Language: Data types: Regular expressions

A regular expression (sometimes shortened as “regex” or “regexp”) is a pattern that can match some set of strings, and optionally capture parts of those strings for further use.

You can use regular expression values with the =~ and !~ match operators, case statements and selectors, node definitions, and certain functions (notably regsubst for editing strings and match for capturing and extracting substrings). Regexes act like any other value, and can be assigned to variables and used in function arguments.


Regular expressions are written as patterns bordered by forward slashes. (Unlike in Ruby, you cannot specify options or encodings after the final slash, like /node .*/m.)

if $host =~ /^www(\d+)\./ {
  notify { "Welcome web server #$1": }

Puppet uses Ruby’s standard regular expression implementation to match patterns.

Alternate forms of regex quoting like Ruby’s %r{^www(\d+)\.} are not allowed. You cannot interpolate variables or expressions into regex values.

Some places in the language accept both real regex values and stringified regexes — that is, the same pattern quoted as a string instead of surrounded by slashes.

Regex options

Regexes in Puppet cannot have options or encodings appended after the final slash. However, you can turn options on or off for portions of the expression using the (?<ENABLED OPTION>:<SUBPATTERN>) and (?-<DISABLED OPTION>:<SUBPATTERN>) notation. The following example enables the i option while disabling the m and x options:

$packages = $operatingsystem ? {
  /(?i-mx:ubuntu|debian)/        => 'apache2',
  /(?i-mx:centos|fedora|redhat)/ => 'httpd',

The following options are allowed:

  • i — Ignore case
  • m — Treat a newline as a character matched by .
  • x — Ignore whitespace and comments in the pattern

Regex capture variables

Within conditional statements and node definitions, any captured substrings from parentheses in a regular expression will be available as numbered variables ($1, $2, etc.) inside the associated code section, and the entire match will be available as $0.

These are not normal variables, and have some special behaviors:

  • The values of the numbered variables do not persist outside the code block associated with the pattern that set them.
  • You can’t manually assign values to a variable with only digits in its name; they can only be set by pattern matching.
  • In nested conditionals, each conditional has its own set of values for the set of numbered variables. At the end of an interior statement, the numbered variables are reset to their previous values for the remainder of the outside statement. (This causes conditional statements to act like local scopes, but only with regard to the numbered variables.)

The Regexp data type

The data type of regular expressions is Regexp.

By default, Regexp matches any regular expression value. If you are looking for a type that matches strings which match arbitrary regular expressions, see the Pattern type.

You can use parameters to restrict which values Regexp will match.


The full signature for Regexp is:


This parameter is optional.

Position Parameter Data Type Default Value Description
1 Specific regular expression Regexp none If specified, this will result in a data type that only matches one specific regular expression value. This doesn’t have any particular practical use.


  • Regexp — matches any regular expression.
  • Regexp[/foo/] — matches the regular expression /foo/ only.

Regexp only matches literal regular expression values. It’s not to be confused with the abstract Pattern data type, which uses a regular expression to match a limited set of String values.