continue;
                }

                if (c == '>')
                {
                    inTag = false;
                    continue;
                }
                if (!inTag) cntrContent++;
            }

            string substr = text.Substring(0, cntr);

            //search for nonclosed tags        
            MatchCollection openedTags = new Regex("<[^/](.|\n)*?>").Matches(substr);
            MatchCollection closedTags = new Regex("<[/](.|\n)*?>").Matches(substr);

            // create stack          
            Stack<string> opentagsStack = new Stack<string>();
            Stack<string> closedtagsStack = new Stack<string>();

            // to be honest, this seemed like a good idea then I got lost along the way 
            // so logic is probably hanging by a thread!! 
            foreach (Match tag in openedTags)
            {
                string openedtag = tag.Value.Substring(1, tag.Value.Length - 2);
                // strip any attributes, sure we can use regex for this!
                if (openedtag.IndexOf(" ") >= 0)
                {
                    openedtag = openedtag.Substring(0, openedtag.IndexOf(" "));
                }

                // ignore brs as self-closed
                if (openedtag.Trim() != "br")
                {
                    opentagsStack.Push(openedtag);
                }
            }

            foreach (Match tag in closedTags)
            {
                string closedtag = tag.Value.Substring(2, tag.Value.Length - 3);
                closedtagsStack.Push(closedtag);
            }

            if (closedtagsStack.Count < opentagsStack.Count)
            {
                while (opentagsStack.Count > 0)
                {
                    string tagstr = opentagsStack.Pop();

                    if (closedtagsStack.Count == 0 || tagstr != closedtagsStack.Peek())
                    {
                        substr += "</" + tagstr + ">";
                    }
                    else
                    {
                        closedtagsStack.Pop();
                    }
                }
            }

            return substr;
        }

        public static string TruncateHTMLSafeishWord(string text, int wordCount)
        {
            bool inTag = false;
            int cntr = 0;
            int cntrWords = 0;
            Char lastc = ' ';

            // loop through html, counting only viewable content
            foreach (Char c in text)
            {
                if (cntrWords == wordCount) break;
                cntr++;
                if (c == '<')
                {
                    inTag = true;
                    continue;
                }

                if (c == '>')
                {
                    inTag = false;
                    continue;
                }
                if (!inTag)
                {
                    // do not count double spaces, and a space not in a tag counts as a word
                    if (c == 32 && lastc != 32)
                        cntrWords++;
                }
            }

            string substr = text.Substring(0, cntr) + " ...";

            //search for nonclosed tags        
            MatchCollection openedTags = new Regex("<[^/](.|\n)*?>").Matches(substr);
            MatchCollection closedTags = new Regex("<[/](.|\n)*?>").Matches(substr);

            // create stack          
            Stack<string> opentagsStack = new Stack<string>();
            Stack<string> closedtagsStack = new Stack<string>();

            foreach (Match tag in openedTags)
            {
                string openedtag = tag.Value.Substring(1, tag.Value.Length - 2);
                // strip any attributes, sure we can use regex for this!
                if (openedtag.IndexOf(" ") >= 0)
                {
                    openedtag = openedtag.Substring(0, openedtag.IndexOf(" "));
                }

                // ignore brs as self-closed
                if (openedtag.Trim() != "br")
                {
                    opentagsStack.Push(openedtag);
                }
            }

            foreach (Match tag in closedTags)
            {
                string closedtag = tag.Value.Substring(2, tag.Value.Length - 3);
                closedtagsStack.Push(closedtag);
            }

            if (closedtagsStack.Count < opentagsStack.Count)
            {
                while (opentagsStack.Count > 0)
                {
                    string tagstr = opentagsStack.Pop();

                    if (closedtagsStack.Count == 0 || tagstr != closedtagsStack.Peek())
                    {
                        substr += "</" + tagstr + ">";
                    }
                    else
                    {
                        closedtagsStack.Pop();
                    }
                }
            }

            return substr;
        }

        public static string TruncateHTMLSafeishCharXML(string text, int charCount)
        {
            // your data, probably comes from somewhere, or as params to a methodint 
            XmlDocument xml = new XmlDocument();
            xml.LoadXml(text);
            // create a navigator, this is our primary tool
            XPathNavigator navigator = xml.CreateNavigator();
            XPathNavigator breakPoint = null;

