$re = '/<div>\s*\n\s*.*?<p class=.*?>(.*?)<\/p>\s*\n\s*.*?\s*\n\s*(?=(\<a class=\"a\"\>))/';
$str = '<div>
<p class="match"> this sentence should match </p>
some text
<a class="a"> some text </a>
</div>
<div>
<p class="match"> this sentence shouldnt match</p>
some text
<a class ="b"> some text </a>
</div>
<div>
<p class="match"> this sentence should match </p>
some text
<a class="a"> some text </a>
</div>';
$subst = "";
$result = preg_replace($re, $subst, $str);
echo "The result of the substitution is ".$result;
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