$re = '~(?:_T\(\"[^\"]+\"\).*?|L\"[^\"]+\".*?)?(?<!#include )(?<!_T\(|L|\\\\)\"(.*?)\"(?<!\\\\\")~';
$str = '"This is a MBCS string"; // "This is a MBCS string" match
_T("This is maybe a unicode string"); // no match
L"This is a unicode string"; // no match
"These both" + "should match"; // "These both" and "should match" match
"This is a \\"quoted\\" string"; // "This is a \\"quoted\\" string" match
_T("Maybe this") + "is a match"; // "is this" match but instead would match ") + "
do_something(_T("This doesn\'t match")) + do_something("but this does match"); // "but this does match" match but isntead it matches ")) + do_something("';
preg_match_all($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