Comment by lvncelot 14 hours ago
> 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
The regex answer is from the very old days of Stackoverflow, before fun was banned. I agree it barely qualifies as answer, but considering that the question has over 4 million page views (which almost puts it in the top 100 most viewed questions all-time), it has reached a lot people. The answer probably had much more influence than any serious answer on that topic. So I'd say the author did a good job.
People have shared it here and on reddit a bunch of times because it's funny. I always found the pragmatic counter-answer about using regex and the comments about how brittle it is to parse XML properly assuming a specific structure to be much more useful.
For anyone wondering about the railroad switch post: https://stackoverflow.com/questions/11227809/why-is-processi...
This is new to me, and a wonderful dive that I wish I was aware of during my OS course. Thanks!
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.
The thing is, even when parsing html "correctly" (whatever that is) regexes will still be used. Sure, There will be a bunch of additional structures and mechanisms involved, but you will be identifying tokens via a bunch of regexes.
So yes, while it is an inspired comidic genius of a rant, and sort of informative in that it opens your eyes to the limitations of regexes, it sort of brushes under the rug all the places that those poor maligned regular expressions will be used when parsing html.
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.
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 -->
<!-- <!-- Ignore <ht> this --> but do count <hr> this —->
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.
I know this is grumpy but this I’ve never liked this answer. It is a perfect encapsulation of the elitism in the SO community—if you’re new, your questions are closed and your answers are edited and downvoted. Meanwhile this is tolerated only because it’s posted by a member with high rep and username recognition.
I think this answer was tolerated when SO wasn't as bad as it is now, and wouldn't be tolerated now from anyone.
As someone who used to write custom crawlers 20 years ago, I can confirm that regular expressions worked great. All my crawlers were custom designed for a page and the sites were mostly generated by some CMS and had consistent HTML. I don't remember having to do much bug fixes that were related to regular expression issues.
I don't suggest writing generic HTML parsers that works with any site, but for custom crawlers they work great.
Not to say that the tools available are the same now as 20 years ago. Today I would probably use puppeteer or some similar tool and query the DOM instead.
An interesting thing is that most webpages are generated using text templates. There's some text processing like escaping special characters, but it's mostly text that happened to be (somewhat) valid HTML.
So extracting information from this text with regexps often makes perfect sense.
I would distinguish between parsing and scraping. Parsing really needs a, well, parser. Otherwise you’ll get things wrong on perfectly well formed input and your program will be brittle and weird.
A scraper is already resigned to being brittle and weird. You’re relying not only on the syntax of the data, but an implicit structure beyond that. This structure is unspecified and may change without notice, so whatever robustness you can achieve will come from being loose with what you accept and trying to guess what changes might be made on the other end. Regex is a decent tool for that.
Funny how differently people can perceive things. That's my least favorite SO answer of all time, and I cringe every time I see it.
It's a very bad answer. First of all, processing HTML with regex can be perfectly acceptable depending on what you're trying to do. Yes, this doesn't include full-blown "parsing" of arbitrary HTML, but there are plenty of ways in which you might want to process or transform HTML that either don't require producing a parse tree, don't require perfect accuracy, or are operating on HTML whose structure is constrained and known in advance. Second, it doesn't even attempt to explain to OP why parsing arbitrary HTML with regex is impossible or poorly-advised.
The OP didn't want his post to be taken over by someone hamming it up with an attempt at creative writing. He wanted a useful answer. Yes, this answer is "quirky" and "whimsical" and "fun" but I read those as euphemisms for "trying to conscript unwilling victims into your personal sense of nerd-humor".
There's nothing that brings joy into this world quite like the guy waiting around to tell people he doesn't like the thing they like.
The whole argument hinges on one word in your post: arbitrary.
I parse my own HTML I produce directly in a context where I fully control the output. It works fine, but parsing other people’s HTML is a lesson in humility. I’ve also done that, but I did it as a one time thing. I parsed a specific point in time, refusing to change that at any point.
It also hinges on another word: parsing. There are things other than parsing that you might want to do. For example, if you want to count the number of `<hr>` tags in an HTML document, that doesn't require parsing it, and can indeed be done with regex.
No you can’t. You can have an unescaped <hr> inside a script tag, for example. The best you can do is a simple string search for “<hr>” and hope it’s returning what you think it might be returning. Regexps are not powerful enough to determine whether any particular instance of “<hr>” is actually an HTML tag.
Like, it’s not a matter of cleverness, either. You can’t code around it. It’s simply not possible.
I prefer the question about CPU pipelines that gets explained using a railroad switch as example. That one does a decent job of answering the question instead of going of on a, how to best put it, mentally deranged one page rant about regexes with the lazy throw away line at the end being the only thing that makes it qualify as an answer at all.