RegEx match open tags except XHTML self-contained

Question

Locked. Comments on this question have been disabled, but it is still accepting new answers and other interactions. Learn more.

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?

Answer 1

Locked. There are disputes about this answer’s content being resolved at this time. It is not currently accepting new interactions.

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 trangession 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?

---

Moderator's Note

This post is locked to prevent inappropriate edits to its content. The post looks exactly as it is supposed to look - there are no problems with its content. Please do not flag it for our attention.

Answer 2

I think the flaw here is that HTML is a Chomsky Type 2 grammar (context free grammar) and RegEx 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 RegEx.

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

Answer 3

Here is a PHP based parser that parses HTML using some ungodly regex. As the author of this project, I can tell you it is possible to parse HTML with regex, but not efficient. If you need a server-side solution (as I did for my wp-Typography WordPress plugin), this works.

Answer 4

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 <a href="#paragraph">here</a>
    <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:

1. <hr />
2. <br/>
3. <br>

I also entered tags with:

1. one attribute
2. more than one attribute
3. attributes which value is bound either into single quotes or into double quotes
4. attributes containing single quotes when the delimiter is a double quote and vice versa
5. "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:

1. the first sub-expression (\w+) matches the tag name
2. the second sub-expression contains the pattern of an attribute. It is composed by:
   1. one or more whitespaces \s+
   2. the name of the attribute (\w+)
   3. zero or more whitespaces \s* (it is possible or not, leaving blanks here)
   4. the "=" symbol
   5. again, zero or more whitespaces
   6. 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.
   7. 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
   8. 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
   9. zero or more whitespaces \s*
   10. the attribute sub-expression ends here, with the specification of zero or more possible occurrences, given by the asterisk.
3. Then, since a tag may end with a whitespace before the ">" symbol, zero or more whitespaces are matched with the \s* subpattern.
4. 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.

Answer 5

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.

Answer 6

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.