Mastering Regex in Ansible: Search, Extract, and Replace

Learn how to use Ansible's regex filters to search, extract, and replace text. Explore practical examples, including extracting kernel versions from package lists.

Regular Expression

A regular expression (regex) is a character pattern describing a set of strings. It is a powerful tool for searching, validating, and manipulating text data. Regular expressions are widely used in programming languages, text editors, and other software tools that work with text data.

A regular expression consists of a sequence of characters defining the matching pattern. These characters can include letters, digits, special characters, and meta-characters. Meta-characters are special characters that have a special meaning in regular expressions, such as . (match any character), * (match zero or more occurrences), and + (check one or more occurrences).

Regular expressions can be used for a variety of tasks, such as:

Searching for specific patterns in text data

Validating input data to ensure it conforms to a particular format

Extracting specific data from text

Replacing parts of the text with other text

Splitting text into regions based on a delimiter or pattern

For example, the regular expression ^[a-z]+@[a-z]+\.[a-z]{2,3}$ matches email addresses that are composed of one or more lowercase letters before the @ symbol, one or more lowercase letters after the @ symbol and before the ., and a two or three-letter lowercase top-level domain after the ..

Regular expressions can be complex and require careful crafting to achieve the desired results. Many online resources and tutorials are available to help learn regular expressions and practice using them.

The Ansible Regular Expression Modules

In Ansible, the regex_search filter is a Jinja2 filter that allows you to search for a regular expression pattern in a string and extract the matching substring. Underneath it, use the Python re module.

The syntax of the regex_search filter is:

``yaml


{{ my_string | regex_search(regex_pattern) }}

Here, my_string is the string to search in, and regex_pattern is the regular expression pattern to search for.

If the search is successful, regex_search returns the first matching substring. If there are multiple matches, only the first match is returned. If there is no match, an empty string is returned.

The regex_search filter takes two arguments: the regular expression pattern to search for, and the string to search within. Here's an example:

`yaml


name: Search for pattern in string
  ansible.builtin.debug:
    msg: "{{ my_string | regex_search('pattern') }}"

In this example, my_string is a variable containing the string to search within and 'pattern' is the regular expression pattern to search for. The regex_search filter will search for the first occurrence of 'pattern' within my_string and return it as the result of the debug task.

For example, suppose you have a variable my_string containing the text "My phone number is 123-456-7890". You can use the regex_search filter to extract the phone number from the string using the regular expression pattern \d{3}-