Golang Regex match and replace first occurrence after a certain string

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 \n 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.

Do you have to capture `MYSTRING=` and then use a backreference in the replacement pattern? Can you hardcode `MYSTRING=`? — Wiktor Stribiżew, Feb 26 '19 at 23:41
There [comes a time](https://stackoverflow.com/questions/1732348/regex-match-open-tags-except-xhtml-self-contained-tags/1732454#1732454) when regular expressions are no longer the best solution to a problem... — Luke Joshua Park, Feb 27 '19 at 00:06

Wiktor Stribiżew · Accepted Answer · 2019-02-27T00:28:41.010

1

You may use ReplaceAllStringFunc and use a regex like

(?m)^bar:(?:\n\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:(?:\n\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

edited Feb 27 '19 at 00:28

answered Feb 26 '19 at 23:52

Wiktor Stribiżew

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Sorry, I think I might have worded my question a bit too confusingly. I meant to say that I need the `MYSTRING=*` replaced with `MYSTRING=stringofmychoice` after a **certain** string has appeared, not after it has appeared a certain number of times. In the example above, that **certain** string would be `bar`. I want to find a string, `bar`, and change the `MYSTRING=*` into `MYSTRING=barstring` after it. – bli00 Feb 27 '19 at 00:21
@thestateofmay That does not change much. – Wiktor Stribiżew Feb 27 '19 at 00:22
Would it be possibly to greedily use the matching between `bar` and `MYSTRING=` instead? I'm trying something like `re := regexp.MustCompile("(bar).(MYSTRING=).*")` but it's not working. – bli00 Feb 27 '19 at 00:43
@thestateofmay ``re := regexp.MustCompile("((?s:bar.*?MYSTRING=)).*")``. Not sure what you mean though. If you plan to use it as a 1-regex solution, [watch out](https://regex101.com/r/L3sXFB/1). Since there is no lookahead support, it is difficult to come up with a working and readable regex for this. – Wiktor Stribiżew Feb 27 '19 at 08:54

Golang Regex match and replace first occurrence after a certain string

1 Answers1