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I need to match all of these opening tags:
<p>
<a href="foo">
But not these:
<br />
<hr class="foo" />
I came up with this and wanted to make sure I've got it right. I am only capturing the a-z.
<([a-z]+) *[^/]*?>
I believe it says:
Find a less-than, then
Find (and capture) a-z one or more times, then
Find zero or more spaces, then
Find any character zero or more times, greedy, except /, then
Find a greater-than
Do I have that right? And more importantly, what do you think?
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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 nerves 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 regex parsers for HTML will instantly transport a programmer's consciousness into a world of ceaseless screaming, he comes, the pestilent slithy regex-infection will devour your HTML parser, application and existence for all time like Visual Basic only worse he comes he comes do not fight he com̡e̶s, ̕h̵is un̨ho͞ly radiańcé destro҉ying all enli̍̈́̂̈́ghtenment, HTML tags lea͠ki̧n͘g fr̶ǫm ̡yo͟ur eye͢s̸ ̛l̕ik͏e liquid pain, the song of re̸gular expression parsing will extinguish the voices of mortal man from the sphere I can see it can you see ̲͚̖͔̙î̩́t̲͎̩̱͔́̋̀ it is beautiful the final snuffing of the lies of Man ALL IS LOŚ͖̩͇̗̪̏̈́T ALL IS LOST the pon̷y he comes he c̶̮omes he comes the ichor permeates all MY FACE MY FACE ᵒh god no NO NOO̼OO NΘ stop the an*̶͑̾̾̅ͫ͏̙̤g͇̫͛͆̾ͫ̑͆l͖͉̗̩̳̟̍ͫͥͨe̠̅s ͎a̧͈͖r̽̾̈́͒͑e not rè̑ͧ̌aͨl̘̝̙̃ͤ͂̾̆ ZA̡͊͠͝LGΌ ISͮ̂҉̯͈͕̹̘̱ TO͇̹̺ͅƝ̴ȳ̳ TH̘Ë͖́̉ ͠P̯͍̭O̚N̐Y̡ H̸̡̪̯ͨ͊̽̅̾̎Ȩ̬̩̾͛ͪ̈́̀́͘ ̶̧̨̱̹̭̯ͧ̾ͬC̷̙̲̝͖ͭ̏ͥͮ͟Oͮ͏̮̪̝͍M̲̖͊̒ͪͩͬ̚̚͜Ȇ̴̟̟͙̞ͩ͌͝S̨̥̫͎̭ͯ̿̔̀ͅ
Have you tried using an XML parser instead?
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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.
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), you can't possibly make this work.
But many will try, and some will even claim success - but until others find the fault and totally mess you up.
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...
There are people that will tell you that the Earth is round (or perhaps that the Earth is an oblate spheroid if they want to use strange words). They are lying.
There are people that will tell you that Regular Expressions shouldn't be recursive. They are limiting you. They need to subjugate you, and they do it by keeping you in ignorance.
You can live in their reality or take the red pill.
Like Lord Marshal (is he a relative of the Marshal .NET class?), I have seen the Underverse Stack Based Regex-Verse and returned with powers knowledge you can't imagine. Yes, I think there were an Old One or two protecting them, but they were watching football on the TV, so it wasn't difficult.
I think the XML case is quite simple. The RegEx (in the .NET syntax), deflated and coded in base64 to make it easier to comprehend by your feeble mind, should be something like this: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The options to set is RegexOptions.ExplicitCapture. The capture group you are looking for is ELEMENTNAME. If the capture group ERROR is not empty then there was a parsing error and the Regex stopped.
If you have problems reconverting it to a human-readable regex, this should help:
static string FromBase64(string str)
{
byte[] byteArray = Convert.FromBase64String(str);
using (var msIn = new MemoryStream(byteArray))
using (var msOut = new MemoryStream()) {
using (var ds = new DeflateStream(msIn, CompressionMode.Decompress)) {
ds.CopyTo(msOut);
}
return Encoding.UTF8.GetString(msOut.ToArray());
}
}
If you are unsure, no, I'm NOT kidding (but perhaps I'm lying). It WILL work. I've built tons of unit tests to test it, and I have even used (part of) the conformance tests. It's a tokenizer, not a full-blown parser, so it will only split the XML into its component tokens. It won't parse/integrate DTDs.
Oh... if you want the source code of the regex, with some auxiliary methods:
regex to tokenize an xml or the full plain regex
In shell, you can parse HTML using sed:
Turing.sed
Write HTML parser (homework)
???
Profit!
Related (why you shouldn't use regex match):
If You Like Regular Expressions So Much, Why Don't You Marry Them?
Regular Expressions: Now You Have Two Problems
Hacking stackoverflow.com's HTML sanitizer
I agree that the right tool to parse XML and especially HTML is a parser and not a regular expression engine. However, like others have pointed out, sometimes using a regex is quicker, easier, and gets the job done if you know the data format.
Microsoft actually has a section of Best Practices for Regular Expressions in the .NET Framework and specifically talks about Consider[ing] the Input Source.
Regular Expressions do have limitations, but have you considered the following?
The .NET framework is unique when it comes to regular expressions in that it supports Balancing Group Definitions.
See Matching Balanced Constructs with .NET Regular Expressions
See .NET Regular Expressions: Regex and Balanced Matching
See Microsoft's docs on Balancing Group Definitions
For this reason, I believe you CAN parse XML using regular expressions. Note however, that it must be valid XML (browsers are very forgiving of HTML and allow bad XML syntax inside HTML). This is possible since the "Balancing Group Definition" will allow the regular expression engine to act as a PDA.
Quote from article 1 cited above:
.NET Regular Expression Engine
As described above properly balanced constructs cannot be described by
a regular expression. However, the .NET regular expression engine
provides a few constructs that allow balanced constructs to be
recognized.
(?<group>) - pushes the captured result on the capture stack with
the name group.
(?<-group>) - pops the top most capture with the name group off the
capture stack.
(?(group)yes|no) - matches the yes part if there exists a group
with the name group otherwise matches no part.
These constructs allow for a .NET regular expression to emulate a
restricted PDA by essentially allowing simple versions of the stack
operations: push, pop and empty. The simple operations are pretty much
equivalent to increment, decrement and compare to zero respectively.
This allows for the .NET regular expression engine to recognize a
subset of the context-free languages, in particular the ones that only
require a simple counter. This in turn allows for the non-traditional
.NET regular expressions to recognize individual properly balanced
constructs.
Consider the following regular expression:
(?=<ul\s+id="matchMe"\s+type="square"\s*>)
(?>
<!-- .*? --> |
<[^>]*/> |
(?<opentag><(?!/)[^>]*[^/]>) |
(?<-opentag></[^>]*[^/]>) |
[^<>]*
)*
(?(opentag)(?!))
Use the flags:
Singleline
IgnorePatternWhitespace (not necessary if you collapse regex and remove all whitespace)
IgnoreCase (not necessary)
Regular Expression Explained (inline)
(?=<ul\s+id="matchMe"\s+type="square"\s*>) # match start with <ul id="matchMe"...
(?> # atomic group / don't backtrack (faster)
<!-- .*? --> | # match xml / html comment
<[^>]*/> | # self closing tag
(?<opentag><(?!/)[^>]*[^/]>) | # push opening xml tag
(?<-opentag></[^>]*[^/]>) | # pop closing xml tag
[^<>]* # something between tags
)* # match as many xml tags as possible
(?(opentag)(?!)) # ensure no 'opentag' groups are on stack
You can try this at A Better .NET Regular Expression Tester.
