Unexpected behavior with 'ansible-playbook --limit' and regular expression with lookahead

I have the following test setup and result:

$ cat hosts
[test]
abc-1
abc-ecs
abc-x
ecs-1
ecs-y

$ cat test-regex.yaml 
- name: test
  hosts:
    - test
  tasks:
    - name: set var
      set_fact:
        myvar: "this is a test"

$ cat hosts | sed "s: ::g" | ggrep -P '^(?!ecs).*'
[test]
abc-1
abc-ecs
abc-x

$ ansible-playbook -i hosts -l ~'^(?!ecs).*' test-regex.yaml -D -C --list-hosts --list-tasks

playbook: test-regex.yaml

  play #1 (test): test  TAGS: []
    pattern: ['test']
    hosts (5):
      abc-ecs
      abc-x
      ecs-1
      ecs-y
      abc-1
    tasks:
      set var   TAGS: []

As shown with ggrep, ^(?!ecs).* does not match ecs-1 and ecs-y, but Ansible says it matches, can you explain?

I would like to apply to hosts starting with ecs only. I tested with Ansnile Core 2.13.3.

Also for testing purpose, can I maintain one file instead of two as shown here?

Solution

The question of the original post asks why -l ~'^(?!ecs).*' matches every host in the test group:

[test]
abc-1
abc-ecs
abc-x
ecs-1
ecs-y

but not those three abc-* hosts only as intended. This is because -l ~'^(?!ecs).*' not only matches hosts, but groups as well, and ansible creates two default groups: all and ungrouped. In this case, the regular expression matches:

test group which contains all 5 hosts listed above
all group which contains all 5 hosts listed above
ungrouped group which contains 0 hosts
abc-1 host
abc-x host
abc-ecs host

The union of all above consists the 5 hosts as shown in the output in the original post, so this is an expected behavior. To get what I intended, which is every hosts not starting with ecs, I should exclude the test and all groups also. The regular expression and the intended result are copied below.

$ ansible-playbook -i hosts -l ~'^(?!ecs|test|all).*' test-regex.yaml -D -C --list-hosts --list-tasks

playbook: test-regex.yaml

  play #1 (test): regexcheck    TAGS: []
    pattern: ['test']
    hosts (3):
      abc-1
      abc-x
      abc-ecs
    tasks:
      set var   TAGS: []

The "mystery" resolved.