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##### SUMMARY
The current documentation has this explanation for the `firstmatch`
parameter of the `lineinfile` action:

```
If set, insertafter and insertbefore find a first line has regular expression matches.
```

I have no idea what `firstmatch` does, based on the above line.


##### ISSUE TYPE
- Documentation Report

##### COMPONENT NAME
lineinfile

##### ANSIBLE VERSION
<!--- Paste verbatim output from "ansible --version" between quotes -->
```paste below
ansible 2.8.4
  config file = None
  configured module search path = [u'/home/user/.ansible/plugins/modules', u'/usr/share/ansible/plugins/modules']
  ansible python module location = /home/user/.virtualenvs/elasticluster/local/lib/python2.7/site-packages/ansible
  executable location = /home/user/.virtualenvs/elasticluster/bin/ansible
  python version = 2.7.14 (default, Sep 23 2017, 22:06:14) [GCC 7.2.0]
```

##### CONFIGURATION
<!--- Paste verbatim output from "ansible-config dump --only-changed" between quotes -->
```paste below

```

- Create a backup file including the timestamp information so you can
        get the original file back if you somehow clobbered it incorrectly.
    type: bool
    default: no
  firstmatch:
    description:
      - Used with C(insertafter) or C(insertbefore).
      - If set, C(insertafter) and C(insertbefore) find a first line has regular expression matches.
    type: bool
    default: no
    version_added: "2.5"
  others:
    description:
      - All arguments accepted by the M(file) module also work here.
    type: str
extends_documentation_fragment:
    - files
    - validate
notes:
  - As of Ansible 2.3, the I(dest) option has been changed to I(path) as default, but I(dest) still works as well.
seealso:
- module: blockinfile
- module: copy
- module: file
- module: replace
- module: template
- module: win_lineinfile
author:
    - Daniel Hokka Zakrissoni (@dhozac)
    - Ahti Kitsik (@ahtik)