replace module. Please explain \1 \2

[John Harmon]

I am trying to use the replace module, and I see all of these examples with \1 and \2 in them.  I assume these are for backrefs or soemthing?  Can someone explain it to me?  I am trying to append "sss" to my passwd and group lines in my nsswitch file.  I haven't tested my code yet, but I would still like to know what \1 \2 mean.  Maybe someone can point me to some documentation as I can't seem to find anything on it myself.

- name: Update nsswitch.conf
  replace:
    path:     /etc/nsswitch.conf
    regexp:   {{item.regexp}}
    replace:  {{item.replace}}
  with_items:
    - {regexp: '^passwd.*'
       replace: "\1 sss" },
    - {regexp: '^group.*'
       replace: "\1 sss" }

[Gordon Messmer]

Yes, those appear to be regex back-references.  The documentation is here:

http://docs.ansible.com/ansible/latest/replace_module.html