Support Captured Groups for path type RegularExpression

[Robbilie]

Nginx Ingress Controller supported using regex captured groups for regex path matching:

https://kubernetes.github.io/ingress-nginx/examples/rewrite/#rewrite-target

I would like to request the functionality for ngf aswell.

An example:

apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
  annotations:
    nginx.ingress.kubernetes.io/use-regex: "true"
    nginx.ingress.kubernetes.io/rewrite-target: /$2
  name: rewrite
  namespace: default
spec:
  ingressClassName: nginx
  rules:
  - host: rewrite.bar.com
    http:
      paths:
      - path: /something(/|$)(.*)
        pathType: ImplementationSpecific
        backend:
          service:
            name: http-svc
            port: 
              number: 80

would become:

apiVersion: gateway.networking.k8s.io/v1
kind: HTTPRoute
metadata:
  name: rewrite
spec:
  parentRefs:
  - name: nginx
  hostnames:
  - rewrite.bar.com
  rules:
  - matches:
    - path:
        type: RegularExpression
        value: /something(/|$)(.*)
    backendRefs:
    - name: http-svc
      port: 80
    filters:
    - type: URLRewrite
      urlRewrite:
        path:
          type: ReplaceFullPath
          replaceFullPath: /$2

(opening the discussion as per #3110 (comment))

[sjberman]

@Robbilie Just out of curiosity, when you attempt to create this Route, what errors are you getting back?

[Robbilie]

that is indeed a little more conclusive :D

All rules are invalid: spec.rules[0].filters[1].urlRewrite.path: Invalid value: {"type":"ReplaceFullPath","replaceFullPath":"/$2"}: cannot contain $

gateway:
https://gist.github.com/Robbilie/77455e1081b73e408221875c8bf86b3e

httproute:
https://gist.github.com/Robbilie/d39b4fb15ddac67117fa4365c2ca92a0

[Robbilie]

in any case, the ngf generates rewrites like this:

rewrite ^ $request_uri;
rewrite ^ /v1/theme break;

while the ingress does them like this:

rewrite "(?i)/keycloak-token-validator/v1(/|$)(.*)" /$2 break;

so for the capture groups to work the whole rewrite generation would have to be changed

[sjberman]

Thanks for sharing. So your main use case for capture groups is for rewriting to the captured group.

[Robbilie]

Yeah :) I want to use very precise routes mapped to spring boot endpoints and not expose a whole sub-path via a prefix

[sjberman]

Perfect. I'll create an enhancement issue for this.

[sjberman]

#5061 issue for tracking this feature request

[sjberman]

Gateway API is proposing regex for rewrites now, which is great timing. We'll keep an eye on this GEP and API so that we can natively support this feature through the Gateway API: kubernetes-sigs/gateway-api#4361

[Robbilie]

i feel like their proposed solution is overly complex or is this just me?

[sjberman]

Maybe slightly, but from my brief look at it, it seems like it's just a slightly different way to accomplish what you suggested.

Your example might look something like:

apiVersion: gateway.networking.k8s.io/v1
kind: HTTPRoute
metadata:
  name: rewrite
spec:
  parentRefs:
  - name: nginx
  hostnames:
  - rewrite.bar.com
  rules:
  - matches:
    - path:
        type: PathPrefix
        value: /something
    backendRefs:
    - name: http-svc
      port: 80
    filters:
    - type: URLRewrite
      urlRewrite:
        regexModifier:
          pathPattern: /something(/|$)(.*)
          pathSubstitution: /$2

[sjberman]

@Robbilie I'm realizing you may be able to accomplish what you want without regex.

Would this work?

apiVersion: gateway.networking.k8s.io/v1
kind: HTTPRoute
metadata:
  name: rewrite
spec:
  parentRefs:
  - name: nginx
  hostnames:
  - rewrite.bar.com
  rules:
  - matches:
    - path:
        type: PathPrefix
        value: /something
    backendRefs:
    - name: http-svc
      port: 80
    filters:
    - type: URLRewrite
      urlRewrite:
        path:
          type: ReplacePrefixMatch
          replacePrefixMatch: /

This should take any path that is prefixed with /something and rewrite it with that prefix removed, retaining the suffix. I believe this is what you're trying to do with your regex, correct?

[Robbilie]

the path match should be very precise to not expose too many endpoints and also route traffic to different services. while that ingress removes a prefix, it does not satisfy my requirements :)