I used the sample source of:
<html>
<body>
<div>
<br />
<ul id="matchMe" type="square">
<li>stuff...</li>
<li>more stuff</li>
<li>
<div>
<span>still more</span>
<ul>
<li>Another >ul<, oh my!</li>
<li>...</li>
</ul>
</div>
</li>
</ul>
</div>
</body>
</html>
This found the match:
<ul id="matchMe" type="square">
<li>stuff...</li>
<li>more stuff</li>
<li>
<div>
<span>still more</span>
<ul>
<li>Another >ul<, oh my!</li>
<li>...</li>
</ul>
</div>
</li>
</ul>
although it actually came out like this:
<ul id="matchMe" type="square"> <li>stuff...</li> <li>more stuff</li> <li> <div> <span>still more</span> <ul> <li>Another >ul<, oh my!</li> <li>...</li> </ul> </div> </li> </ul>
Lastly, I really enjoyed Jeff Atwood's article: Parsing Html The Cthulhu Way. Funny enough, it cites the answer to this question that currently has over 4k votes.
I suggest using QueryPath for parsing XML and HTML in PHP. It's basically much the same syntax as jQuery, only it's on the server side.
While the answers that you can't parse HTML with regexes are correct, they don't apply here. The OP just wants to parse one HTML tag with regexes, and that is something that can be done with a regular expression.
The suggested regex is wrong, though:
<([a-z]+) *[^/]*?>
If you add something to the regex, by backtracking it can be forced to match silly things like <a >>, [^/] is too permissive. Also note that <space>*[^/]* is redundant, because the [^/]* can also match spaces.
My suggestion would be
<([a-z]+)[^>]*(?<!/)>
Where (?<! ... ) is (in Perl regexes) the negative look-behind. It reads "a <, then a word, then anything that's not a >, the last of which may not be a /, followed by >".
Note that this allows things like <a/ > (just like the original regex), so if you want something more restrictive, you need to build a regex to match attribute pairs separated by spaces.
Sun Tzu, an ancient Chinese strategist, general, and philosopher, said:
It is said that if you know your enemies and know yourself, you can win a hundred battles without a single loss.
If you only know yourself, but not your opponent, you may win or may lose.
If you know neither yourself nor your enemy, you will always endanger yourself.
In this case your enemy is HTML and you are either yourself or regex. You might even be Perl with irregular regex. Know HTML. Know yourself.
I have composed a haiku describing the nature of HTML.
HTML has
complexity exceeding
regular language.
I have also composed a haiku describing the nature of regex in Perl.
The regex you seek
is defined within the phrase
<([a-zA-Z]+)(?:[^>]*[^/]*)?>
Try:
<([^\s]+)(\s[^>]*?)?(?<!/)>
It is similar to yours, but the last > must not be after a slash, and also accepts h1.
<?php
$selfClosing = explode(',', 'area,base,basefont,br,col,frame,hr,img,input,isindex,link,meta,param,embed');
$html = '
<p>foo</p>
<hr/>
<br/>
<div>name</div>';
$dom = new DOMDocument();
$dom->loadHTML($html);
$els = $dom->getElementsByTagName('*');
foreach ( $els as $el ) {
$nodeName = strtolower($el->nodeName);
if ( !in_array( $nodeName, $selfClosing ) ) {
var_dump( $nodeName );
}
}
Output:
string(4) "html"
string(4) "body"
string(1) "p"
string(1) "a"
string(3) "div"
Basically just define the element node names that are self closing, load the whole html string into a DOM library, grab all elements, loop through and filter out ones which aren't self closing and operate on them.
I'm sure you already know by now that you shouldn't use regex for this purpose.
I don't know your exact need for this, but if you are also using .NET, couldn't you use Html Agility Pack?
Excerpt:
It is a .NET code library that allows
you to parse "out of the web" HTML
files. The parser is very tolerant
with "real world" malformed HTML.
You want the first > not preceded by a /. Look here for details on how to do that. It's referred to as negative lookbehind.
However, a naïve implementation of that will end up matching <bar/></foo> in this example document
<foo><bar/></foo>
Can you provide a little more information on the problem you're trying to solve? Are you iterating through tags programatically?
If you need this for PHP:
The PHP DOM functions won't work properly unless it is properly formatted XML. No matter how much better their use is for the rest of mankind.
simplehtmldom is good, but I found it a bit buggy, and it is is quite memory heavy [Will crash on large pages.]
I have never used querypath, so can't comment on its usefulness.
Another one to try is my DOMParser which is very light on resources and I've been using happily for a while. Simple to learn & powerful.
For Python and Java, similar links were posted.
For the downvoters - I only wrote my class when the XML parsers proved unable to withstand real use. Religious downvoting just prevents useful answers from being posted - keep things within perspective of the question, please.
Here's the solution:
<?php
// here's the pattern:
$pattern = '/<(\w+)(\s+(\w+)\s*\=\s*(\'|")(.*?)\\4\s*)*\s*(\/>|>)/';
// a string to parse:
$string = 'Hello, try clicking here
<br/>and check out.<hr />
<h2>title</h2>
<a name ="paragraph" rel= "I\'m an anchor"></a>
Fine, <span title=\'highlight the "punch"\'>thanks<span>.
<div class = "clear"></div>
<br>';
// let's get the occurrences:
preg_match_all($pattern, $string, $matches, PREG_PATTERN_ORDER);
// print the result:
print_r($matches[0]);
?>
To test it deeply, I entered in the string auto-closing tags like:
<hr />
<br/>
<br>
I also entered tags with:
one attribute
more than one attribute
attributes which value is bound either into single quotes or into double quotes
attributes containing single quotes when the delimiter is a double quote and vice versa
"unpretty" attributes with a space before the "=" symbol, after it and both before and after it.
Should you find something which does not work in the proof of concept above, I am available in analyzing the code to improve my skills.
<EDIT>
I forgot that the question from the user was to avoid the parsing of self-closing tags.
In this case the pattern is simpler, turning into this:
$pattern = '/<(\w+)(\s+(\w+)\s*\=\s*(\'|")(.*?)\\4\s*)*\s*>/';
The user #ridgerunner noticed that the pattern does not allow unquoted attributes or attributes with no value. In this case a fine tuning brings us the following pattern:
$pattern = '/<(\w+)(\s+(\w+)(\s*\=\s*(\'|"|)(.*?)\\5\s*)?)*\s*>/';
</EDIT>
Understanding the pattern
If someone is interested in learning more about the pattern, I provide some line:
the first sub-expression (\w+) matches the tag name
the second sub-expression contains the pattern of an attribute. It is composed by:
one or more whitespaces \s+
the name of the attribute (\w+)
zero or more whitespaces \s* (it is possible or not, leaving blanks here)
the "=" symbol
again, zero or more whitespaces
the delimiter of the attribute value, a single or double quote ('|"). In the pattern, the single quote is escaped because it coincides with the PHP string delimiter. This sub-expression is captured with the parentheses so it can be referenced again to parse the closure of the attribute, that's why it is very important.
the value of the attribute, matched by almost anything: (.*?); in this specific syntax, using the greedy match (the question mark after the asterisk) the RegExp engine enables a "look-ahead"-like operator, which matches anything but what follows this sub-expression
here comes the fun: the \4 part is a backreference operator, which refers to a sub-expression defined before in the pattern, in this case, I am referring to the fourth sub-expression, which is the first attribute delimiter found
zero or more whitespaces \s*
the attribute sub-expression ends here, with the specification of zero or more possible occurrences, given by the asterisk.
Then, since a tag may end with a whitespace before the ">" symbol, zero or more whitespaces are matched with the \s* subpattern.
The tag to match may end with a simple ">" symbol, or a possible XHTML closure, which makes use of the slash before it: (/>|>). The slash is, of course, escaped since it coincides with the regular expression delimiter.
Small tip: to better analyze this code it is necessary looking at the source code generated since I did not provide any HTML special characters escaping.
Whenever I need to quickly extract something from an HTML document, I use Tidy to convert it to XML and then use XPath or XSLT to get what I need.
