Replace regex in rewriter

[jvoisin]

No need for brittle regex when matching plain strings or domain names. This should save some negligible amount of heap memory as well as tremendously speeding up the matching.

[fguillot]

In PR #3009, I added a unit test for GetRefererForURL(). Your refactoring is not backward compatible:

--- FAIL: TestGetRefererForURL (0.00s)
    --- FAIL: TestGetRefererForURL/Pixiv_Image_URL (0.00s)
        referer_override_test.go:62: GetRefererForURL(https://i.pximg.net/img-master/example.jpg): expected https://www.pixiv.net, got
    --- FAIL: TestGetRefererForURL/SSPai_CDN_URL (0.00s)
        referer_override_test.go:62: GetRefererForURL(https://cdnfile.sspai.com/example.png): expected https://sspai.com, got
    --- FAIL: TestGetRefererForURL/Piokok_URL (0.00s)
        referer_override_test.go:62: GetRefererForURL(https://sp1.piokok.com/example.jpg): expected https://sp1.piokok.com, got
    --- FAIL: TestGetRefererForURL/Weibo_Video_URL (0.00s)
        referer_override_test.go:62: GetRefererForURL(https://f.video.weibocdn.com/example.mp4): expected https://weibo.com, got
    --- FAIL: TestGetRefererForURL/HelloGithub_Image_URL (0.00s)
        referer_override_test.go:62: GetRefererForURL(https://img.hellogithub.com/example.png): expected https://hellogithub.com, got

[jvoisin]

You're right, I should have added tests in the first place :/

[fguillot]

I'm wondering if it's worth to have benchmark unit tests in your refactoring. It's hard to tell how the new implementation compare to the previous one in term of performance.