How do I quote square brackets in Ansible variable value for regexp parameter?

Question

I use the ansible.builtin.lineinfile module to modify a PHP-FPM pool configuration file. How should I quote the square brackets (annot.: or other special characters) in a value of a variable for the regexp parameter?

Without quoting the square brackets are interpreted as bracket expression - a list of characters to be matching. Without Ansible I would use sed to add a backslash.

The following example is for a Debian default pool conf file.

- name: Change PHP-FPM pool parameters
  become: true
  ansible.builtin.lineinfile:
    path: "/etc/php/{{ php_fpm_version }}/fpm/pool.d/www.conf"
    regexp: "^;?{{ item.para }} ="
    line: '{{ item.para }} = {{ item.value }}'
  loop: "{{ php_fpm_pool_parameters }}"
  vars:
    php_fpm_pool_parameters:
    - para: 'pm.max_spare_servers'
      value: '7'
    - para: 'pm.max_requests'
      value: '500'
    - para: 'env[PATH]'
      value: '/usr/bin:/bin'
    - para: 'php_admin_value[memory_limit]'
      value: '4096M'
    - para: 'php_admin_value[max_input_time]'
      value: '600'

Answer 1

You need to escape the regex special characters in your string so that they are taken literally. FYI, escaping is not different in ansible than outside (i.e. you add the escape char which is \).

Fortunately, there is a regex_escape filter made just for that.

The following (untested) should suit your needs. Note: for extra security I marked the space preceding the = sign as optional and extended the regexp to match the entire line.

- name: Change PHP-FPM pool parameters
  become: true
  ansible.builtin.lineinfile:
    path: "/etc/php/{{ php_fpm_version }}/fpm/pool.d/www.conf"
    regexp: "^;?{{ item.para | regex_escape }} ?=.*$"
    line: '{{ item.para }} = {{ item.value }}'
  loop: "{{ php_fpm_pool_parameters }}"
  vars:
    php_fpm_pool_parameters:
    - para: 'pm.max_spare_servers'
      value: '7'
    - para: 'pm.max_requests'
      value: '500'
    - para: 'env[PATH]'
      value: '/usr/bin:/bin'
    - para: 'php_admin_value[memory_limit]'
      value: '4096M'
    - para: 'php_admin_value[max_input_time]'
      value: '600'