add Function extname()

README.md

@@ -1368,6 +1368,21 @@ userlist:
 ensure_resources('user', hiera_hash('userlist'), {'ensure' => 'present'})
 ```
 
+#### `stdlib::extname`
+
+Returns the Extension (the Portion of Filename in Path starting from the last Period).
+
+Example usage:
+
+```puppet
+stdlib::extname('test.rb')       => '.rb'
+stdlib::extname('a/b/d/test.rb') => '.rb'
+stdlib::extname('test')          => ''
+stdlib::extname('.profile')      => ''
+```
+
+*Type*: rvalue.
+
 #### `fact`
 
 Return the value of a given fact. Supports the use of dot-notation for referring to structured facts. If a fact requested does not exist, returns Undef.
@@ -1969,12 +1984,12 @@ Arguments: A numeric or a string representing a number.
 
 #### `num2bool`
 
-Converts a number, or a string representation of a number, into a true Boolean. 
+Converts a number, or a string representation of a number, into a true Boolean.
 Zero or anything non-numeric becomes `false`.
 Numbers greater than zero become `true`.
 
 Since Puppet 5.0.0, the same can be achieved with the Puppet type system.
-See the [`Boolean.new`](https://puppet.com/docs/puppet/latest/function.html#conversion-to-boolean) 
+See the [`Boolean.new`](https://puppet.com/docs/puppet/latest/function.html#conversion-to-boolean)
 function in Puppet for the many available type conversions.
 
     Boolean(0) # false
@@ -2236,7 +2251,7 @@ Replaces consecutive repeats (such as 'aaaa') in a string with a single characte
 Converts certain strings to a Boolean. This attempts to convert strings that contain the values '1', 'true', 't', 'y', or 'yes' to `true`. Strings that contain values '0', 'false', 'f', 'n', or 'no', or that are an empty string or undefined are converted to `false`. Any other value causes an error. These checks are case insensitive.
 
 Since Puppet 5.0.0, the same can be achieved with the Puppet type system.
-See the [`Boolean.new`](https://puppet.com/docs/puppet/latest/function.html#conversion-to-boolean) 
+See the [`Boolean.new`](https://puppet.com/docs/puppet/latest/function.html#conversion-to-boolean)
 function in Puppet for the many available type conversions.
 
     Boolean('false'), Boolean('n'), Boolean('no') # all false

lib/puppet/functions/stdlib/extname.rb

@@ -0,0 +1,26 @@
+# Returns the Extension (the Portion of Filename in Path starting from the
+# last Period).
+#
+# If Path is a Dotfile, or starts with a Period, then the starting Dot is not
+# dealt with the Start of the Extension.
+#
+# An empty String will also be returned, when the Period is the last Character
+# in Path.
+
+Puppet::Functions.create_function(:'stdlib::extname') do
+  # @param filename The Filename
+  # @return [String] The Extension starting from the last Period
+  # @example Determining the Extension of a Filename
+  #   stdlib::extname('test.rb')       => '.rb'
+  #   stdlib::extname('a/b/d/test.rb') => '.rb'
+  #   stdlib::extname('test')          => ''
+  #   stdlib::extname('.profile')      => ''
+  dispatch :extname do
+    param 'String', :filename
+    return_type 'String'
+  end
+
+  def extname(filename)
+    File.extname(filename)
+  end
+end

spec/functions/extname_spec.rb

@@ -0,0 +1,16 @@
+require 'spec_helper'
+
+describe 'stdlib::extname' do
+  it { is_expected.not_to eq(nil) }
+  it { is_expected.to run.with_params.and_raise_error(ArgumentError, %r{'stdlib::extname' expects 1 argument, got none}) }
+  it { is_expected.to run.with_params('one', 'two').and_raise_error(ArgumentError, %r{'stdlib::extname' expects 1 argument, got 2}) }
+  it { is_expected.to run.with_params([]).and_raise_error(ArgumentError, %r{'stdlib::extname' parameter 'filename' expects a String value, got Array}) }
+  it { is_expected.to run.with_params('test.rb').and_return('.rb') }
+  it { is_expected.to run.with_params('a/b/d/test.rb').and_return('.rb') }
+  it { is_expected.to run.with_params('test').and_return('') }
+  it { is_expected.to run.with_params('.profile').and_return('') }
+
+  context 'with UTF8 and double byte characters' do
+    it { is_expected.to run.with_params('file_√ạĺűē/竹.rb').and_return('.rb') }
+  end
+end