How can I strip HTML tags from a string in ASP.NET

问题:

Using ASP.NET, how can I strip the HTML tags from a given string reliably (i.e. not using regex)? I am looking for something like PHP\'s strip_tags.

Example:

<ul><li>Hello</li></ul>

Output:

\"Hello\"

I am trying not to reinvent the wheel, but I have not found anything that meets my needs so far.

回答1:

If it is just stripping all HTML tags from a string, this works reliably with regex as well. Replace:

<[^>]*(>|$)

with the empty string, globally. Don\'t forget to normalize the string afterwards, replacing:

[\\s\\r\\n]+

with a single space, and trimming the result. Optionally replace any HTML character entities back to the actual characters.

Note:

1. There is a limitation: HTML and XML allow > in attribute values. This solution will return broken markup when encountering such values.
2. The solution is technically safe, as in: The result will never contain anything that could be used to do cross site scripting or to break a page layout. It is just not very clean.
3. As with all things HTML and regex:
   Use a proper parser if you must get it right under all circumstances.

回答2:

Go download HTMLAgilityPack, now! ;) Download LInk

This allows you to load and parse HTML. Then you can navigate the DOM and extract the inner values of all attributes. Seriously, it will take you about 10 lines of code at the maximum. It is one of the greatest free .net libraries out there.

Here is a sample:

            string htmlContents = new System.IO.StreamReader(resultsStream,Encoding.UTF8,true).ReadToEnd();

            HtmlAgilityPack.HtmlDocument doc = new HtmlAgilityPack.HtmlDocument();
            doc.LoadHtml(htmlContents);
            if (doc == null) return null;

            string output = \"\";
            foreach (var node in doc.DocumentNode.ChildNodes)
            {
                output += node.InnerText;
            }

回答3:

Regex.Replace(htmlText, \"<.*?>\", string.Empty);

回答4:

protected string StripHtml(string Txt)
{
    return Regex.Replace(Txt, \"<(.|\\\\n)*?>\", string.Empty);
}    

Protected Function StripHtml(Txt as String) as String
    Return Regex.Replace(Txt, \"<(.|\\n)*?>\", String.Empty)
End Function

回答5:

I\'ve posted this on the asp.net forums, and it still seems to be one of the easiest solutions out there. I won\'t guarantee it\'s the fastest or most efficient, but it\'s pretty reliable. In .NET you can use the HTML Web Control objects themselves. All you really need to do is insert your string into a temporary HTML object such as a DIV, then use the built-in \'InnerText\' to grab all text that is not contained within tags. See below for a simple C# example:

System.Web.UI.HtmlControls.HtmlGenericControl htmlDiv = new System.Web.UI.HtmlControls.HtmlGenericControl(\"div\");
htmlDiv.InnerHtml = htmlString;
String plainText = htmlDiv.InnerText;

回答6:

I have written a pretty fast method in c# which beats the hell out of the Regex. It is hosted in an article on CodeProject.

Its advantages are, among better performance the ability to replace named and numbered HTML entities (those like & and &203;) and comment blocks replacement and more.

Please read the related article on CodeProject.

Thank you.

回答7:

For those of you who can\'t use the HtmlAgilityPack, .NETs XML reader is an option. This can fail on well formatted HTML though so always add a catch with regx as a backup. Note this is NOT fast, but it does provide a nice opportunity for old school step through debugging.

public static string RemoveHTMLTags(string content)
    {
        var cleaned = string.Empty;
        try
        {
            StringBuilder textOnly = new StringBuilder();
            using (var reader = XmlNodeReader.Create(new System.IO.StringReader(\"<xml>\" + content + \"</xml>\")))
            {
                while (reader.Read())
                {
                    if (reader.NodeType == XmlNodeType.Text)
                        textOnly.Append(reader.ReadContentAsString());
                }
            }
            cleaned = textOnly.ToString();
        }
        catch
        {
            //A tag is probably not closed. fallback to regex string clean.
            string textOnly = string.Empty;
            Regex tagRemove = new Regex(@\"<[^>]*(>|$)\");
            Regex compressSpaces = new Regex(@\"[\\s\\r\\n]+\");
            textOnly = tagRemove.Replace(content, string.Empty);
            textOnly = compressSpaces.Replace(textOnly, \" \");
            cleaned = textOnly;
        }

        return cleaned;
    }

回答8:

string result = Regex.Replace(anytext, @\"<(.|\\n)*?>\", string.Empty);

