$re = '/"MachineSID" *: *"([\d-]+?)"/';
$str = '{ "ConfigName" : "SiteName", "Name" : "ABCTest" } "Windows" : { "MachineSID" : "15371-868-9808907-15424" }';
preg_match_all($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