diff --git a/Source/Core/DolphinQt/Config/Mapping/IOWindow.cpp b/Source/Core/DolphinQt/Config/Mapping/IOWindow.cpp
index 802744d0c0..194a9d00cb 100644
--- a/Source/Core/DolphinQt/Config/Mapping/IOWindow.cpp
+++ b/Source/Core/DolphinQt/Config/Mapping/IOWindow.cpp
@@ -4,6 +4,7 @@
 
 #include "DolphinQt/Config/Mapping/IOWindow.h"
 
+#include <optional>
 #include <thread>
 
 #include <QComboBox>
@@ -11,7 +12,9 @@
 #include <QGroupBox>
 #include <QHBoxLayout>
 #include <QLabel>
+#include <QLineEdit>
 #include <QListWidget>
+#include <QMessageBox>
 #include <QPlainTextEdit>
 #include <QPushButton>
 #include <QSlider>
@@ -25,22 +28,174 @@
 #include "DolphinQt/QtUtils/BlockUserInputFilter.h"
 
 #include "InputCommon/ControlReference/ControlReference.h"
+#include "InputCommon/ControlReference/ExpressionParser.h"
 #include "InputCommon/ControllerEmu/ControllerEmu.h"
 #include "InputCommon/ControllerInterface/ControllerInterface.h"
 
 constexpr int SLIDER_TICK_COUNT = 100;
 
+namespace
+{
+// TODO: Make sure these functions return colors that will be visible in the current theme.
+QTextCharFormat GetSpecialCharFormat()
+{
+  QTextCharFormat format;
+  format.setFontWeight(QFont::Weight::Bold);
+  return format;
+}
+
+QTextCharFormat GetLiteralCharFormat()
+{
+  QTextCharFormat format;
+  format.setForeground(QBrush{Qt::darkMagenta});
+  return format;
+}
+
+QTextCharFormat GetInvalidCharFormat()
+{
+  QTextCharFormat format;
+  format.setUnderlineStyle(QTextCharFormat::WaveUnderline);
+  format.setUnderlineColor(Qt::darkRed);
+  return format;
+}
+
+QTextCharFormat GetControlCharFormat()
+{
+  QTextCharFormat format;
+  format.setForeground(QBrush{Qt::darkGreen});
+  return format;
+}
+
+QTextCharFormat GetVariableCharFormat()
+{
+  QTextCharFormat format;
+  format.setForeground(QBrush{Qt::darkYellow});
+  return format;
+}
+
+QTextCharFormat GetBarewordCharFormat()
+{
+  QTextCharFormat format;
+  format.setForeground(QBrush{Qt::darkBlue});
+  return format;
+}
+
+QTextCharFormat GetCommentCharFormat()
+{
+  QTextCharFormat format;
+  format.setForeground(QBrush{Qt::darkGray});
+  return format;
+}
+}  // namespace
+
+ControlExpressionSyntaxHighlighter::ControlExpressionSyntaxHighlighter(QTextDocument* parent,
+                                                                       QLineEdit* result)
+    : QSyntaxHighlighter(parent), m_result_text(result)
+{
+}
+
+void ControlExpressionSyntaxHighlighter::highlightBlock(const QString&)
+{
+  // TODO: This is going to result in improper highlighting with non-ascii characters:
+  ciface::ExpressionParser::Lexer lexer(document()->toPlainText().toStdString());
+
+  std::vector<ciface::ExpressionParser::Token> tokens;
+  const auto tokenize_status = lexer.Tokenize(tokens);
+
+  using ciface::ExpressionParser::TokenType;
+
+  const auto set_block_format = [this](int start, int count, const QTextCharFormat& format) {
+    if (start + count <= currentBlock().position() ||
+        start >= currentBlock().position() + currentBlock().length())
+    {
+      // This range is not within the current block.
+      return;
+    }
+
+    int block_start = start - currentBlock().position();
+
+    if (block_start < 0)
+    {
+      count += block_start;
+      block_start = 0;
+    }
+
+    setFormat(block_start, count, format);
+  };
+
+  for (auto& token : tokens)
+  {
+    std::optional<QTextCharFormat> char_format;
+
+    switch (token.type)
+    {
+    case TokenType::TOK_INVALID:
+      char_format = GetInvalidCharFormat();
+      break;
+    case TokenType::TOK_LPAREN:
+    case TokenType::TOK_RPAREN:
+    case TokenType::TOK_COMMA:
+      char_format = GetSpecialCharFormat();
+      break;
+    case TokenType::TOK_LITERAL:
+      char_format = GetLiteralCharFormat();
+      break;
+    case TokenType::TOK_CONTROL:
+      char_format = GetControlCharFormat();
+      break;
+    case TokenType::TOK_BAREWORD:
+      char_format = GetBarewordCharFormat();
+      break;
+    case TokenType::TOK_VARIABLE:
+      char_format = GetVariableCharFormat();
+      break;
+    case TokenType::TOK_COMMENT:
+      char_format = GetCommentCharFormat();
+      break;
+    default:
+      if (token.IsBinaryOperator())
+        char_format = GetSpecialCharFormat();
+      break;
+    }
+
+    if (char_format.has_value())
+      set_block_format(int(token.string_position), int(token.string_length), *char_format);
+  }
+
+  // This doesn't need to be run for every "block", but it works.
+  if (ciface::ExpressionParser::ParseStatus::Successful != tokenize_status)
+  {
+    m_result_text->setText(tr("Invalid Token."));
+  }
+  else
+  {
+    ciface::ExpressionParser::RemoveInertTokens(&tokens);
+    const auto parse_status = ciface::ExpressionParser::ParseTokens(tokens);
+
+    m_result_text->setText(
+        QString::fromStdString(parse_status.description.value_or(_trans("Success."))));
+
+    if (ciface::ExpressionParser::ParseStatus::Successful != parse_status.status)
+    {
+      const auto token = *parse_status.token;
+      set_block_format(int(token.string_position), int(token.string_length),
+                       GetInvalidCharFormat());
+    }
+  }
+}
+
 IOWindow::IOWindow(QWidget* parent, ControllerEmu::EmulatedController* controller,
                    ControlReference* ref, IOWindow::Type type)
     : QDialog(parent), m_reference(ref), m_controller(controller), m_type(type)
 {
   CreateMainLayout();
-  ConnectWidgets();
 
