regex – firxworx

This post covers how to use ansible’s regex_replace filter to strip leading and/or trailing slashes from file paths and URL fragments.

The final example demonstrates how to generate a valid filename from a file or URL path by removing leading and trailing slashes, and replacing any remaining slashes with underscores. This could be useful for a variety of applications from backup scripts to web scraping.

Stripping slashes

Regexes and jinja2 expressions in ansible can be a pain in the ass, especially when it comes to escaping the right thing.

The key to the following examples is a double-escape of the forward slash character. They have been tested on ansible v.2.3.1.0.

Remove leading slashes

{{ variable_name | regex_replace('^\\/', '') }}

Remove trailing slashes

{{ variable_name | regex_replace('\\/$', '') }}

Stripping both leading and trailing slashes

This example makes use of the | (OR) to combine the previous two examples into one regex:

{{ variable_name | regex_replace('^\\/|\\/$', '')

Here’s a quick debug task that demonstrates the above in action:

- debug:
    msg: "Look Ma! No leading or trailing slashes! -- {{ item | regex_replace('^\\/|\\/$', '') }}"
  with_items:
    - "/home/and/lots/of/folders/trailing_and_leading/"
    - "no_leading/but/there/is/trailing/"
    - "/leading/but/no_trailing"
    - "no_leading/and/no_trailing"

Creating a valid filename from a path

To create a valid filename from a path, we need to remove leading and trailing slashes, then replace any remaining slashes with underscores. This sort of thing can be useful for naming backup files, data obtained from URL scraping, etc.

This is accomplished by adding a second regex_replace to the previous example that replaces all slashes with underscores, e.g. regex_replace('\\/', '_').

The combined regex that creates a valid filename from given a path:

{{ variable_name | regex_replace('^\\/|\\/$', '') | regex_replace('\\/', '_') }}

If variable_name held a value like “/var/www” or “/var/www/” it would result in: “var_www”.

In case you want to replace the slashes in the path with a character other than an underscore, you can adjust the examples to use any valid filename character instead of a slash (e.g. the ‘^’ character).

Working around a regex_replace bug in the shell module

In my working version of ansible (2.3.x) the regex_replace filter is ignored (!) when it is applied to variables in a tasks using the shell module. The value of unfiltered variables are substituted into the shell command fine, and using any other filter works fine too.

Workaround: employ the set_fact module to build a new fact (variable) based on the original variable, applying the regex_replace filter here as required. Reference the new fact in the shell module to take advantage of the pre-filtered values.

In the following example, assume that the hypothetical {{ list_of_paths }} variable contains a list of strings containing file/dir/URL paths. The set_fact module builds the new {{ paths }} fact such that it contains a “pi” item corresponding to every item in the original list. Each of these items has a “stripped” property containing the filtered value and a “path” property containing the original unfiltered value.

- name: workaround example
  set_fact: 
    paths: "{{ paths|default([]) + [{'pi': {'path': item, 'stripped': item|regex_replace('^\\/|\\/$', '')|regex_replace('\\/', '_')}}] }}"
      with_items: "{{ list_of_paths }}"

When looping over {{ paths }} in a shell task (e.g. via with_items), the filtered slash-free values for items can be referenced via {{ item.stripped }}. The original unfiltered path can be referenced via {{ item.path }}.

Tag: regex

Stripping leading and trailing slashes from paths in Ansible