In your case, something like this:
//p/a[#href='foo']
I used a open source tool called HTMLParser before. It's designed to parse HTML in various ways and serves the purpose quite well. It can parse HTML as different treenode and you can easily use its API to get attributes out of the node. Check it out and see if this can help you.
I like to parse HTML with regular expressions. I don't attempt to parse idiot HTML that is deliberately broken. This code is my main parser (Perl edition):
$_ = join "",<STDIN>; tr/\n\r \t/ /s; s/</\n</g; s/>/>\n/g; s/\n ?\n/\n/g;
s/^ ?\n//s; s/ $//s; print
It's called htmlsplit, splits the HTML into lines, with one tag or chunk of text on each line. The lines can then be processed further with other text tools and scripts, such as grep, sed, Perl, etc. I'm not even joking :) Enjoy.
It is simple enough to rejig my slurp-everything-first Perl script into a nice streaming thing, if you wish to process enormous web pages. But it's not really necessary.
HTML Split
Some better regular expressions:
/(<.*?>|[^<]+)\s*/g # Get tags and text
/(\w+)="(.*?)"/g # Get attibutes
They are good for XML / XHTML.
With minor variations, it can cope with messy HTML... or convert the HTML -> XHTML first.
The best way to write regular expressions is in the Lex / Yacc style, not as opaque one-liners or commented multi-line monstrosities. I didn't do that here, yet; these ones barely need it.
There are some nice regexes for replacing HTML with BBCode here. For all you nay-sayers, note that he's not trying to fully parse HTML, just to sanitize it. He can probably afford to kill off tags that his simple "parser" can't understand.
For example:
$store =~ s/http:/http:\/\//gi;
$store =~ s/https:/https:\/\//gi;
$baseurl = $store;
if (!$query->param("ascii")) {
$html =~ s/\s\s+/\n/gi;
$html =~ s/<pre(.*?)>(.*?)<\/pre>/\[code]$2\[\/code]/sgmi;
}
$html =~ s/\n//gi;
$html =~ s/\r\r//gi;
$html =~ s/$baseurl//gi;
$html =~ s/<h[1-7](.*?)>(.*?)<\/h[1-7]>/\n\[b]$2\[\/b]\n/sgmi;
$html =~ s/<p>/\n\n/gi;
$html =~ s/<br(.*?)>/\n/gi;
$html =~ s/<textarea(.*?)>(.*?)<\/textarea>/\[code]$2\[\/code]/sgmi;
$html =~ s/<b>(.*?)<\/b>/\[b]$1\[\/b]/gi;
$html =~ s/<i>(.*?)<\/i>/\[i]$1\[\/i]/gi;
$html =~ s/<u>(.*?)<\/u>/\[u]$1\[\/u]/gi;
$html =~ s/<em>(.*?)<\/em>/\[i]$1\[\/i]/gi;
$html =~ s/<strong>(.*?)<\/strong>/\[b]$1\[\/b]/gi;
$html =~ s/<cite>(.*?)<\/cite>/\[i]$1\[\/i]/gi;
$html =~ s/<font color="(.*?)">(.*?)<\/font>/\[color=$1]$2\[\/color]/sgmi;
$html =~ s/<font color=(.*?)>(.*?)<\/font>/\[color=$1]$2\[\/color]/sgmi;
$html =~ s/<link(.*?)>//gi;
$html =~ s/<li(.*?)>(.*?)<\/li>/\[\*]$2/gi;
$html =~ s/<ul(.*?)>/\[list]/gi;
$html =~ s/<\/ul>/\[\/list]/gi;
$html =~ s/<div>/\n/gi;
$html =~ s/<\/div>/\n/gi;
$html =~ s/<td(.*?)>/ /gi;
$html =~ s/<tr(.*?)>/\n/gi;
$html =~ s/<img(.*?)src="(.*?)"(.*?)>/\[img]$baseurl\/$2\[\/img]/gi;
$html =~ s/<a(.*?)href="(.*?)"(.*?)>(.*?)<\/a>/\[url=$baseurl\/$2]$4\[\/url]/gi;
$html =~ s/\[url=$baseurl\/http:\/\/(.*?)](.*?)\[\/url]/\[url=http:\/\/$1]$2\[\/url]/gi;
$html =~ s/\[img]$baseurl\/http:\/\/(.*?)\[\/img]/\[img]http:\/\/$1\[\/img]/gi;
$html =~ s/<head>(.*?)<\/head>//sgmi;
$html =~ s/<object>(.*?)<\/object>//sgmi;
$html =~ s/<script(.*?)>(.*?)<\/script>//sgmi;
$html =~ s/<style(.*?)>(.*?)<\/style>//sgmi;
$html =~ s/<title>(.*?)<\/title>//sgmi;
$html =~ s/<!--(.*?)-->/\n/sgmi;
$html =~ s/\/\//\//gi;
$html =~ s/http:\//http:\/\//gi;
$html =~ s/https:\//https:\/\//gi;
$html =~ s/<(?:[^>'"]*|(['"]).*?\1)*>//gsi;
$html =~ s/\r\r//gi;
$html =~ s/\[img]\//\[img]/gi;
$html =~ s/\[url=\//\[url=/gi;
About the question of the regular expression methods to parse (x)HTML, the answer to all of the ones who spoke about some limits is: you have not been trained enough to rule the force of this powerful weapon, since nobody here spoke about recursion.
A regular expression-agnostic colleague notified me this discussion, which is not certainly the first on the web about this old and hot topic.
After reading some posts, the first thing I did was looking for the "?R" string in this thread. The second was to search about "recursion".
No, holy cow, no match found. Since nobody mentioned the main mechanism a parser is built onto, I was soon aware that nobody got the point.
If an (x)HTML parser needs recursion, a regular expression parser without recursion is not enough for the purpose. It's a simple construct.
The black art of regular expressions is hard to master, so maybe there are further possibilities we left out while trying and testing our personal solution to capture the whole web in one hand... Well, I am sure about it :)
Here's the magic pattern:
$pattern = "/<([\w]+)([^>]*?)(([\s]*\/>)|(>((([^<]*?|<\!\-\-.*?\-\->)|(?R))*)<\/\\1[\s]*>))/s";
Just try it. It's written as a PHP string, so the "s" modifier makes classes include newlines.
Here's a sample note on the PHP manual I wrote in January: Reference
(Take care. In that note I wrongly used the "m" modifier; it should be erased, notwithstanding it is discarded by the regular expression engine, since no ^ or $ anchoring was used).
Now, we could speak about the limits of this method from a more informed point of view:
according to the specific implementation of the regular expression engine, recursion may have a limit in the number of nested patterns parsed, but it depends on the language used
although corrupted, (x)HTML does not drive into severe errors. It is not sanitized.
Anyhow, it is only a regular expression pattern, but it discloses the possibility to develop of a lot of powerful implementations.
I wrote this pattern to power the recursive descent parser of a template engine I built in my framework, and performances are really great, both in execution times or in memory usage (nothing to do with other template engines which use the same syntax).
<\s*(\w+)[^/>]*>
The parts explained:
<: Starting character
\s*: It may have whitespaces before the tag name (ugly, but possible).
(\w+): tags can contain letters and numbers (h1). Well, \w also matches '_', but it does not hurt I guess. If curious, use ([a-zA-Z0-9]+) instead.
[^/>]*: Anything except > and / until closing >
>: Closing >
UNRELATED
And to the fellows, who underestimate regular expressions, saying they are only as powerful as regular languages:
anbanban which is not regular and not even context free, can be matched with ^(a+)b\1b\1$
Backreferencing FTW!
As many people have already pointed out, HTML is not a regular language which can make it very difficult to parse. My solution to this is to turn it into a regular language using a tidy program and then to use an XML parser to consume the results. There are a lot of good options for this. My program is written using Java with the jtidy library to turn the HTML into XML and then Jaxen to xpath into the result.
