I want to use the same hostname let's say example.com with multiple Ingress resources running in different namespaces i.e monitoring and myapp. I'm using Kubernetes nginx-ingress controller.

haproxy-ingress.yaml

apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
  name: haproxy-ingress
  namespace: myapp
  annotations:
    kubernetes.io/ingress.class: "nginx"  
spec:
  tls:
  - hosts:
    # fill in host here
    - example.com
    
  rules:
    - host: example.com
      http:
        paths:
          - path: /
            pathType: Prefix
            backend:
              service:
                name: haproxy
                port:
                  number: 80

grafana-ingress.yaml

apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
  name: grafana-ingress
  namespace: monitoring
  annotations:
    nginx.ingress.kubernetes.io/use-regex: "true"
spec:
  tls:
  - hosts:
    - example.com
    
  rules:
    - host: example.com
      http:
        paths:
          # only match /grafana and paths under /grafana/
          - path: /grafana(/|$)(.*)
            pathType: Prefix
            backend:
              service:
                name: grafana
                port:
                  number: 3000

When I'm doing curl example.com then it is redirecting me to the deployment running in namespace one(as expected) but when I'm doing curl example.com/grafana then still it is redirecting me to namespace one deployment.

Please help.

Yes it is possible.

There can be two issues in your case.

One is you don't need the regex path for grafana ingress. Simple /grafana path will be fine with path type Prefix as with path type Prefix any /grafana/... will be redirected associated service. So the manifest file will be:

apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
  name: grafana-ingress
  namespace: monitoring
spec:
  tls:
  - hosts:
    - example.com
    
  rules:
    - host: example.com
      http:
        paths:
          - path: /grafana
            pathType: Prefix
            backend:
              service:
                name: grafana
                port:
                  number: 3000

And the second issue can be the related service or deployment might not be the under same namespace monitoring. Please make sure the deployment/service/secret or other resources needed for grafana remains under the same namespace monitoring.