$re = '/((?:<br\s*\/?>)+)|(?<!<br)\s+(?!\/?>)/';
$str = '<strong>Blabla1</strong> Blaabla2<br /> Blaabla3 <strong>Blaabla4</strong> Blaabla5 Blaabla6<br /><br /> Blaabla7 <span style=\'color:#B22222;\'>Blaabla8</span> Blaabla9';
$subst = "$1\n";
$result = preg_replace($re, $subst, $str);
echo "The result of the substitution is ".$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