If you're simply trying to find those tags (without ambitions of parsing) try this regular expression:
/<[^/]*?>/g
I wrote it in 30 seconds, and tested here:
http://gskinner.com/RegExr/
It matches the types of tags you mentioned, while ignoring the types you said you wanted to ignore.
It seems to me you're trying to match tags without a "/" at the end. Try this:
<([a-zA-Z][a-zA-Z0-9]*)[^>]*(?<!/)>
It's true that when programming it's usually best to use dedicated parsers and APIs instead of regular expressions when dealing with HTML, especially if accuracy is paramount (e.g., if your processing might have security implications). However, I don’t ascribe to a dogmatic view that XML-style markup should never be processed with regular expressions. There are cases when regular expressions are a great tool for the job, such as when making one-time edits in a text editor, fixing broken XML files, or dealing with file formats that look like but aren’t quite XML. There are some issues to be aware of, but they're not insurmountable or even necessarily relevant.
A simple regex like <([^>"']|"[^"]*"|'[^']*')*> is usually good enough, in cases such as those I just mentioned. It's a naive solution, all things considered, but it does correctly allow unencoded > symbols in attribute values. If you're looking for, e.g., a table tag, you could adapt it as </?table\b([^>"']|"[^"]*"|'[^']*')*>.
Just to give a sense of what a more "advanced" HTML regex would look like, the following does a fairly respectable job of emulating real-world browser behavior and the HTML5 parsing algorithm:
</?([A-Za-z][^\s>/]*)(?:=\s*(?:"[^"]*"|'[^']*'|[^\s>]+)|[^>])*(?:>|$)
The following matches a fairly strict definition of XML tags (although it doesn't account for the full set of Unicode characters allowed in XML names):
<(?:([_:A-Z][-.:\w]*)(?:\s+[_:A-Z][-.:\w]*\s*=\s*(?:"[^"]*"|'[^']*'))*\s*/?|/([_:A-Z][-.:\w]*)\s*)>
Granted, these don't account for surrounding context and a few edge cases, but even such things could be dealt with if you really wanted to (e.g., by searching between the matches of another regex).
At the end of the day, use the most appropriate tool for the job, even in the cases when that tool happens to be a regex.
Although it's not suitable and effective to use regular expressions for that purpose sometimes regular expressions provide quick solutions for simple match problems and in my view it's not that horrbile to use regular expressions for trivial works.
There is a definitive blog post about matching innermost HTML elements written by Steven Levithan.
If you only want the tag names, it should be possible to do this via a regular expression.
<([a-zA-Z]+)(?:[^>]*[^/] *)?>
should do what you need. But I think the solution of "moritz" is already fine. I didn't see it in the beginning.
For all downvoters: In some cases it just makes sense to use a regular expression, because it can be the easiest and quickest solution. I agree that in general you should not parse HTML with regular expressions.
But regular expressions can be a very powerful tool when you have a subset of HTML where you know the format and you just want to extract some values. I did that hundreds of times and almost always achieved what I wanted.
The OP doesn't seem to say what he needs to do with the tags. For example, does he need to extract inner text, or just examine the tags?
I'm firmly in the camp that says a regular expression is not the be-all, end-all text parser. I've written a large amount of text-parsing code including this code to parse HTML tags.
While it's true I'm not all that great with regular expressions, I consider regular expressions just too rigid and hard to maintain for this sort of parsing.
This may do:
<.*?[^/]>
Or without the ending tags:
<[^/].*?[^/]>
What's with the flame wars on HTML parsers? HTML parsers must parse (and rebuild!) the entire document before it can categorize your search. Regular expressions may be a faster / elegant in certain circumstances. My 2 cents...
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I need to match all of these opening tags:
<p>
<a href="foo">
But not these:
<br />
<hr class="foo" />
I came up with this and wanted to make sure I've got it right. I am only capturing the a-z.
<([a-z]+) *[^/]*?>
I believe it says:
Find a less-than, then
Find (and capture) a-z one or more times, then
Find zero or more spaces, then
Find any character zero or more times, greedy, except /, then
Find a greater-than
Do I have that right? And more importantly, what do you think?
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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 nerves 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 regex parsers for HTML will instantly transport a programmer's consciousness into a world of ceaseless screaming, he comes, the pestilent slithy regex-infection will devour your HTML parser, application and existence for all time like Visual Basic only worse he comes he comes do not fight he com̡e̶s, ̕h̵is un̨ho͞ly radiańcé destro҉ying all enli̍̈́̂̈́ghtenment, HTML tags lea͠ki̧n͘g fr̶ǫm ̡yo͟ur eye͢s̸ ̛l̕ik͏e liquid pain, the song of re̸gular expression parsing will extinguish the voices of mortal man from the sphere I can see it can you see ̲͚̖͔̙î̩́t̲͎̩̱͔́̋̀ it is beautiful the final snuffing of the lies of Man ALL IS LOŚ͖̩͇̗̪̏̈́T ALL IS LOST the pon̷y he comes he c̶̮omes he comes the ichor permeates all MY FACE MY FACE ᵒh god no NO NOO̼OO NΘ stop the an*̶͑̾̾̅ͫ͏̙̤g͇̫͛͆̾ͫ̑͆l͖͉̗̩̳̟̍ͫͥͨe̠̅s ͎a̧͈͖r̽̾̈́͒͑e not rè̑ͧ̌aͨl̘̝̙̃ͤ͂̾̆ ZA̡͊͠͝LGΌ ISͮ̂҉̯͈͕̹̘̱ TO͇̹̺ͅƝ̴ȳ̳ TH̘Ë͖́̉ ͠P̯͍̭O̚N̐Y̡ H̸̡̪̯ͨ͊̽̅̾̎Ȩ̬̩̾͛ͪ̈́̀́͘ ̶̧̨̱̹̭̯ͧ̾ͬC̷̙̲̝͖ͭ̏ͥͮ͟Oͮ͏̮̪̝͍M̲̖͊̒ͪͩͬ̚̚͜Ȇ̴̟̟͙̞ͩ͌͝S̨̥̫͎̭ͯ̿̔̀ͅ
Have you tried using an XML parser instead?
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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.
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), you can't possibly make this work.
But many will try, and some will even claim success - but until others find the fault and totally mess you up.
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...
There are people that will tell you that the Earth is round (or perhaps that the Earth is an oblate spheroid if they want to use strange words). They are lying.
There are people that will tell you that Regular Expressions shouldn't be recursive. They are limiting you. They need to subjugate you, and they do it by keeping you in ignorance.
You can live in their reality or take the red pill.
Like Lord Marshal (is he a relative of the Marshal .NET class?), I have seen the Underverse Stack Based Regex-Verse and returned with powers knowledge you can't imagine. Yes, I think there were an Old One or two protecting them, but they were watching football on the TV, so it wasn't difficult.
I think the XML case is quite simple. The RegEx (in the .NET syntax), deflated and coded in base64 to make it easier to comprehend by your feeble mind, should be something like this: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The options to set is RegexOptions.ExplicitCapture. The capture group you are looking for is ELEMENTNAME. If the capture group ERROR is not empty then there was a parsing error and the Regex stopped.
If you have problems reconverting it to a human-readable regex, this should help:
static string FromBase64(string str)
{
byte[] byteArray = Convert.FromBase64String(str);
using (var msIn = new MemoryStream(byteArray))
using (var msOut = new MemoryStream()) {
using (var ds = new DeflateStream(msIn, CompressionMode.Decompress)) {
ds.CopyTo(msOut);
}
return Encoding.UTF8.GetString(msOut.ToArray());
}
}
If you are unsure, no, I'm NOT kidding (but perhaps I'm lying). It WILL work. I've built tons of unit tests to test it, and I have even used (part of) the conformance tests. It's a tokenizer, not a full-blown parser, so it will only split the XML into its component tokens. It won't parse/integrate DTDs.
