Golang正则表达式匹配并替换某个字符串后的第一个匹配项

Following up on my previous question about using Golang's regex to replace between strings. I now have a bit of complexity added to it. Here is what the contexts of my file looks like:

foo:
    blahblah
    MYSTRING=*
bar:
    blah
    blah
    MYSTRING=*

I need to replace what's between MYSTRING= and with a string of my choice (like previous stated in the original post). I can do that with:

var re = regexp.MustCompile(`(MYSTRING=).*`)
s := re.ReplaceAllString(content, `${1}stringofmychoice`)

But now I need to match and replace only after a certain occurrence. So that the contents of my file can look something like this:

foo:
    blahblah
    MYSTRING=foostring
bar:
    blah
    blah
    MYSTRING=barstring

ReplaceAllString obviously replaces everything, which is not what I want. Is there a way to only match and replace the first occurrence after a certain string?

For a bit of background about all of this. I'm trying to write a program to edit the contents of a given docker-compose.yml file and its environment variables. I need to edit the environment variable MYSTRING differently depending on what service it's listed under. In the example above, the two different services would be foo and bar.

You may use ReplaceAllStringFunc and use a regex like

(?m)^bar:(?:
\s{4}.*)+

See the regex demo. It will match a bar block indented with four whitespaces. Then, after a match is obtained, you may use a regular ReplaceAllString on the match.

See the Go demo:

package main

import (
    "fmt"
    "regexp"
)

const sample = `foo:
    blahblah
    MYSTRING=*
bar:
    blah
    blah
    MYSTRING=*`

func main() {
    re := regexp.MustCompile(`(?m)^bar:(?:
\s{4}.*)+`)
    re_2 := regexp.MustCompile(`(MYSTRING=).*`)
    s := re.ReplaceAllStringFunc(sample, func(m string) string {
                return re_2.ReplaceAllString(m, `${1}stringofmychoice`)
        })
    fmt.Println(s)
}

Here, the second occurrence is changed in the bar block:

foo:
    blahblah
    MYSTRING=*
bar:
    blah
    blah
    MYSTRING=stringofmychoice