What's up with all those equals signs anyway?

>>todsac+(OP)
The real punchline is that this is a perfect example of "just enough knowledge to be dangerous." Whoever processed these emails knew enough to know emails aren't plain text, but not enough to know that quoted-printable decoding isn't something you hand-roll with find-and-replace. It's the same class of bug as manually parsing HTML with regex, it works right up until it doesn't, and then you get congressional evidence full of mystery equals signs.

>>ruhith+Vh
> It's the same class of bug as manually parsing HTML with regex, it works right up until it doesn't

I'm sure you already know this one, but for anyone else reading this I can share my favourite StackOverflow answer of all time: https://stackoverflow.com/a/1732454

>>lvncel+Ko
It took me years to notice, but did you catch that the answer actually subtly misinterprets what the question is asking for?

Guy (in my reading) appears to talk about matching an entire HTML document with regex. Indeed, that is not possible due to the grammars involved. But that is not what was being asked.

What was being asked is whether the individual HTML tags can be parsed via regex. And to my understanding those are very much workable, and there's no grammar capability mismatch either.

>>perchi+oi1
I think even for single opening tags like asked there are impossible edge cases.

For example, this is perfectly valid XHTML:

    <a href="/" title="<a /> />"></a>

>>tiagod+gl1
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.

>>comex+Zv1
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.

>>kstrau+QT1
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.

zlacker