Oh... if you want the source code of the regex, with some auxiliary methods:
regex to tokenize an xml or the full plain regex
In shell, you can parse HTML using sed:
Turing.sed
Write HTML parser (homework)
???
Profit!
Related (why you shouldn't use regex match):
If You Like Regular Expressions So Much, Why Don't You Marry Them?
Regular Expressions: Now You Have Two Problems
Hacking stackoverflow.com's HTML sanitizer
I agree that the right tool to parse XML and especially HTML is a parser and not a regular expression engine. However, like others have pointed out, sometimes using a regex is quicker, easier, and gets the job done if you know the data format.
Microsoft actually has a section of Best Practices for Regular Expressions in the .NET Framework and specifically talks about Consider[ing] the Input Source.
Regular Expressions do have limitations, but have you considered the following?
The .NET framework is unique when it comes to regular expressions in that it supports Balancing Group Definitions.
See Matching Balanced Constructs with .NET Regular Expressions
See .NET Regular Expressions: Regex and Balanced Matching
See Microsoft's docs on Balancing Group Definitions
For this reason, I believe you CAN parse XML using regular expressions. Note however, that it must be valid XML (browsers are very forgiving of HTML and allow bad XML syntax inside HTML). This is possible since the "Balancing Group Definition" will allow the regular expression engine to act as a PDA.
Quote from article 1 cited above:
.NET Regular Expression Engine
As described above properly balanced constructs cannot be described by
a regular expression. However, the .NET regular expression engine
provides a few constructs that allow balanced constructs to be
recognized.
(?<group>) - pushes the captured result on the capture stack with
the name group.
(?<-group>) - pops the top most capture with the name group off the
capture stack.
(?(group)yes|no) - matches the yes part if there exists a group
with the name group otherwise matches no part.
These constructs allow for a .NET regular expression to emulate a
restricted PDA by essentially allowing simple versions of the stack
operations: push, pop and empty. The simple operations are pretty much
equivalent to increment, decrement and compare to zero respectively.
This allows for the .NET regular expression engine to recognize a
subset of the context-free languages, in particular the ones that only
require a simple counter. This in turn allows for the non-traditional
.NET regular expressions to recognize individual properly balanced
constructs.
Consider the following regular expression:
(?=<ul\s+id="matchMe"\s+type="square"\s*>)
(?>
<!-- .*? --> |
<[^>]*/> |
(?<opentag><(?!/)[^>]*[^/]>) |
(?<-opentag></[^>]*[^/]>) |
[^<>]*
)*
(?(opentag)(?!))
Use the flags:
Singleline
IgnorePatternWhitespace (not necessary if you collapse regex and remove all whitespace)
IgnoreCase (not necessary)
Regular Expression Explained (inline)
(?=<ul\s+id="matchMe"\s+type="square"\s*>) # match start with <ul id="matchMe"...
(?> # atomic group / don't backtrack (faster)
<!-- .*? --> | # match xml / html comment
<[^>]*/> | # self closing tag
(?<opentag><(?!/)[^>]*[^/]>) | # push opening xml tag
(?<-opentag></[^>]*[^/]>) | # pop closing xml tag
[^<>]* # something between tags
)* # match as many xml tags as possible
(?(opentag)(?!)) # ensure no 'opentag' groups are on stack
You can try this at A Better .NET Regular Expression Tester.
I used the sample source of:
<html>
<body>
<div>
<br />
<ul id="matchMe" type="square">
<li>stuff...</li>
<li>more stuff</li>
<li>
<div>
<span>still more</span>
<ul>
<li>Another >ul<, oh my!</li>
<li>...</li>
</ul>
</div>
</li>
</ul>
</div>
</body>
</html>
This found the match:
<ul id="matchMe" type="square">
<li>stuff...</li>
<li>more stuff</li>
<li>
<div>
<span>still more</span>
<ul>
<li>Another >ul<, oh my!</li>
<li>...</li>
</ul>
</div>
</li>
</ul>
although it actually came out like this:
<ul id="matchMe" type="square"> <li>stuff...</li> <li>more stuff</li> <li> <div> <span>still more</span> <ul> <li>Another >ul<, oh my!</li> <li>...</li> </ul> </div> </li> </ul>
Lastly, I really enjoyed Jeff Atwood's article: Parsing Html The Cthulhu Way. Funny enough, it cites the answer to this question that currently has over 4k votes.
I suggest using QueryPath for parsing XML and HTML in PHP. It's basically much the same syntax as jQuery, only it's on the server side.
While the answers that you can't parse HTML with regexes are correct, they don't apply here. The OP just wants to parse one HTML tag with regexes, and that is something that can be done with a regular expression.
The suggested regex is wrong, though:
<([a-z]+) *[^/]*?>
If you add something to the regex, by backtracking it can be forced to match silly things like <a >>, [^/] is too permissive. Also note that <space>*[^/]* is redundant, because the [^/]* can also match spaces.
My suggestion would be
<([a-z]+)[^>]*(?<!/)>
Where (?<! ... ) is (in Perl regexes) the negative look-behind. It reads "a <, then a word, then anything that's not a >, the last of which may not be a /, followed by >".
Note that this allows things like <a/ > (just like the original regex), so if you want something more restrictive, you need to build a regex to match attribute pairs separated by spaces.
Sun Tzu, an ancient Chinese strategist, general, and philosopher, said:
It is said that if you know your enemies and know yourself, you can win a hundred battles without a single loss.
If you only know yourself, but not your opponent, you may win or may lose.
If you know neither yourself nor your enemy, you will always endanger yourself.
In this case your enemy is HTML and you are either yourself or regex. You might even be Perl with irregular regex. Know HTML. Know yourself.
I have composed a haiku describing the nature of HTML.
HTML has
complexity exceeding
regular language.
I have also composed a haiku describing the nature of regex in Perl.
The regex you seek
is defined within the phrase
<([a-zA-Z]+)(?:[^>]*[^/]*)?>
Try:
<([^\s]+)(\s[^>]*?)?(?<!/)>
It is similar to yours, but the last > must not be after a slash, and also accepts h1.
<?php
$selfClosing = explode(',', 'area,base,basefont,br,col,frame,hr,img,input,isindex,link,meta,param,embed');
$html = '
<p>foo</p>
<hr/>
<br/>
<div>name</div>';
$dom = new DOMDocument();
$dom->loadHTML($html);
$els = $dom->getElementsByTagName('*');
foreach ( $els as $el ) {
$nodeName = strtolower($el->nodeName);
if ( !in_array( $nodeName, $selfClosing ) ) {
var_dump( $nodeName );
}
}
Output:
string(4) "html"
string(4) "body"
string(1) "p"
string(1) "a"
string(3) "div"
Basically just define the element node names that are self closing, load the whole html string into a DOM library, grab all elements, loop through and filter out ones which aren't self closing and operate on them.
I'm sure you already know by now that you shouldn't use regex for this purpose.
I don't know your exact need for this, but if you are also using .NET, couldn't you use Html Agility Pack?
Excerpt:
It is a .NET code library that allows
you to parse "out of the web" HTML
files. The parser is very tolerant
with "real world" malformed HTML.
You want the first > not preceded by a /. Look here for details on how to do that. It's referred to as negative lookbehind.
However, a naïve implementation of that will end up matching <bar/></foo> in this example document
<foo><bar/></foo>
Can you provide a little more information on the problem you're trying to solve? Are you iterating through tags programatically?
If you need this for PHP:
The PHP DOM functions won't work properly unless it is properly formatted XML. No matter how much better their use is for the rest of mankind.
simplehtmldom is good, but I found it a bit buggy, and it is is quite memory heavy [Will crash on large pages.]
I have never used querypath, so can't comment on its usefulness.
