Golang Regex to replace domain to proxy URL

I want to replace all links of a webpage to a reverse proxy domain.

The rules are

https://test.com/xxx --> https_test_com.proxy.com/xxx
http://sub.test.com/xxx --> http_sub_test_com.proxy.com/xxx

How to achieve it by regex in golang?

The type of response body is []byte, and character encoding of it is UTF-8.
I have tried in this way. But it cannot replace all the dot to underscore in the origin domain. The length of subdomain is variable, that means the number of dot can vary

respBytes := []byte(`_.Xc=function(a){var b=window.google&&window.google.logUrl?"":"https://www.google.com";b+="/gen_204?";b+=a.j(2040-b.length);
		<cite class="iUh30 Zu0yb tjvcx">https://cloud.google.com</cite></div><div class="eFM0qc"><a class="fl" href="https://webcache.googleusercontent.com/search?q=cache:80SWJ_cSDhwJ:https://cloud.google.com/+&cd=1&hl=en&ct=clnk&gl=au" ping="/url?sa=t&source=web&rct=j&url=https://webcache.googleusercontent.com/search%3Fq%3Dcache:80SWJ_cSDhwJ:https://cloud.google.com/%2B%26cd%3D1%26hl%3Den%26ct%3Dclnk%26gl%3Dau&ved=2ahUKEwia5ovYsv3xAhXS4jgGHad0BJYQIDAAegQIBRAG"><span>Cached</span></a></li><li class="action-menu-item OhScic zsYMMe" role="menuitem"><a class="fl" href="/search?q=related:https://cloud.google.com/+google+cloud&sa=X&ved=2ahUKEwia5ovYsv3xAhXS4jgGHad0BJYQHzAAegQIBRAH">
		`)
proxyURI := "proxy.com"
var re = regexp.MustCompile(`(http
展开收缩
*):\/\/([a-zA-Z0-9_\-.:]*)`)
content := re.ReplaceAll(respBytes, []byte("_."+proxyURI))

Here's how you can do this:

func replaceAndPrint() {
	src := `
<a href="https://test.com/xxx">link 1</a>
<a href="https://test.com/yyy">link 2</a>
`
	r := regexp.MustCompile("\"https://(test\\.com.*)\"")
	result := r.ReplaceAllString(src, "http://sub.$1")
	fmt.Println(result)
}

Output:

<a href=http://sub.test.com/xxx>link 1</a>
<a href=http://sub.test.com/yyy>link 2</a>

Explanation:
regexp.MustCompile's argument defines a capturing group (inside a pair of parentheses). The value of that capturing group is referenced by $1 in the call to r.ReplaceAllString.

UPDATE:

Sorry, misread the example.

Here's an updated version:

func replaceAndPrint2() {
	src := `
<a href="http://test.com/xxx">link 1</a>
<a href="https://sub1.sub2.test.com/yyy">link 2</a>
`
	r := regexp.MustCompile("(\\.|://)([^./]*)")
	replacer := strings.NewReplacer("://", "_", ".", "_")
	res := r.ReplaceAllStringFunc(src, func(g string) string {
		if g == ".com" {
			return replacer.Replace(g) + ".proxy.com"
		}
		return replacer.Replace(g)
	})
	fmt.Println(res)
}

Output:

<a href="http_test_com.proxy.com/xxx">link 1</a>
<a href="https_sub1_sub2_test_com.proxy.com/yyy">link 2</a>