$re = '/struct_separate_cart\.cart[a-zA-Z0-9\[\.\]\_]+\.shipping\.discount/';
$str = 'struct_separate_cart.cart[LOCAL.this_vendor].shipping.discount';
preg_match($re, $str, $matches, PREG_OFFSET_CAPTURE, 0);
// Print the entire match result
var_dump($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