Another one to try is my DOMParser which is very light on resources and I've been using happily for a while. Simple to learn & powerful.
For Python and Java, similar links were posted.
For the downvoters - I only wrote my class when the XML parsers proved unable to withstand real use. Religious downvoting just prevents useful answers from being posted - keep things within perspective of the question, please.
Here's the solution:
<?php
// here's the pattern:
$pattern = '/<(\w+)(\s+(\w+)\s*\=\s*(\'|")(.*?)\\4\s*)*\s*(\/>|>)/';
// a string to parse:
$string = 'Hello, try clicking here
<br/>and check out.<hr />
<h2>title</h2>
<a name ="paragraph" rel= "I\'m an anchor"></a>
Fine, <span title=\'highlight the "punch"\'>thanks<span>.
<div class = "clear"></div>
<br>';
// let's get the occurrences:
preg_match_all($pattern, $string, $matches, PREG_PATTERN_ORDER);
// print the result:
print_r($matches[0]);
?>
To test it deeply, I entered in the string auto-closing tags like:
<hr />
<br/>
<br>
I also entered tags with:
one attribute
more than one attribute
attributes which value is bound either into single quotes or into double quotes
attributes containing single quotes when the delimiter is a double quote and vice versa
"unpretty" attributes with a space before the "=" symbol, after it and both before and after it.
Should you find something which does not work in the proof of concept above, I am available in analyzing the code to improve my skills.
<EDIT>
I forgot that the question from the user was to avoid the parsing of self-closing tags.
In this case the pattern is simpler, turning into this:
$pattern = '/<(\w+)(\s+(\w+)\s*\=\s*(\'|")(.*?)\\4\s*)*\s*>/';
The user #ridgerunner noticed that the pattern does not allow unquoted attributes or attributes with no value. In this case a fine tuning brings us the following pattern:
$pattern = '/<(\w+)(\s+(\w+)(\s*\=\s*(\'|"|)(.*?)\\5\s*)?)*\s*>/';
</EDIT>
Understanding the pattern
If someone is interested in learning more about the pattern, I provide some line:
the first sub-expression (\w+) matches the tag name
the second sub-expression contains the pattern of an attribute. It is composed by:
one or more whitespaces \s+
the name of the attribute (\w+)
zero or more whitespaces \s* (it is possible or not, leaving blanks here)
the "=" symbol
again, zero or more whitespaces
the delimiter of the attribute value, a single or double quote ('|"). In the pattern, the single quote is escaped because it coincides with the PHP string delimiter. This sub-expression is captured with the parentheses so it can be referenced again to parse the closure of the attribute, that's why it is very important.
the value of the attribute, matched by almost anything: (.*?); in this specific syntax, using the greedy match (the question mark after the asterisk) the RegExp engine enables a "look-ahead"-like operator, which matches anything but what follows this sub-expression
here comes the fun: the \4 part is a backreference operator, which refers to a sub-expression defined before in the pattern, in this case, I am referring to the fourth sub-expression, which is the first attribute delimiter found
zero or more whitespaces \s*
the attribute sub-expression ends here, with the specification of zero or more possible occurrences, given by the asterisk.
Then, since a tag may end with a whitespace before the ">" symbol, zero or more whitespaces are matched with the \s* subpattern.
The tag to match may end with a simple ">" symbol, or a possible XHTML closure, which makes use of the slash before it: (/>|>). The slash is, of course, escaped since it coincides with the regular expression delimiter.
Small tip: to better analyze this code it is necessary looking at the source code generated since I did not provide any HTML special characters escaping.
Whenever I need to quickly extract something from an HTML document, I use Tidy to convert it to XML and then use XPath or XSLT to get what I need.
In your case, something like this:
//p/a[#href='foo']
I used a open source tool called HTMLParser before. It's designed to parse HTML in various ways and serves the purpose quite well. It can parse HTML as different treenode and you can easily use its API to get attributes out of the node. Check it out and see if this can help you.
I like to parse HTML with regular expressions. I don't attempt to parse idiot HTML that is deliberately broken. This code is my main parser (Perl edition):
$_ = join "",<STDIN>; tr/\n\r \t/ /s; s/</\n</g; s/>/>\n/g; s/\n ?\n/\n/g;
s/^ ?\n//s; s/ $//s; print
It's called htmlsplit, splits the HTML into lines, with one tag or chunk of text on each line. The lines can then be processed further with other text tools and scripts, such as grep, sed, Perl, etc. I'm not even joking :) Enjoy.
It is simple enough to rejig my slurp-everything-first Perl script into a nice streaming thing, if you wish to process enormous web pages. But it's not really necessary.
HTML Split
Some better regular expressions:
/(<.*?>|[^<]+)\s*/g # Get tags and text
/(\w+)="(.*?)"/g # Get attibutes
They are good for XML / XHTML.
With minor variations, it can cope with messy HTML... or convert the HTML -> XHTML first.
The best way to write regular expressions is in the Lex / Yacc style, not as opaque one-liners or commented multi-line monstrosities. I didn't do that here, yet; these ones barely need it.
There are some nice regexes for replacing HTML with BBCode here. For all you nay-sayers, note that he's not trying to fully parse HTML, just to sanitize it. He can probably afford to kill off tags that his simple "parser" can't understand.
For example:
$store =~ s/http:/http:\/\//gi;
$store =~ s/https:/https:\/\//gi;
$baseurl = $store;
if (!$query->param("ascii")) {
$html =~ s/\s\s+/\n/gi;
$html =~ s/<pre(.*?)>(.*?)<\/pre>/\[code]$2\[\/code]/sgmi;
}
$html =~ s/\n//gi;
$html =~ s/\r\r//gi;
$html =~ s/$baseurl//gi;
$html =~ s/<h[1-7](.*?)>(.*?)<\/h[1-7]>/\n\[b]$2\[\/b]\n/sgmi;
$html =~ s/<p>/\n\n/gi;
$html =~ s/<br(.*?)>/\n/gi;
$html =~ s/<textarea(.*?)>(.*?)<\/textarea>/\[code]$2\[\/code]/sgmi;
$html =~ s/<b>(.*?)<\/b>/\[b]$1\[\/b]/gi;
$html =~ s/<i>(.*?)<\/i>/\[i]$1\[\/i]/gi;
$html =~ s/<u>(.*?)<\/u>/\[u]$1\[\/u]/gi;
$html =~ s/<em>(.*?)<\/em>/\[i]$1\[\/i]/gi;
$html =~ s/<strong>(.*?)<\/strong>/\[b]$1\[\/b]/gi;
$html =~ s/<cite>(.*?)<\/cite>/\[i]$1\[\/i]/gi;
$html =~ s/<font color="(.*?)">(.*?)<\/font>/\[color=$1]$2\[\/color]/sgmi;
$html =~ s/<font color=(.*?)>(.*?)<\/font>/\[color=$1]$2\[\/color]/sgmi;
$html =~ s/<link(.*?)>//gi;
$html =~ s/<li(.*?)>(.*?)<\/li>/\[\*]$2/gi;
$html =~ s/<ul(.*?)>/\[list]/gi;
$html =~ s/<\/ul>/\[\/list]/gi;
$html =~ s/<div>/\n/gi;
$html =~ s/<\/div>/\n/gi;
$html =~ s/<td(.*?)>/ /gi;
$html =~ s/<tr(.*?)>/\n/gi;
$html =~ s/<img(.*?)src="(.*?)"(.*?)>/\[img]$baseurl\/$2\[\/img]/gi;
$html =~ s/<a(.*?)href="(.*?)"(.*?)>(.*?)<\/a>/\[url=$baseurl\/$2]$4\[\/url]/gi;
$html =~ s/\[url=$baseurl\/http:\/\/(.*?)](.*?)\[\/url]/\[url=http:\/\/$1]$2\[\/url]/gi;
$html =~ s/\[img]$baseurl\/http:\/\/(.*?)\[\/img]/\[img]http:\/\/$1\[\/img]/gi;
$html =~ s/<head>(.*?)<\/head>//sgmi;
$html =~ s/<object>(.*?)<\/object>//sgmi;
$html =~ s/<script(.*?)>(.*?)<\/script>//sgmi;
$html =~ s/<style(.*?)>(.*?)<\/style>//sgmi;
$html =~ s/<title>(.*?)<\/title>//sgmi;
$html =~ s/<!--(.*?)-->/\n/sgmi;
$html =~ s/\/\//\//gi;
$html =~ s/http:\//http:\/\//gi;
$html =~ s/https:\//https:\/\//gi;
$html =~ s/<(?:[^>'"]*|(['"]).*?\1)*>//gsi;
$html =~ s/\r\r//gi;
$html =~ s/\[img]\//\[img]/gi;
$html =~ s/\[url=\//\[url=/gi;
About the question of the regular expression methods to parse (x)HTML, the answer to all of the ones who spoke about some limits is: you have not been trained enough to rule the force of this powerful weapon, since nobody here spoke about recursion.
