Extract a string in C++

I know such question are asked many times but I have a real hard time to extract a string from a large string. I have a string that looks like this:

GET /analysis HTTP/1.1

Host: localhost:4433

User-Agent: curl/7.47.0

Accept: */*

Authorization: Basic MTIzYWxpY2U6bWVyY3VyeQ==

Content-Length: 40

Content-Type: application/x-www-form-urlencoded



{"u_id": 62, "g_id": 14, "a_type": "LR"}

I wanted to extract MTIzYWxpY2U6bWVyY3VyeQ== but MTIzYWxpY2U6bWVyY3VyeQ== could be anything but Authorization: Basic would be definitely there I do not have boost library. I also do not want any whitespace character around the extracted string.