回答9:

For those who are complining about Michael Tiptop\'s solution not working, here is the .Net4+ way of doing it:

public static string StripTags(this string markup)
{
    try
    {
        StringReader sr = new StringReader(markup);
        XPathDocument doc;
        using (XmlReader xr = XmlReader.Create(sr,
                           new XmlReaderSettings()
                           {
                               ConformanceLevel = ConformanceLevel.Fragment
                               // for multiple roots
                           }))
        {
            doc = new XPathDocument(xr);
        }

        return doc.CreateNavigator().Value; // .Value is similar to .InnerText of  
                                           //  XmlDocument or JavaScript\'s innerText
    }
    catch
    {
        return string.Empty;
    }
}

回答10:

I\'ve looked at the Regex based solutions suggested here, and they don\'t fill me with any confidence except in the most trivial cases. An angle bracket in an attribute is all it would take to break, let alone mal-formmed HTML from the wild. And what about entities like &? If you want to convert HTML into plain text, you need to decode entities too.

So I propose the method below.

Using HtmlAgilityPack, this extension method efficiently strips all HTML tags from an html fragment. Also decodes HTML entities like &. Returns just the inner text items, with a new line between each text item.

public static string RemoveHtmlTags(this string html)
{
        if (String.IsNullOrEmpty(html))
            return html;

        var doc = new HtmlAgilityPack.HtmlDocument();
        doc.LoadHtml(html);

        if (doc.DocumentNode == null || doc.DocumentNode.ChildNodes == null)
        {
            return WebUtility.HtmlDecode(html);
        }

        var sb = new StringBuilder();

        var i = 0;

        foreach (var node in doc.DocumentNode.ChildNodes)
        {
            var text = node.InnerText.SafeTrim();

            if (!String.IsNullOrEmpty(text))
            {
                sb.Append(text);

                if (i < doc.DocumentNode.ChildNodes.Count - 1)
                {
                    sb.Append(Environment.NewLine);
                }
            }

            i++;
        }

        var result = sb.ToString();

        return WebUtility.HtmlDecode(result);
}

public static string SafeTrim(this string str)
{
    if (str == null)
        return null;

    return str.Trim();
}

If you are really serious, you\'d want to ignore the contents of certain HTML tags too (<script>, <style>, <svg>, <head>, <object> come to mind!) because they probably don\'t contain readable content in the sense we are after. What you do there will depend on your circumstances and how far you want to go, but using HtmlAgilityPack it would be pretty trivial to whitelist or blacklist selected tags.

If you are rendering the content back to an HTML page, make sure you understand XSS vulnerability & how to prevent it - i.e. always encode any user-entered text that gets rendered back onto an HTML page (> becomes > etc).

回答11:

For the second parameter,i.e. keep some tags, you may need some code like this by using HTMLagilityPack:

public string StripTags(HtmlNode documentNode, IList keepTags)
{
    var result = new StringBuilder();
        foreach (var childNode in documentNode.ChildNodes)
        {
            if (childNode.Name.ToLower() == \"#text\")
            {
                result.Append(childNode.InnerText);
            }
            else
            {
                if (!keepTags.Contains(childNode.Name.ToLower()))
                {
                    result.Append(StripTags(childNode, keepTags));
                }
                else
                {
                    result.Append(childNode.OuterHtml.Replace(childNode.InnerHtml, StripTags(childNode, keepTags)));
                }
            }
        }
        return result.ToString();
    }

More explanation on this page: http://nalgorithm.com/2015/11/20/strip-html-tags-of-an-html-in-c-strip_html-php-equivalent/

回答12:

using System.Text.RegularExpressions;

string str = Regex.Replace(HttpUtility.HtmlDecode(HTMLString), \"<.*?>\", string.Empty);

回答13:

Simply use string.StripHTML();