            // find the text node we need:
            while (navigator.MoveToFollowing(XPathNodeType.Text))
            {
                string lastText = navigator.Value.Substring(0, Math.Min(charCount, navigator.Value.Length));
                charCount -= navigator.Value.Length;
                if (charCount <= 0)
                {
                    // truncate the last text. Here goes your "search word boundary" code:        
                    navigator.SetValue(lastText);
                    breakPoint = navigator.Clone();
                    break;
                }
            }

            // first remove text nodes, because Microsoft unfortunately merges them without asking
            while (navigator.MoveToFollowing(XPathNodeType.Text))
            {
                if (navigator.ComparePosition(breakPoint) == XmlNodeOrder.After)
                {
                    navigator.DeleteSelf();
                }
            }

            // moves to parent, then move the rest
            navigator.MoveTo(breakPoint);
            while (navigator.MoveToFollowing(XPathNodeType.Element))
            {
                if (navigator.ComparePosition(breakPoint) == XmlNodeOrder.After)
                {
                    navigator.DeleteSelf();
                }
            }

            // moves to parent
            // then remove *all* empty nodes to clean up (not necessary):
            // TODO, add empty elements like <br />, <img /> as exclusion
            navigator.MoveToRoot();
            while (navigator.MoveToFollowing(XPathNodeType.Element))
            {
                while (!navigator.HasChildren && (navigator.Value ?? "").Trim() == "")
                {
                    navigator.DeleteSelf();
                }
            }

            // moves to parent
            navigator.MoveToRoot();
            return navigator.InnerXml;
        }

        [TestMethod]
        public void TestTruncateHTMLSafeish()
        {
            // Case where we just make it to start of HREF (so effectively an empty link)

            // 'simple' nested none attributed tags
            Assert.AreEqual(@"<h1>1234</h1><b><i>56789</i>012</b>",
            TruncateHTMLSafeishChar(
                @"<h1>1234</h1><b><i>56789</i>012345</b>",
                12));

            // In middle of a!
            Assert.AreEqual(@"<h1>1234</h1><a href=""testurl""><b>567</b></a>",
            TruncateHTMLSafeishChar(
                @"<h1>1234</h1><a href=""testurl""><b>5678</b></a><i><strong>some italic nested in string</strong></i>",
                7));

            // more
            Assert.AreEqual(@"<div><b><i><strong>1</strong></i></b></div>",
            TruncateHTMLSafeishChar(
                @"<div><b><i><strong>12</strong></i></b></div>",
                1));

            // br
            Assert.AreEqual(@"<h1>1 3 5</h1><br />6",
            TruncateHTMLSafeishChar(
                @"<h1>1 3 5</h1><br />678<br />",
                6));
        }

        [TestMethod]
        public void TestTruncateHTMLSafeishWord()
        {
            // zero case
            Assert.AreEqual(@" ...",
                            TruncateHTMLSafeishWord(
                                @"",
                               5));

            // 'simple' nested none attributed tags
            Assert.AreEqual(@"<h1>one two <br /></h1><b><i>three  ...</i></b>",
            TruncateHTMLSafeishWord(
                @"<h1>one two <br /></h1><b><i>three </i>four</b>",
                3), "we have added ' ...' to end of summary");

            // In middle of a!
            Assert.AreEqual(@"<h1>one two three </h1><a href=""testurl""><b class=""mrclass"">four  ...</b></a>",
            TruncateHTMLSafeishWord(
                @"<h1>one two three </h1><a href=""testurl""><b class=""mrclass"">four five </b></a><i><strong>some italic nested in string</strong></i>",
                4));

