$re = '/\W*(?i)\b(?<!\.)(?!GOOD_DATABASE)\w+[.]\S*(?-i)\W*/m';
$str = 'SELECT
COL1,
COl2,
COL3
FROM GOOD_DATABASE.SCHEMA1.TAB1 table1
JOIN GOOD_DATABASE.SCHEMA2.TAB2 table2
JOIN BAD_DATABSE.SCHEMA1.TAB3
JOIN BAD_DATABASE.SCHEMA3.TAB4';
preg_match_all($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