Regular expression HTML tag javascript
Question
I want to verify if the code that enter is a HTML code ( is necessary to start with <html> and end with </html> )
I try to do this
var reghtml = new RegExp("(<html>*\\n+</html>)");
but I have a problem is necessary to make a \\n in the code, I need to verify the first and end tag ( = <html> and </html> ) and if he make something between them is necessary to start with < and end with >
is there any solution ?
2 answers
solution1
2 2016-11-26 23:02:20
You shouldn't use regular-expressions to validate HTML (let alone parse it) because HTML is not a " Regular Language ".
So here's an example of a false-negative case which would cause any regular expression you could write to attempt to validate HTML to mark it as invalid:
<html>
<head>
<!-- </html> -->
</head>
<body>
<p>This is valid HTML</p>
</body>
</html>
And because you can nest comments in HTML (and SGML and XML) you can't write a straightforward regex for this particular case either:
<html>
<head>
<!-- <!-- <!-- <!-- </html> -->
</head>
<body>
<p>This is valid HTML</p>
</body>
</html>
And here's a false-positive (assuming you don't use the ^$ regex anchors):
<p>illegal element</p>
<html>
<img>illegal text node</img>
</html>
<p>another illegal element</p>
Granted, there are more powerful implementations of of regular-expressions that add rudiminary support for things like counting-depth, but then you're in for a world of hurt.
The correct way to validate HTML is to use a HTML DOM library. In .NET this is HtmlAgilityPack. In browser-based JavaScript it's even simpler: just use the browser's built-in parser ( innerHTML ):
(stolen from Check if HTML snippet is valid with Javascript )
function isValidHtml(html) {
var doc = document.implementation.createHTMLDocuiment("");
doc.documentElement.innerHTML = html;
return ( doc.documentElement.innerHTML === html );
}
solution2
1 2016-11-26 22:52:25
Here a pattern for you. It checks if the first level has a valid opening and closing tag. The first level has to have closing tags, you can't do <html><img /></html> , for that you can remove the whole closing tag checking pattern part.
var validHtml = '\\ <html itemscope>\\ <head></head>\\ <body style="background: red;">\\ Everything is fine\\ </body>\\ </html>\\ ', invalidHtml = '\\ <html itemscope>\\ <head></foot>\\ <body>\\ Nothing is fine\\ </body>\\ </html>\\ ', pattern = /^\\s*<html(?:\\s[^>]*)?>(?:\\s*<(\\w+)(?:\\s[^>]+)?>(?:.|\\s)*<\\/\\1>\\s*)*<\\/html>\\s*$/i; console.log(pattern.test(validHtml) ? 'valid' : 'invalid'); console.log(pattern.test(invalidHtml) ? 'valid' : 'invalid');
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