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HTML Tag Stripper

, 8 Nov 2007 CPOL
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A fast way to strip the HTML tags from an HTML fragment and leave only the visible text

Introduction

This article explains a simple method of stripping HTML tags that is similar to the PHP strip_tags() function. This is usually useful in CMS systems where you need to store the text-only version of, for example, an article in order to allow a full-text search through all articles.

Background

Stripping the tags, in this problem's context, means keeping only the visible text of an HTML document or HTML fragment. This means excluding all HTML comments and all HTML <script>, <style> and <noscript> blocks.

I must also mention the fact that the text resulting from this stripping can be processed even more by replacing named HTML entities such as &quot;, &amp;, &copy;, &nbsp;, etc. and unnamed HTML entities such as &#355; with their corresponding characters. Just set the method's respective parameters, i.e. replaceNamedEntities and replaceNumberedEntities, to true. Bear in mind, however, that these can slow the execution time down significantly.

Using the Code

There is only one method involved in this operation. I called it, without any inspiration, HtmlStripTags. It accepts three parameters:

  • htmlContent: the HTML content to process
  • replaceNamedEntities: whether to replace the HTML named entities such as &nbsp; and others
  • replaceNumberedEntities: whether to replace the HTML numbered entities, i.e. Unicode HTML representations such as &#355;
public static string HtmlStripTags(string htmlContent, 
    bool replaceNamedEntities, bool replaceNumberedEntities)
{
    if (htmlContent == null)
        return null;
    htmlContent = htmlContent.Trim();
    if (htmlContent == string.Empty)
        return string.Empty;

    int bodyStartTagIdx = htmlContent.IndexOf("<body", 
        StringComparison.CurrentCultureIgnoreCase);
    int bodyEndTagIdx = htmlContent.IndexOf("</body>", 
        StringComparison.CurrentCultureIgnoreCase);

    int startIdx = 0, endIdx = htmlContent.Length - 1;
    if (bodyStartTagIdx >= 0)
        startIdx = bodyStartTagIdx;
    if (bodyEndTagIdx >= 0)
        endIdx = bodyEndTagIdx;

    bool insideTag = false,
        insideAttributeValue = false,
        insideHtmlComment = false,
        insideScriptBlock = false,
        insideNoScriptBlock = false,
        insideStyleBlock = false;
    char attributeValueDelimiter = '"';

    StringBuilder sb = new StringBuilder(htmlContent.Length);
    for (int i = startIdx; i <= endIdx; i++)
    {

        // html comment block
        if (!insideHtmlComment)
        {
            if (i + 3 < htmlContent.Length &&
                htmlContent[i] == '<' &&
                ...
                ...
                ...

Points of Interest

I avoided using Regular Expressions in order to achieve maximum performance. RegExs are not yet .NET Framework's strongest point. Moreover, I considered this task simple enough to not require such a universal tool. I ran benchmark tests comparing this implementation to another one presented here at CodeProject.com, Covert HTML to Plain Text, which uses mainly Regular Expressions. I found that when parsing large HTML contents (80+ KB) without replacing any HTML entities, it could yet give 5x times better performance, i.e. ~ 5 microseconds on a Intel Core Duo @ 1,83GHz with 1GB RAM. Of course, it is less elegant than using Regular Expressions. I simply maximized the performance.

History

  • 18 July, 2007 -- Original version posted
  • 8 November, 2007 -- Article content and downloads updated

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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About the Author

Andrei Ion Rînea
Software Developer (Senior) IBM, Business Analytics
Romania Romania
No Biography provided

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Comments and Discussions

 
QuestionEasiest way Pin
Patryk Moura20-Aug-13 12:15
memberPatryk Moura20-Aug-13 12:15 
AnswerRe: Easiest way Pin
PIEBALDconsult20-Aug-13 12:25
professionalPIEBALDconsult20-Aug-13 12:25 
QuestionInfinite loop Pin
Paul C Smith28-Jun-13 2:47
memberPaul C Smith28-Jun-13 2:47 
GeneralMy vote of 4 Pin
rcardare4-Jul-11 6:36
memberrcardare4-Jul-11 6:36 
GeneralNice Code Pin
sweet_gangster_boy13-May-10 9:53
membersweet_gangster_boy13-May-10 9:53 
Questionpossible to skip some tags? Pin
SelArom1-Jun-09 19:38
memberSelArom1-Jun-09 19:38 
Questioncool code Pin
Thea Ganoe8-Nov-07 13:55
memberThea Ganoe8-Nov-07 13:55 
AnswerRe: cool code Pin
Andrei Rinea8-Nov-07 23:13
memberAndrei Rinea8-Nov-07 23:13 
Questionwhat is so special on position 4163 Pin
Kaldor Amir24-Jul-07 21:15
memberKaldor Amir24-Jul-07 21:15 
AnswerRe: what is so special on position 4163 Pin
Andrei Rinea24-Jul-07 21:54
memberAndrei Rinea24-Jul-07 21:54 
QuestionWhy Not RegEx? PinPopular
thund3rstruck21-Jul-07 18:50
memberthund3rstruck21-Jul-07 18:50 
AnswerRe: Why Not RegEx? Pin
Uwe Keim22-Jul-07 0:53
sitebuilderUwe Keim22-Jul-07 0:53 
AnswerRe: Why Not RegEx? Pin
Andrei Rinea22-Jul-07 11:24
memberAndrei Rinea22-Jul-07 11:24 
GeneralRe: Why Not RegEx? Pin
thund3rstruck22-Jul-07 18:29
memberthund3rstruck22-Jul-07 18:29 
GeneralRe: Why Not RegEx? Pin
Andrei Rinea23-Jul-07 0:03
memberAndrei Rinea23-Jul-07 0:03 
AnswerRe: Why Not RegEx? Pin
Gleb Dolgich11-Nov-07 9:29
memberGleb Dolgich11-Nov-07 9:29 
GeneralRe: Why Not RegEx? Pin
Andrei Rinea12-Nov-07 0:25
memberAndrei Rinea12-Nov-07 0:25 
GeneralRe: Why Not RegEx? Pin
sebbie(r)13-Jun-08 0:07
membersebbie(r)13-Jun-08 0:07 
GeneralRe: Why Not RegEx? Pin
Kasim P13-Nov-07 4:12
memberKasim P13-Nov-07 4:12 
AnswerRe: Why Not RegEx? Pin
Daniel Penrod22-Nov-07 14:57
memberDaniel Penrod22-Nov-07 14:57 
GeneralAs is, requires .NET 2 or above Pin
Fred_Smith21-Jul-07 13:12
memberFred_Smith21-Jul-07 13:12 
GeneralRe: As is, requires .NET 2 or above Pin
Andrei Rinea22-Jul-07 11:10
memberAndrei Rinea22-Jul-07 11:10 
GeneralAlso see... Pin
Ravi Bhavnani19-Jul-07 2:55
memberRavi Bhavnani19-Jul-07 2:55 

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