<?php
$text = file_get_contents('dio.php');
$patternOld = '#<hr\s+id=("|\')system-readmore("|\')\s*\/*>?#i';
$pattern = '#(?:\s*<(?<tag>[\w-]+)>)?<hr\s+id=["\']system-readmore["\']\s*\/*>(?:<\/(?P=tag)>\s*)?#i';
$tagPos = preg_match($pattern, $text);
$tagPosOld = preg_match($patternOld, $text);
list ($introtextOld, $fulltextOld) = preg_split($patternOld, $text, 2);
list ($introtext, $fulltext) = preg_split($pattern, $text, 2);
print_r([
'new' => [
'tagPos' => $tagPos,
'intro' => $introtext,
'more' => $fulltext,
],
'old' => [
'tagPos' => $tagPosOld,
'intro' => $introtextOld,
'more' => $fulltextOld,
],
]);
<div>Das ist ein Test</div><div><hr id="system-readmore" /></div>
<div>Mehr Text</div>
