$re = '/(?!<p\s*+class="translate")<p[^>]*+>[^<>]*+<\Kstrong/m';
$str = '<p class="translate" style="font-size: 15px; color: black; line-height: 18px; font-style: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; background-color: transparent; font-family: Verdana; text-align: right;"><strong><span style="color: blue;">Lorem</span><br><span style="color: navy;">ipsum</span></strong>
</p></div>
<p style="font-size: 15px; color: black; line-height: 18px; font-style: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; background-color: transparent; font-family: Verdana; text-align: right;"><strong><span style="color: blue;">Lorem</span><br><span style="color: navy;">ipsum</span></strong>
</p>
stronger
strongest
<span style="color:blue;font-weight:bold;">';
preg_match_all($re, $str, $matches, PREG_SET_ORDER, 0);
// Print the entire match result
var_dump($matches);
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php