$re = '/<(.+?)>[^<]+<\/\1>/m';
$str = '<EightSquaredSchema xmlns:xs="http://www.w3.org/2001/XMLSchema" xmlns:xdt="http://www.w3.org/2005/xpath-datatypes" xmlns:fn="http://www.w3.org/2005/xpath-functions">
<A>
<nested-A>valid text</nested-A>
Remove text not inside valid xml braces
<nested-B>more valid text here</nested-B>
</A>
</EightSquaredSchema>
';
preg_match_all($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