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comex | 26 days ago

If you already know where the start of the opening tag is, then I think a regex is capable of finding the end of that same opening tag, even in cases like yours. In that sense, it’s possible to use a regex to parse a single tag. What’s not possible is finding opening tags within a larger fragment of HTML.

discuss

kstrauser|26 days ago

For any given regex, an opponent can craft a string which is valid HTML but that the regex cannot parse. There are a million edge cases like:

  <!—- Don't count <hr> this! -—> but do count <hr> this -->

and

  <!-- <!-- Ignore <ht> this --> but do count <hr> this —->

Now your regex has to include balanced comment markers. Solve that

You need a context-free grammar to correctly parse HTML with its quoting rules, and escaping, and embedded scripts and CDATA, etc. etc. etc. I don't think any common regex libraries are as powerful as CFGs.

Basically, you can get pretty far with regexes, but it's provably (like in a rigorous compsci kinda way) impossible to correctly parse all valid HTML with only regular expressions.

marcosdumay|26 days ago

HTML comments do not nest. The obvious tokenizer you can create with regular expressions is the correct one.

DemocracyFTW2|25 days ago

I don't think your comment assumes the right givens. I just tried in Vivaldi (i.e. Chrome) and this snippet:

    <!doctype html>
    A<!—- Don't count <hr> this! -—> but do count <hr> that -->Z

gets fixed and rendered as

    <!DOCTYPE html>
    <html><head></head><body>A<!--—- Don't count <hr--> this! -—&gt; but do count <hr> that --&gt;Z</body></html>

Another surprise is that

    <!doctype html>
    A<!—- Don't count this! -— but do count that -->Z

gets rewritten to

    <!DOCTYPE html>
    <html><head></head><body>A<!--—- Don't count this! -— but do count that ---->Z</body></html>

Note the insertion of extra `--` minus-hyphens.

This is what MDN (https://developer.mozilla.org/en-US/docs/Web/HTML/Guides/Com...) has to say:

Comments start with the string ``, generally with text in between. This text cannot start with the string `>` or `->`, cannot contain the strings `-->` or `--!>`, nor end with the string `<!-`, though `<!` is allowed. [...] The above is true for XML comments as well. In addition, in XML, such as in SVG or MathML markup, a comment cannot contain the character sequence `--`.

Meaning that you can recognize HTML comments with (one branch of) a RegEx—you start wherever you see `<!--` and consume everything up to one of the listed alternatives. No nesting required.

Be it said that I find the precise rules too convoluted for what they do. Especially XML's prohibition on `--` in comments is ridiculous taken on its own. First you tell me that a comment ends with three characters `-->`, and then you tell me I can't use the specific substring `--`, either? And why can't I use `--!>`?

An interesting bit here is that AFAIK the `<!` syntax was used in SGML as one of the alternatives to write a 'lone tag', so instead of `<hr></hr>` or `<hr/>` (XHTML) or `<hr>` (HTML) you could write `<!hr>` to denote a tag with no content. We should have kept this IMO.

*EDIT* On the quoted HTML source you see things like `-—` (hyphen-minus, em-dash). This is how the Vivaldi DevTools render it; my text editor and HN comment system did not alter these characters. I have no idea whether Chrome's rendering engine internally uses these em-dashes or whether it's just a quirk in DevTool text output.