html - RegEx match open tags except XHTML self-contained tags

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96

You can't parse [X]HTML with regex. Because HTML can't be parsed by regex. Regex is not a tool that can be used to correctly parse HTML. As I have answered in HTML-and-regex questions here so many times before, the use of regex will not allow you to consume HTML. Regular expressions are a tool that is insufficiently sophisticated to understand the constructs employed by HTML. HTML is not a regular language and hence cannot be parsed by regular expressions. Regex queries are not equipped to break down HTML into its meaningful parts. so many times but it is not getting to me. Even enhanced irregular regular expressions as used by Perl are not up to the task of parsing HTML. You will never make me crack. HTML is a language of sufficient complexity that it cannot be parsed by regular expressions. Even Jon Skeet cannot parse HTML using regular expressions. Every time you attempt to parse HTML with regular expressions, the unholy child weeps the blood of virgins, and Russian hackers pwn your webapp. Parsing HTML with regex summons tainted souls into the realm of the living. HTML and regex go together like love, marriage, and ritual infanticide. The <center> cannot hold it is too late. The force of regex and HTML together in the same conceptual space will destroy your mind like so much watery putty. If you parse HTML with regex you are giving in to Them and their blasphemous ways which doom us all to inhuman toil for the One whose Name cannot be expressed in the Basic Multilingual Plane, he comes. HTML-plus-regexp will liquify the n​erves of the sentient whilst you observe, your psyche withering in the onslaught of horror. Rege̿̔̉x-based HTML parsers are the cancer that is killing StackOverflow it is too late it is too late we cannot be saved the transgression of a chi͡ld ensures regex will consume all living tissue (except for HTML which it cannot, as previously prophesied) dear lord help us how can anyone survive this scourge using regex to parse HTML has doomed humanity to an eternity of dread torture and security holes using regex as a tool to process HTML establishes a breach between this world and the dread realm of c͒ͪo͛ͫrrupt entities (like SGML entities, but more corrupt) a mere glimpse of the world of reg​ex parsers for HTML will ins​tantly transport a programmer's consciousness into a world of ceaseless screaming, he comes, the pestilent slithy regex-infection wil​l devour your HT​ML parser, application and existence for all time like Visual Basic only worse he comes he comes do not fi​ght he com̡e̶s, ̕h̵i​s un̨ho͞ly radiańcé destro҉ying all enli̍̈́̂̈́ghtenment, HTML tags lea͠ki̧n͘g fr̶ǫm ̡yo​͟ur eye͢s̸ ̛l̕ik͏e liq​uid pain, the song of re̸gular exp​ression parsing will exti​nguish the voices of mor​tal man from the sp​here I can see it can you see ̲͚̖͔̙î̩́t̲͎̩̱͔́̋̀ it is beautiful t​he final snuffing of the lie​s of Man ALL IS LOŚ͖̩͇̗̪̏̈́T ALL I​S LOST the pon̷y he comes he c̶̮omes he comes the ich​or permeates all MY FACE MY FACE ᵒh god no NO NOO̼O​O NΘ stop the an​*̶͑̾̾​̅ͫ͏̙̤g͇̫͛͆̾ͫ̑͆l͖͉̗̩̳̟̍ͫͥͨe̠̅s ͎a̧͈͖r̽̾̈́͒͑e n​ot rè̑ͧ̌aͨl̘̝̙̃ͤ͂̾̆ ZA̡͊͠͝LGΌ ISͮ̂҉̯͈͕̹̘̱ TO͇̹̺ͅƝ̴ȳ̳ TH̘Ë͖́̉ ͠P̯͍̭O̚​N̐Y̡ H̸̡̪̯ͨ͊̽̅̾̎Ȩ̬̩̾͛ͪ̈́̀́͘ ̶̧̨̱̹̭̯ͧ̾ͬC̷̙̲̝͖ͭ̏ͥͮ͟Oͮ͏̮̪̝͍M̲̖͊̒ͪͩͬ̚̚͜Ȇ̴̟̟͙̞ͩ͌͝S̨̥̫͎̭ͯ̿̔̀ͅ


Have you tried using an XML parser instead?


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81

While arbitrary HTML with only a regex is impossible, it's sometimes appropriate to use them for parsing a limited, known set of HTML.

If you have a small set of HTML pages that you want to scrape data from and then stuff into a database, regexes might work fine. For example, I recently wanted to get the names, parties, and districts of Australian federal Representatives, which I got off of the Parliament's web site. This was a limited, one-time job.

Regexes worked just fine for me, and were very fast to set up.

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76

I think the flaw here is that HTML is a Chomsky Type 2 grammar (context free grammar) and a regular expression is a Chomsky Type 3 grammar (regular grammar). Since a Type 2 grammar is fundamentally more complex than a Type 3 grammar (see the Chomsky hierarchy), it is mathematically impossible to parse XML with a regular expression.

But many will try, and some will even claim success - but until others find the fault and totally mess you up.

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66

Don't listen to these guys. You totally can parse context-free grammars with regex if you break the task into smaller pieces. You can generate the correct pattern with a script that does each of these in order:

  1. Solve the Halting Problem.
  2. Square a circle.
  3. Work out the Traveling Salesman Problem in O(log n) or less. If it's any more than that, you'll run out of RAM and the engine will hang.
  4. The pattern will be pretty big, so make sure you have an algorithm that losslessly compresses random data.
  5. Almost there - just divide the whole thing by zero. Easy-peasy.

I haven't quite finished the last part myself, but I know I'm getting close. It keeps throwing CthulhuRlyehWgahnaglFhtagnExceptions for some reason, so I'm going to port it to VB 6 and use On Error Resume Next. I'll update with the code once I investigate this strange door that just opened in the wall. Hmm.

P.S. Pierre de Fermat also figured out how to do it, but the margin he was writing in wasn't big enough for the code.

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56

Disclaimer: use a parser if you have the option. That said...

This is the regex I use (!) to match HTML tags:

<(?:"[^"]*"['"]*|'[^']*'['"]*|[^'">])+> 

It may not be perfect, but I ran this code through a lot of HTML. Note that it even catches strange things like <a name="badgenerator"">, which show up on the web.

I guess to make it not match self contained tags, you'd either want to use Kobi's negative look-behind:

<(?:"[^"]*"['"]*|'[^']*'['"]*|[^'">])+(?<!/\s*)> 

or just combine if and if not.

To downvoters: This is working code from an actual product. I doubt anyone reading this page will get the impression that it is socially acceptable to use regexes on HTML.

Caveat: I should note that this regex still breaks down in the presence of CDATA blocks, comments, and script and style elements. Good news is, you can get rid of those using a regex...

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