The replace module can be used to replace markup in a file on the managed node (e.g. the target system). Or, the connection, delegate_to, or local_action modules/parameters can be used to run the task on the control node (that's your Ansible server).

The ansible-doc replace command can be used to show the Ansible documention on the replace module.

Let's say /tmp/foo.txt contains the following text.

Hello

In this example, every line beginning with Hello in /tmp/foo.txt will be updated to contain just the text World.

- name: "replace lines beginning with 'Hello' followed by anything with 'World' in foo.txt"
  replace:
    path: "/tmp/foo.txt:
    regexp: "^Hello.*"
    replace: "World"

If the file was successfully updated, the play should indicate changed.

TASK [replace lines beginning with 'Hello' followed by anything with 'World' in foo.txt]
changed: [server1.example.com]

If the file was not updated, the play should indicate ok. This is the expected behavior if there are no lines to replace in the file.

TASK [replace lines beginning with 'Hello' followed by anything with 'World' in foo.txt]
ok: [server1.example.com]