Implement maintainer feedback

Refactor the code for readability and add a missing test case

lib/expand_variables.rb

@@ -5,27 +5,35 @@ module ExpandVariables
 
   class << self
     def expand(value, variables)
-      variables_hash = nil
-
-      value.gsub(VARIABLES_REGEXP) do
-        variables_hash ||= transform_variables(variables)
-        variables_hash[Regexp.last_match(1) || Regexp.last_match(2)]
+      replace_with(value, variables) do |vars_hash, last_match|
+        match_or_blank_value(vars_hash, last_match)
       end
     end
 
     def expand_existing(value, variables)
+      replace_with(value, variables) do |vars_hash, last_match|
+        match_or_original_value(vars_hash, last_match)
+      end
+    end
+
+    private
+
+    def replace_with(value, variables)
       variables_hash = nil
 
       value.gsub(VARIABLES_REGEXP) do
         variables_hash ||= transform_variables(variables)
-        variables_hash.fetch(
-          Regexp.last_match(1) || Regexp.last_match(2),
-          Regexp.last_match(0)
-        )
+        yield(variables_hash, Regexp.last_match)
       end
     end
 
-    private
+    def match_or_blank_value(variables, last_match)
+      variables[last_match[1] || last_match[2]]
+    end
+
+    def match_or_original_value(variables, last_match)
+      match_or_blank_value(variables, last_match) || last_match[0]
+    end
 
     def transform_variables(variables)
       # Lazily initialise variables

spec/lib/expand_variables_spec.rb

@@ -113,6 +113,13 @@ RSpec.describe ExpandVariables do
               variables: [
                 { key: 'variable', value: 'value' }
               ]
+            },
+            "complex expansions with missing variable for Windows": {
+              value: 'key%variable%%variable2%',
+              result: 'keyvalue',
+              variables: [
+                { key: 'variable', value: 'value' }
+              ]
             }
           }
         end