How to write regular expression in postman?

Question

Software testing

Good day to all.

Help needed.

It is necessary to extract from the link everything between the signs = and ", i.e. the payment id.

"url": "https://lalala.ru/payment_ru.html?mdOrder=12abcd11-aaaa-8888-88cc-c1cc00c6cc1c"

Here is the regular expression itself: (?<==)(.*)(?=")
Here is the code itself, which I am trying to register in Postman in variables

payment_id = resp_data.url.match(/(?<==)(.*)(?=")/);

The problem is that the variable is not being assigned. If to unpack, then there null. What am I doing wrong?

Thank you all in advance for your help.