Let's say you are using the vars plugin to define the foo variable.

vars:
  foo: "Line One\nLine Two\nLine Three"

The debug module can be used to output the foo variable.

- name: display the content of the 'foo' variable
  debug: 
    var: foo

Which should return the following.

TASK [display the content of the 'foo' variable]
ok: [server1.example.com] => {
    "foo": "Line One\nLine Two\nLine Three"
}

regex_search can be used to perform a regular expression search for a string matching a pattern. Here is how to use regex_search, matching the first line containing "Two".

- name: display the content of the 'foo' variable, matching 'Two'
  debug: 
    msg: "{{ foo | regex_search('.*two.*', ignorecase=True) }}"

Which should now return the following.

TASK [display the content of the 'foo' variable, matching 'Two']
ok: [server1.example.com] => {
    "msg": "Line Two"
}

By default, regex_search will only return the first match. In this example, the first line that contains "Line" will be returned, meaning "Line Two" and "Line Three" are not returned.

- name: display the content of the 'foo' variable, matching 'Line'
  debug: 
    msg: "{{ foo | regex_search('.*Line.*') }}"

One option is to set multiline to true.

- name: display the content of the 'foo' variable, matching 'Line'
  debug: 
    msg: "{{ foo | regex_search('.*Line.*', multiline=True, ignorecase=True) }}"

Or, regex_findall can be used to return every pattern match, not just the first match.

TASK [display the content of the 'foo' variable, matching 'Line']
ok: [server1.example.com] => {
    "msg": "Line One"
}

Let's say the foo variable contains a value of "Hello World". Here is how you would search for the first line in the "out" variable that contain Hello World, using the foo variable.

- name: display the content of the 'out' variable, matching the 'foo' variable
  debug: 
    msg: "{{ out | regex_search('(.*' + foo + '.*)') }}"