Validating HTML Tags in a String in C#

Question

Assume that we have the following HTML strings.

string A = " <table width=325><tr><td width=325>test</td></tr></table>"
string B = " <<table width=325><tr><td width=325>test</td></table>"

How can we validate A or B in C# according to HTML specifications?

A should return true whereas B should return false.

You could parse the string and add up all of the < and > characters. If either of them is an odd amount you could assume it's invalid (for this case) — Cory Danielson
– Cory Danielson, Commented Sep 22, 2011 at 18:43
I think both of the html you have provided completely incorrect according to spec. — tugberk
– tugberk, Commented Sep 22, 2011 at 18:49
There is a number of HTML tags in HTML4 and HTML5 that do not require the use of a closing tag for valid HTML: optgroup, option, p, tbody, td, tr, tfoot, thead, th — alex
– alex, Commented Sep 22, 2011 at 18:53

Robotnik · Accepted Answer · 2021-06-29 06:05:26Z

15

For this specific case you can use HTML Agility Pack to assert if the HTML is well formed or if you have tags not opened.

var htmlDoc = new HtmlDocument();

htmlDoc.LoadHtml(
    "WAVEFORM</u> YES, <u>NEGATIVE AUSCULTATION OF EPIGASTRUM</u> YES,");

foreach (var error in htmlDoc.ParseErrors)
{
    // Prints: TagNotOpened
    Console.WriteLine(error.Code);
    // Prints: Start tag <u> was not found
    Console.WriteLine(error.Reason); 
}

Checking a HTML string for unopened tags

edited Jun 29, 2021 at 6:05

Robotnik

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answered Sep 23, 2011 at 6:05

sikender

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6 Comments

Mike Chamberlain Over a year ago

+1 for using an existing library instead of trying to hack it together yourself.

Andrew Bullock Over a year ago

just a warning, whilst the above example works, this: "<p>example<p>" doesnt trigger any parser errors.

Jereme Over a year ago

This also does not throw any parser errors: <script type="text/javascript" src="/Scripts/clipboard.min.js" <="" script="">

Alexander Over a year ago

@AndrewBullock That is because it indeed is valid HTML.

Alexander Over a year ago

@JeremeGuenther Can you link to the github bug ticket that you surely created when you stumbled over this issue?

|

thekip · Accepted Answer · 2011-09-22 18:42:57Z

1

One point to start with is checking if it's valid XML.

by the way, I think both your examples are incorrect as you've left out the </tr> from both.

answered Sep 22, 2011 at 18:42

thekip

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5 Comments

alex Over a year ago

HTML is not valid XML, XHTML is.

thekip Over a year ago

@alex, that's true but at least it could be a start as all other checks would be really hard

alex Over a year ago

There is a number of HTML tags in HTML4 and HTML5 that do not require the use of a closing tag for valid HTML: optgroup, option, p, tbody, td, tr, tfoot, thead, th

Arve Systad Over a year ago

@thekip As long as there is one single HTML element with no closing tag, the entire XML-validity check would be pointless. And there's a pretty good chance there is one.

Cas Sakal Over a year ago

XML is much more strict than HTML and XMLReader does not give proper results.

rohit89 · Accepted Answer · 2016-02-23 12:26:35Z

0

http://web.archive.org/web/20110820163031/http://markbeaton.com/SoftwareInfo.aspx?ID=81a0ecd0-c41c-48da-8a39-f10c8aa3f931

Github link: https://github.com/markbeaton/TidyManaged

This guy has written a .NET wrapper for HTMLTidy. I haven't used it but it may be what you are looking for.

edited Feb 23, 2016 at 12:26

answered Sep 22, 2011 at 18:55

rohit89

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