RegexSubstitution in declarativeNetRequest

[Browser Extenstion]

I'm using declarativeNetRequest to redirect some domain. This is example of the rule:
[
  {
    "id": 1,
    "priority": 1,
    "action": {
      "type": "redirect",
      "redirect": {
        "regexSubstitution": "https://new-domain.com?string=\\0"  
      }
    },
    "condition": {
      "regexFilter": "^https://domain.com(.*)",
      "resourceTypes": ["main_frame"]
    }
  }
]

The format of url in condition regexFilter is (it always contains word "string"): https://domain.com/some_data?string=........
So I want to redirect with regexSubstitution the data comes after string=.
In other words:
https://domain.com/new/test/1?string=my_test_with_regex&success=hope_true --> https://new-domain.com?string=my_test_with_regex&success=hope_true

Is it possible?
If yes, how I can use regex with "\\0" variable?

[wOxxOm]

\\0 inserts everything matched by regexFilter i.e. in your case the entire source URL.

You should use \\1 to insert the first parenthesized group. However, since regex is slow and very restricted in this API it's best to apply a faster condition first via requestDomains (Chrome 101+) and urlFilter:

[{
  "id": 1,

  "action": {
    "type": "redirect",
    "redirect": {

      "transform": {
        "host": "new-domain.com",
        "path": ""
      }
    },
  },
  "condition": {
    "urlFilter": "?string=",
    "resourceTypes": ["main_frame"],
    "requestDomains": ["domain.com"]
  }
}]

Note that this example redirects only main_frame requests i.e. navigation, not XHR or fetch or images.

[Browser Extenstion]

This solution works only if string is a first argument (?string=)
It not works for &string=.

In the question I wrote that I need to redirect to another new-domain. But, actually the "new domain" comes from the string.
It something like this:
https://domain.com?param&string=https://some-other-domain.com/additional_information.

So I need to redirect the request to https://some-other-domain.com/additional_information.

[wOxxOm]

Then you'll need to use regex:

[{
  "id": 1,
  "action": {
    "type": "redirect",
    "redirect": {

      "regexSubstitution": "\\1"
    }
  },
  "condition": {
    "regexFilter": "[?&]string=([^&]+)",

    "resourceTypes": ["main_frame"],
    "requestDomains": ["domain.com"]
  }
}]

Note that this will only work if the parameter value is not percent-encoded, otherwise the URL will be invalid like https%3a%2f%2fother-domain.

[Browser Extenstion]

Unfortunately, it does not work.

  "id": 1,
  "action": {
    "type": "redirect",
    "redirect": {
      "regexSubstitution": "\\1"
    }
  },
  "condition": {
    "regexFilter": "[?&]string=([^&]+)",

    "resourceTypes": ["main_frame"],

    "requestDomains": ["example.com"]
  }


https://example.com/?string=https://google.com --> https://example.com/https://google.com
And not to https://google.com as expected.

[wOxxOm]

It means regexFilter should match the entire source URL e.g. "^.*?[?&]string=([^&]+).*"