Golang正则表达式匹配并替换某个字符串后的第一个匹配项

Question

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.

Answer 1

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