$re = '/<Placemark>\s*?<name>(?:[7-9]\d|[1-2]\d\d)(?!00|50)\d\d.*?<\/Placemark>/s';
$str = 'before...
<Placemark>
<name>7700</name>
<description><![CDATA[converted by:</br><a href="http://gridreferencefinder.com/">GridReferenceFinder.com</a></br>]]></description>
<Point>
<coordinates>-0.99153654,52.225002,0</coordinates>
</Point>
</Placemark>
after...
---------
before...
<Placemark>
<name>7701</name>
<description><![CDATA[converted by:</br><a href="http://gridreferencefinder.com/">GridReferenceFinder.com</a></br>]]></description>
<Point>
<coordinates>-0.99153654,52.225002,0</coordinates>
</Point>
</Placemark>
after...
--------
before...
<Placemark>
<name>27650</name>
<description><![CDATA[converted by:</br><a href="http://gridreferencefinder.com/">GridReferenceFinder.com</a></br>]]></description>
<Point>
<coordinates>-0.99153654,52.225002,0</coordinates>
</Point>
</Placemark>
after...
--------
before...
<Placemark>
<name>27651</name>
<description><![CDATA[converted by:</br><a href="http://gridreferencefinder.com/">GridReferenceFinder.com</a></br>]]></description>
<Point>
<coordinates>-0.99153654,52.225002,0</coordinates>
</Point>
</Placemark>
after...
end.';
preg_match_all($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