diff --git a/Source/Core/InputCommon/ControlReference/ExpressionParser.cpp b/Source/Core/InputCommon/ControlReference/ExpressionParser.cpp
diff --git a/Source/Core/InputCommon/ControlReference/ExpressionParser.h b/Source/Core/InputCommon/ControlReference/ExpressionParser.h
@@ -6,6 +6,7 @@
 
 #include <memory>
 #include <string>
+#include <utility>
 #include "InputCommon/ControllerInterface/Device.h"
 
 namespace ciface
@@ -46,26 +47,24 @@ class ControlFinder
   bool is_input;
 };
 
-class ExpressionNode;
 class Expression
 {
 public:
-  Expression() : node(nullptr) {}
-  Expression(ExpressionNode* node);
-  ~Expression();
-  ControlState GetValue() const;
-  void SetValue(ControlState state);
-  int num_controls;
-  ExpressionNode* node;
+  virtual ~Expression() = default;
+  virtual ControlState GetValue() const = 0;
+  virtual void SetValue(ControlState state) = 0;
+  virtual int CountNumControls() const = 0;
+  virtual void UpdateReferences(ControlFinder& finder) = 0;
+  virtual operator std::string() const = 0;
 };
 
 enum class ParseStatus
 {
   Successful,
   SyntaxError,
-  NoDevice,
+  EmptyExpression,
 };
 
-ParseStatus ParseExpression(const std::string& expr, ControlFinder& finder, Expression** expr_out);
+std::pair<ParseStatus, std::unique_ptr<Expression>> ParseExpression(const std::string& expr);
 }
 }
diff --git a/Source/Core/InputCommon/ControllerEmu/ControlGroup/ControlGroup.cpp b/Source/Core/InputCommon/ControllerEmu/ControlGroup/ControlGroup.cpp
@@ -54,8 +54,12 @@ void ControlGroup::LoadConfig(IniFile::Section* sec, const std::string& defdev,
 
   for (auto& c : controls)
   {
-    // control expression
-    sec->Get(group + c->name, &c->control_ref->expression, "");
+    {
+      // control expression
+      std::string expression;
+      sec->Get(group + c->name, &expression, "");
+      c->control_ref->SetExpression(std::move(expression));
+    }
 
     // range
     sec->Get(group + c->name + "/Range", &c->control_ref->range, 100.0);
@@ -109,7 +113,7 @@ void ControlGroup::SaveConfig(IniFile::Section* sec, const std::string& defdev,
   for (auto& c : controls)
   {
     // control expression
-    sec->Set(group + c->name, c->control_ref->expression, "");
+    sec->Set(group + c->name, c->control_ref->GetExpression(), "");
 
     // range
     sec->Set(group + c->name + "/Range", c->control_ref->range * 100.0, 100.0);
@@ -128,6 +132,6 @@ void ControlGroup::SaveConfig(IniFile::Section* sec, const std::string& defdev,
 
 void ControlGroup::SetControlExpression(int index, const std::string& expression)
 {
-  controls.at(index)->control_ref->expression = expression;
+  controls.at(index)->control_ref->SetExpression(expression);
 }
 }  // namespace ControllerEmu