The recursion in regular expressions is the only way to allow the parsing of HTML code with nested tags of indefinite depth.
It seems it's not yet a spreaded practice; not so much contents are available on the web regarding regexp recursion, and until now no user contribute notes have been published on this manual page.
I made several tests with complex patterns to get tags with specific attributes or namespaces, studying the recursion of a subpattern only instead of the full pattern.
Here's an example that may power a fast LL parser with recursive descent (http://en.wikipedia.org/wiki/Recursive_descent_parser):
$pattern = "/<([\w]+)([^>]*?) (([\s]*\/>)| (>((([^<]*?|<\!\-\-.*?\-\->)| (?R))*)<\/\\1[\s]*>))/xsm";
The performances of a preg_match or preg_match_all function call over an avarage (x)HTML document are quite fast and may drive you to chose this way instead of classic DOM object methods, which have a lot of limits and are usually poor in performance with their workarounds, too.
I post a sample application in a brief function (easy to be turned into OOP), which returns an array of objects:
<?php
// test function:
function parse($html) {
// I have split the pattern in two lines not to have long lines alerts by the PHP.net form:
$pattern = "/<([\w]+)([^>]*?)(([\s]*\/>)|".
"(>((([^<]*?|<\!\-\-.*?\-\->)|(?R))*)<\/\\1[\s]*>))/sm";
preg_match_all($pattern, $html, $matches, PREG_OFFSET_CAPTURE);
$elements = array();
foreach ($matches[0] as $key => $match) {
$elements[] = (object)array(
'node' => $match[0],
'offset' => $match[1],
'tagname' => $matches[1][$key][0],
'attributes' => isset($matches[2][$key][0]) ? $matches[2][$key][0] : '',
'omittag' => ($matches[4][$key][1] > -1), // boolean
'inner_html' => isset($matches[6][$key][0]) ? $matches[6][$key][0] : ''
);
}
return $elements;
}
// random html nodes as example:
$html = <<<EOD
<div id="airport">
<div geo:position="1.234324,3.455546" class="index">
<!-- comment test:
<div class="index_top" />
-->
<div class="element decorator">
<ul class="lister">
<li onclick="javascript:item.showAttribute('desc');">
<h3 class="outline">
<a href="http://php.net/manual/en/regexp.reference.recursive.php" onclick="openPopup()">Link</a>
</h3>
<div class="description">Sample description</div>
</li>
</ul>
</div>
<div class="clean-line"></div>
</div>
</div>
<div id="omittag_test" rel="rootChild" />
EOD;
// application:
$elements = parse($html);
if (count($elements) > 0) {
echo "Elements found: <b>".count($elements)."</b><br />";
foreach ($elements as $element) {
echo "<p>Tpl node: <pre>".htmlentities($element->node)."</pre>
Tagname: <tt>".$element->tagname."</tt><br />
Attributes: <tt>".$element->attributes."</tt><br />
Omittag: <tt>".($element->omittag ? 'true' : 'false')."</tt><br />
Inner HTML: <pre>".htmlentities($element->inner_html)."</pre></p>";
}
}
?>