$re = '~(?s)<body>\K\s*<tag(?:(?!<inner_block=25>).)+</tag>~';
$str = '<body>
<tag>
<number=1>
<inner_block=5>
</tag>
<tag>
<number=12>
<inner_block=15>
</tag>
<tag>
<number=21>
<inner_block=25>
</tag>
<tag>
<number=29>
<inner_block=35>
</tag>
</body>';
$subst = "";
$result = preg_replace($re, $subst, $str, 1);
echo "The result of the substitution is ".$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