A regular expression-agnostic colleague notified me this discussion, which is not certainly the first on the web about this old and hot topic.
After reading some posts, the first thing I did was looking for the "?R" string in this thread. The second was to search about "recursion".
No, holy cow, no match found. Since nobody mentioned the main mechanism a parser is built onto, I was soon aware that nobody got the point.
If an (x)HTML parser needs recursion, a regular expression parser without recursion is not enough for the purpose. It's a simple construct.
The black art of regular expressions is hard to master, so maybe there are further possibilities we left out while trying and testing our personal solution to capture the whole web in one hand... Well, I am sure about it :)
Here's the magic pattern:
$pattern = "/<([\w]+)([^>]*?)(([\s]*\/>)|(>((([^<]*?|<\!\-\-.*?\-\->)|(?R))*)<\/\\1[\s]*>))/s";
Just try it. It's written as a PHP string, so the "s" modifier makes classes include newlines.
Here's a sample note on the PHP manual I wrote in January: Reference
(Take care. In that note I wrongly used the "m" modifier; it should be erased, notwithstanding it is discarded by the regular expression engine, since no ^ or $ anchoring was used).
Now, we could speak about the limits of this method from a more informed point of view:
according to the specific implementation of the regular expression engine, recursion may have a limit in the number of nested patterns parsed, but it depends on the language used
although corrupted, (x)HTML does not drive into severe errors. It is not sanitized.
Anyhow, it is only a regular expression pattern, but it discloses the possibility to develop of a lot of powerful implementations.
I wrote this pattern to power the recursive descent parser of a template engine I built in my framework, and performances are really great, both in execution times or in memory usage (nothing to do with other template engines which use the same syntax).
<\s*(\w+)[^/>]*>
The parts explained:
<: Starting character
\s*: It may have whitespaces before the tag name (ugly, but possible).
(\w+): tags can contain letters and numbers (h1). Well, \w also matches '_', but it does not hurt I guess. If curious, use ([a-zA-Z0-9]+) instead.
[^/>]*: Anything except > and / until closing >
>: Closing >
UNRELATED
And to the fellows, who underestimate regular expressions, saying they are only as powerful as regular languages:
anbanban which is not regular and not even context free, can be matched with ^(a+)b\1b\1$
Backreferencing FTW!
As many people have already pointed out, HTML is not a regular language which can make it very difficult to parse. My solution to this is to turn it into a regular language using a tidy program and then to use an XML parser to consume the results. There are a lot of good options for this. My program is written using Java with the jtidy library to turn the HTML into XML and then Jaxen to xpath into the result.
If you're simply trying to find those tags (without ambitions of parsing) try this regular expression:
/<[^/]*?>/g
I wrote it in 30 seconds, and tested here:
http://gskinner.com/RegExr/
It matches the types of tags you mentioned, while ignoring the types you said you wanted to ignore.
It seems to me you're trying to match tags without a "/" at the end. Try this:
<([a-zA-Z][a-zA-Z0-9]*)[^>]*(?<!/)>
It's true that when programming it's usually best to use dedicated parsers and APIs instead of regular expressions when dealing with HTML, especially if accuracy is paramount (e.g., if your processing might have security implications). However, I don’t ascribe to a dogmatic view that XML-style markup should never be processed with regular expressions. There are cases when regular expressions are a great tool for the job, such as when making one-time edits in a text editor, fixing broken XML files, or dealing with file formats that look like but aren’t quite XML. There are some issues to be aware of, but they're not insurmountable or even necessarily relevant.
A simple regex like <([^>"']|"[^"]*"|'[^']*')*> is usually good enough, in cases such as those I just mentioned. It's a naive solution, all things considered, but it does correctly allow unencoded > symbols in attribute values. If you're looking for, e.g., a table tag, you could adapt it as </?table\b([^>"']|"[^"]*"|'[^']*')*>.
Just to give a sense of what a more "advanced" HTML regex would look like, the following does a fairly respectable job of emulating real-world browser behavior and the HTML5 parsing algorithm:
</?([A-Za-z][^\s>/]*)(?:=\s*(?:"[^"]*"|'[^']*'|[^\s>]+)|[^>])*(?:>|$)
The following matches a fairly strict definition of XML tags (although it doesn't account for the full set of Unicode characters allowed in XML names):
<(?:([_:A-Z][-.:\w]*)(?:\s+[_:A-Z][-.:\w]*\s*=\s*(?:"[^"]*"|'[^']*'))*\s*/?|/([_:A-Z][-.:\w]*)\s*)>
Granted, these don't account for surrounding context and a few edge cases, but even such things could be dealt with if you really wanted to (e.g., by searching between the matches of another regex).
At the end of the day, use the most appropriate tool for the job, even in the cases when that tool happens to be a regex.
Although it's not suitable and effective to use regular expressions for that purpose sometimes regular expressions provide quick solutions for simple match problems and in my view it's not that horrbile to use regular expressions for trivial works.
There is a definitive blog post about matching innermost HTML elements written by Steven Levithan.
If you only want the tag names, it should be possible to do this via a regular expression.
<([a-zA-Z]+)(?:[^>]*[^/] *)?>
should do what you need. But I think the solution of "moritz" is already fine. I didn't see it in the beginning.
For all downvoters: In some cases it just makes sense to use a regular expression, because it can be the easiest and quickest solution. I agree that in general you should not parse HTML with regular expressions.
But regular expressions can be a very powerful tool when you have a subset of HTML where you know the format and you just want to extract some values. I did that hundreds of times and almost always achieved what I wanted.
The OP doesn't seem to say what he needs to do with the tags. For example, does he need to extract inner text, or just examine the tags?
I'm firmly in the camp that says a regular expression is not the be-all, end-all text parser. I've written a large amount of text-parsing code including this code to parse HTML tags.
While it's true I'm not all that great with regular expressions, I consider regular expressions just too rigid and hard to maintain for this sort of parsing.
This may do:
<.*?[^/]>
Or without the ending tags:
<[^/].*?[^/]>
What's with the flame wars on HTML parsers? HTML parsers must parse (and rebuild!) the entire document before it can categorize your search. Regular expressions may be a faster / elegant in certain circumstances. My 2 cents...
Currently finalising the coding for my comment system, and it want it to work a little how Stack Overflow works with their posts etc, I would like my users to be able to use BOLD, Italic and Underscore only, and to do that I would use following:
_ Text _ * BOLD * -Italic-
Now, firstly I would like to know a way of stripping a comment completely clean of any tags, html entities and such, so for example, if a user was to use any html / php tags, they would be removed from the input.
I am currently using Strip_tags, but that can leave the output looking quite nasty, even if an abusive or blatent XSS/Injection attempt has been made, I would still like the plain-text to be outputted in full, and not chopped up as strip_tags seems to make an absolute mess when it comes to that.