            // start of h1
            Assert.AreEqual(@"<h1>one two three  ...</h1>",
            TruncateHTMLSafeishWord(
                @"<h1>one two three </h1><a href=""testurl""><b>four five </b></a><i><strong>some italic nested in string</strong></i>",
                3));

            // more than words available
            Assert.AreEqual(@"<h1>one two three </h1><a href=""testurl""><b>four five </b></a><i><strong>some italic nested in string</strong></i> ...",
            TruncateHTMLSafeishWord(
                @"<h1>one two three </h1><a href=""testurl""><b>four five </b></a><i><strong>some italic nested in string</strong></i>",
                99));
        }

        [TestMethod]
        public void TestTruncateHTMLSafeishWordXML()
        {
            // zero case
            Assert.AreEqual(@" ...",
                            TruncateHTMLSafeishWord(
                                @"",
                               5));

            // 'simple' nested none attributed tags
            string output = TruncateHTMLSafeishCharXML(
                @"<body><h1>one two </h1><b><i>three </i>four</b></body>",
                13);
            Assert.AreEqual(@"<body>\r\n  <h1>one two </h1>\r\n  <b>\r\n    <i>three</i>\r\n  </b>\r\n</body>", output,
             "XML version, no ... yet and addeds '\r\n  + spaces?' to format document");

            // In middle of a!
            Assert.AreEqual(@"<h1>one two three </h1><a href=""testurl""><b class=""mrclass"">four  ...</b></a>",
            TruncateHTMLSafeishCharXML(
                @"<body><h1>one two three </h1><a href=""testurl""><b class=""mrclass"">four five </b></a><i><strong>some italic nested in string</strong></i></body>",
                4));

            // start of h1
            Assert.AreEqual(@"<h1>one two three  ...</h1>",
            TruncateHTMLSafeishCharXML(
                @"<h1>one two three </h1><a href=""testurl""><b>four five </b></a><i><strong>some italic nested in string</strong></i>",
                3));

            // more than words available
            Assert.AreEqual(@"<h1>one two three </h1><a href=""testurl""><b>four five </b></a><i><strong>some italic nested in string</strong></i> ...",
            TruncateHTMLSafeishCharXML(
                @"<h1>one two three </h1><a href=""testurl""><b>four five </b></a><i><strong>some italic nested in string</strong></i>",
                99));
        }
    }
}

Answer 1

This is complicated and, as far as I can see, none of the PHP solutions is perfect. What if the text is:

substr("Hello, my <strong>name is <em>Sam</em>. I&acute;m a 
  web developer.  And this text is very long and all the text 
  is inside the sam html tag..</strong>",0,26)."..."

You will actually have to iterate through the whole text to find the end of the starting strong-tag.

My advice to you is to strip all html in the summary. Remember to use html-sanitizing if you are showing users own html-code!

Good luck :)

Answer 2

Ok. This should work (dirty code alert):

        string blah = "hoi <strong>dit <em>is test bla meer tekst</em></strong>";
        int aantalChars = 10;


        bool inTag = false;
        int cntr = 0;
        int cntrContent = 0;
        foreach (Char c in blah)
        {
            if (cntrContent == aantalChars) break;



            cntr++;
            if (c == '<')
            {
                inTag = true;
                continue;
            }
            else if (c == '>')
            {
                inTag = false;
                continue;
            }

            if (!inTag) cntrContent++;
        }

        string substr = blah.Substring(0, cntr);

        //search for nonclosed tags
        MatchCollection openedTags = new Regex("<[^/](.|\n)*?>").Matches(substr);
        MatchCollection closedTags = new Regex("<[/](.|\n)*?>").Matches(substr);

        for (int i =openedTags.Count - closedTags.Count; i >= 1; i--)
        {
            string closingTag = "</" + openedTags[closedTags.Count + i - 1].Value.Substring(1);
            substr += closingTag;
        }