Extract a regex capturing group with rego - regex

Question

I'm trying to build a rule based file access "engine", that evaluates whether a service is allowed to access a file or not via rego rules/data.
Here's what I have so far
data
{
"bucketname": "randombucket"
"bucketpaths": {
"service1": "/base/path/in/bucket"
}
"rules": {
"service1": {
"GET": ["/storage/service1/(.*)"]
// ^ This is basically a relative path and I'd like to extract it
}
}
}
rego rule
package org.test
import future.keywords.if
import future.keywords.in
default allow := false
default ttl := 60
basePath = data["bucketpaths"][input.ServiceType]
default relativePath := ""
allow if {
allowedPaths := data["rules"][input.ServiceType][input.Method]
some allowedPath in allowedPaths
regex.match(allowedPath, input.Path)
# here I would like to do three things:
# 1) check if the regex matches
# 2) extract the capuring group
# 3) set the value into relativePath
}
fullPath = concat(basePath, relativePath)
bucketName = data["bucketname"]
input
{
"ServiceType": "service1",
"Method": "GET",
"Path": "/storage/service1/a/relative/path"
}
Here's a link to the rego playground: https://play.openpolicyagent.org/p/3p7VAxwTcR
Disclaimer, I'm a noob to opa/rego, so these statements probably don't make any sense to experienced people...
I already tried using regex.find_all_string_submatch_n but I just cannot wrap my head around two things basically
I'm inside of an if statement, which is obviously not meant to set values - but how else would I do it?
How do I "just run a piece of code" to evaluate stuff "somewhere"? To - for example - evaluate the regex and just assign the result