php正则组平衡有关问题

php正则组平衡问题
Compilation failed: syntax error in subpattern name (missing terminator) at offset 46

PHP code

<!--

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-->$str = "<div class="a">
<div>cc</div>a<span>ab</span>c<div class="b">xxxy</div>
</div>";
preg_match_all("#((?'open'<div>]*>)((?:.(?!?div))*.)*?)((?'-open'</div>[^>]*>)((?:.(?!?div))*.)*?)#is",$str,$matches); 
print_r($matches);

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