Remove style attribute from HTML tags

Something like this should work (untested code warning):

<?php

$html = '<p style="asd">qwe</p><br /><p class="qwe">qweqweqwe</p>';

$domd = new DOMDocument();
libxml_use_internal_errors(true);
$domd->loadHTML($html);
libxml_use_internal_errors(false);

$domx = new DOMXPath($domd);
$items = $domx->query("//p[@style]");

foreach($items as $item) {
  $item->removeAttribute("style");
}

echo $domd->saveHTML();

The pragmatic regex (<[^>]+) style=".*?" will solve this problem in all reasonable cases. The part of the match that is not the first captured group should be removed, like this:

$output = preg_replace('/(<[^>]+) style=".*?"/i', '$1', $input);

Match a < followed by one or more "not >" until we come to space and the style="..." part. The /i makes it work even with STYLE="...". Replace this match with $1, which is the captured group. It will leave the tag as is, if the tag doesn't include style="...".