$re = '/\b(?:https?:\/\/)?[^\/:]+\/.*?checkout\/b\?bag=?([A-Za-z0-9]*)$/';
$str = 'https://jasongarnerppf3f6.volusionplatformdev.com/checkout/b?bag=5ab1235aebd29702dc547df7';
preg_match_all($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