What I will then do, is replace the asterisks with bold html tags, and so on AFTER stripping the content clean of html tags.
How do people suggest I do this, currently this is the comment sanitize function
function cleanNonSQL( $str )
{
return strip_tags( stripslashes( trim( $str ) ) );
}
PHP tags are surrounded by <? and ?>, or maybe <% and %>on some ages-old installations, so removing PHP tags can be managed by a regex:
$cleaned=preg_replace('/\<\?.*?\?\>/', '', $dirty);
$cleaned=preg_replace('/\<\%.*?\%\>/', '', $cleaned);
Next you take care of the HTML tags: These are surrounded by < and >. Again you can do this with a regex
$cleaned=preg_replace('/\<.*?\>/','',$cleaned);
This will transform
$dirty="blah blah blah <?php echo $this; ?> foo foo foo <some> html <tag> and <another /> bar bar";
into
$cleaned="blah blah blah foo foo foo html and bar bar";
You could try using regular expressions to strip the tags, such as:
preg_replace("/\<(.+?)\>/", '', $str);
Not sure if that's what you're looking for, but it will remove anything inside < and >. You can also make it a little more foolproof by requiring the first character after the < to be a letter.
The correct way is not to delete html tags from your user's comment, but to tell the browser that the following text should not be interpreted as HTML, Javascript, whatever. Imagine someone wants to post example code like we do here on stackoverflow. If you just bluntly remove any parts of a comment that seem to be code, you will mess up the user's comment.
The solution is to use htmlentities which will escape symbols used for html markup in the comment so that it will actually show up as just text in the browser.
For example the browser will interpret a < as the beginning of a html tag. if you just want the browser to display a <, you have to write < in the source code. htmlentities will convert all the relevant symbols into their html entities for you.
Longer Example
echo htmlentities("<b>this text should not be bold</b><?php echo PHP_SELF;?>");
Outputs
<b>this text should not be bold</b><?php echo PHP_SELF;?>
The browser will output
<b>this text should not be bold</b><?php echo PHP_SELF;?>
Consider the following real life example with the solution, you accepted. Imagine a user writing this comment.
i'm in a bad mood today :<. but your blog made me really happy :>
You will now do your preg_replace("/\<(.+?)\>/", '', $comment); on the text and it will remove half the comment:
i'm in a bad mood today :
If that's what you wanted, never mind this answer. If you don't, use htmlentities.
If you want to save the comment as a file and not have the server interpret PHP code inside it, save it with an extension like '.html' or '.txt', so that the web server won't call the PHP interpreter in the first place. There is usually no need to escape PHP code.
I have the following regex used to check HTML code:
/<.+(onclick|onload)[^=>]*=[^>]+>/si
This regex is supposed to detect if there are tags with onclick or onload attributes somewhere in the HTML. It does so in most cases, however the ".+" part is a huge performance problem on big texts (and also source of some bugs as it's too greedy). I've tried to fix it and make it smarter but failed so far - "smarter" one misses some examples like this:
<img alt="<script>" src="http://someurl.com/image.jpg"; onload="alert(42)" width="1" height="1"/>
Now, I know I should not parse HTML with regexes and unmentionable horrors happen if I do. However, in this particular case I can not replace it with the proper code (e.g. real HTML parser). Is it still possible to fix this regex or there's no way to do it?
i would strongly recommend that you be researching alternatives to regex matching - the onclick/load js handler code may comprise arbitrary occurrences of > and < as relops or inside js comments. this applies to the code of other js handlers on the same element before or after the onclick/load handlers as well. the whole tag containing the match might be inside a html comment (though you might want to match these occurrences too or strip the html comments before).
however, having hinted to dire straits you appear to be aware of, the standard disclaimers against 'html regex matching' do not fully apply as you only need matches inside tags. try scanning for
on(click|load)[[:space:]]*=[[:space:]]*('[^']*'|"[^']*")
and add some logic to search the text surrounding any matches for the enclosing tags. if you're brave, try this one:
<(([^'">]+(('[^']*'|"[^"']*")[^'">]+)*)|([^'">]+('[^']*'|"[^"']*"))+)on(click|load)[[:space:]]*=[[:space:]]*('[^']*'|"[^']*")
it matches alternating sequences of text inside and outside of pairs of quotes between the tag opener < and the onclick/load-attribute. the outermost alternative caters for the special case of no whitespace between a closing quote and the onclick/load-attribute.
hope this helps
WordPress spits posts in this format:
<h2>Some header</h>
<p>First paragraph of the post</p>
<p>Second paragraph of the post</p>
etc.
To get my cool styling on the first paragraph (it's one of those things that looks good only sparingly) I need to hook into the get_posts function to filter its output with a preg_replace.
The goal is to get the above code to look like:
<h2>Some header</h>
<p class="first">First paragraph of the post</p>
<p>Second paragraph of the post</p>
I have this so far but it's not even working (the error is: "preg_replace() [function.preg-replace]: Unknown modifier ']'")
$output=preg_replace('<p[^>]*>', '<p class="first">', $content);
I can't use CSS3 meta-selectors because I need to support IE6, and I can't apply the :first-line meta-selector (this is one that IE6 supports) on the parent container because it would hit the H2 instead of the first P.
You may find it easier and more reliable to use an HTML parser such as this one. HTML is notoriously difficult to parse reliably (technically, impossible) with regular expressions, and the parser will give you a very simple means to find the nodes you're interested in. The first page of the doc has a tab labelled "How to modify HTML elements".
Two right possibilities :
Do that in Javascript. Using jQuery, for example, it's a matter of one line : $("h2").next().addClass("first")
Use an HTML parser. Indeed, regexp are not a good tool to do what you want to do. Since loading a whole HTML parser for just this purpose is overkill, you'd really better be using Javascript.
The wrong way
Of course, in order to anwser the question, here is the best way I can't think of to make it happends with regexp. Though, I don't recommend it.
preg_replace('#(</h2>\s*<p[^>]*)>#im', '$1 class="first">', '<h2>Some header</h> <p>First paragraph of the post</p> <p>Second paragraph of the post</p> ');
What we do is:
using preg_replace so we can use advanced regexp to replace the code;
using "m" and "i" flag so the regexp does not bother about line break or case;
using </h2>\s* to match the closing "h2" tags and all the spaces/line breaks after;
using *<p[^>]* to match the "p" tag and its current attributs;
using parenthesis to save that;
using "$1" to replace to replace the matched string we the part we save;
adding the class and closing the ">".
The first draw back I can think of is that it doesn't handle the case where a class already exists.
Of, and by the way, you have <h2>...</h> instead of <h2>...</h2>. I don't know if it's a typo but I assumed it was. Replace in the regexp accordingly if it's not.
The problem is that the first character of the regex in a preg_* function is taken as a modifier delimiter. What you'd need is something like:
$output = preg_replace('~<p\b([^>]*)>~', '<p class="first" \1>', $content, 1);
This also puts back any extra attributes the <p> may have.
Overall, though, it's cleaner to do with CSS selectors and a JS fallback for IE.
EDIT: Added replacement limit and word break.
in this particular case regexp solution would be fairly easy
echo preg_replace('~</h2>\s*<p~', "$0 class='first'", $html);
Reading through the answers there are some that will work but all have drawbacks of either using an external parsing library or possibly matching tags other than the P tag or also matching its attributes.
I ended up using this solution with the str_replace_once function from here:
str_replace_once('<p>', '<p class="first">', $content);
Simple enough and it works just as intended. Here's full WordPress code snippet to filter the first paragraph any time the_content() is called:
add_filter('the_content', 'first_p_style');
function first_p_style($content) {
$output=str_replace_once('<p>', '<p class="first">', $content);
return ($output);
}
Thanks for all the answers!