This regex(using negative lookahead) should work -
\/([a-zA-Z]+?=(?:(?!\/[a-zA-Z]+?=.*?).)*)
You can test it and breakdown the regex Here
Testing it-
$name = "/serialNumber=wsZ0axJazQw/eeFdiFKbpoVW/7uORUSh/OU=GT20702350/OU=See www.rapidssl.com/resources/cps (c)13/OU=Domain Control Validated - RapidSSL(R)/CN=mail.example.com";
$regex = "/\/([a-zA-Z]+?=(?:(?!\/[a-zA-Z]+?=.*?).)*)/";
if(preg_match_all($regex, $name, $matches)){
var_dump($matches[1]);
}
/**
OUTPUT
**/
array
0 => string 'serialNumber=wsZ0axJazQw/eeFdiFKbpoVW/7uORUSh' (length=45)
1 => string 'OU=GT20702350' (length=13)
2 => string 'OU=See www.rapidssl.com/resources/cps (c)13' (length=43)
3 => string 'OU=Domain Control Validated - RapidSSL(R)' (length=41)
4 => string 'CN=mail.example.com' (length=19)