Are Ansible patterns a feature of YAML? - ansible

Question

Ansible patterns is a useful feature when targeting a playbook for a subset hosts of an inventory.
Here is an example:
---
- hosts: all
tasks:
- name: create a file on a remote machine
file:
dest: /tmp/file
state: '{{file_state}}'
- hosts: web
tasks:
- name: create file on web machines
file:
dest: /tmp/web-file
state: '{{file_state}}'
- hosts: all:!db
tasks:
- name: create file on web machines
file:
dest: /tmp/web-not-db-file
state: '{{file_state}}'
- hosts: all:&backup:!web
tasks:
- name: create file on web machines
file:
dest: /tmp/backup-file
state: '{{file_state}}'
Credits: Robert Starmer
So far, I know Ansible utilizes YAML and Jinja2 for parsing its configuration files.
Ansible patterns are not using Jinja2 because they are not surrounded by {{ }} blocks.
Now my question is, are they a custom modification done by Ansible itself?
If yes, is using such modifications documented by Ansible anywhere?

Answer 1

Now my question is, are they a custom modification done by Ansible itself? If yes, is using such modifications documented by Ansible anywhere?
I think your question may be based on some fundamental misunderstandings.
YAML is just way of encoding data structures -- such as dictionaries and lists -- in a text format, so that it's easy (-ish) for people to read/edit them. What an application does with the data presented in a YAML file is entirely application dependent.
So when you write something like:
- hosts: all
This does "mean" anything in YAML other than "here's a list of one item, which is a dictionary with a single key hosts with the value all".
Of course, when Ansible reads a playbook and finds a top-level list, it knows that this is a play, and it expects the play to have a hosts key. It interprets the value of the hosts key using the ansible pattern logic.
Any other application could read that YAML file and decide to do something entirely different with the content.

Answer 2

you're looking for the host pattern documentation:
https://docs.ansible.com/ansible/latest/user_guide/intro_patterns.html#intro-patterns
edit: I did not notice you already linked to this page.
Yes, this is neither yaml nor jinja2, and it's fully documented on the page linked, so I'm not sure we need more than that.