Only check whether a line present in a file (ansible)

You need to add backrefs=yes if you don't want to change your regular expression.

- name: test lineinfile
  lineinfile: backup=yes state=present dest=/etc/foo.txt
              regexp='^#bar' backrefs=yes
              line='bar'

This changes the behavior of lineinfile from:

 find
 if found
   replace line found
 else
   add line

to:

 find
 if found
   replace line found

In other words, this makes operation idempotent.

The reason you get an error is because you're accessing a variable that has never been set. You seem to be using the existance of the extra-var as an indication that you want composer install to run (i.e. you're never passing --extra-vars="composer-install=false"), so you could go with is defined:

- shell: composer install
  when: composer-install is defined

But the variables can be passed through filters, which can be useful for this case, because it still allows you to pass true/false while not defining the variable at all still works:

- shell: composer install
  when: composer-install|default(false)

Some more on conditionals can be found here: http://docs.ansible.com/playbooks_conditionals.html

The jinja2 filters are very useful for more than one reason, so more on those here: http://docs.ansible.com/playbooks_variables.html#jinja2-filters

And finally the complete list of built-in jinja2 filters here: http://jinja.pocoo.org/docs/dev/templates/#builtin-filters