   setWindowTitle(type == IOWindow::Type::Input ? tr("Configure Input") : tr("Configure Output"));
   setWindowFlags(windowFlags() & ~Qt::WindowContextHelpButtonHint);
 
   ConfigChanged();
+
+  ConnectWidgets();
 }
 
 void IOWindow::CreateMainLayout()
@@ -51,18 +206,54 @@ void IOWindow::CreateMainLayout()
   m_option_list = new QListWidget();
   m_select_button = new QPushButton(tr("Select"));
   m_detect_button = new QPushButton(tr("Detect"));
-  m_or_button = new QPushButton(tr("| OR"));
-  m_and_button = new QPushButton(tr("&& AND"));
-  m_add_button = new QPushButton(tr("+ ADD"));
-  m_not_button = new QPushButton(tr("! NOT"));
   m_test_button = new QPushButton(tr("Test"));
-  m_expression_text = new QPlainTextEdit();
   m_button_box = new QDialogButtonBox();
   m_clear_button = new QPushButton(tr("Clear"));
   m_apply_button = new QPushButton(tr("Apply"));
   m_range_slider = new QSlider(Qt::Horizontal);
   m_range_spinbox = new QSpinBox();
 
+  m_parse_text = new QLineEdit();
+  m_parse_text->setReadOnly(true);
+
+  m_expression_text = new QPlainTextEdit();
+  m_expression_text->setFont(QFontDatabase::systemFont(QFontDatabase::FixedFont));
+  new ControlExpressionSyntaxHighlighter(m_expression_text->document(), m_parse_text);
+
+  m_operators_combo = new QComboBox();
+  m_operators_combo->addItem(tr("Operators"));
+  m_operators_combo->insertSeparator(1);
+  if (m_type == Type::Input)
+  {
+    m_operators_combo->addItem(tr("! Not"));
+    m_operators_combo->addItem(tr("* Multiply"));
+    m_operators_combo->addItem(tr("/ Divide"));
+    m_operators_combo->addItem(tr("% Modulo"));
+    m_operators_combo->addItem(tr("+ Add"));
+    m_operators_combo->addItem(tr("- Subtract"));
+    m_operators_combo->addItem(tr("> Greater-than"));
+    m_operators_combo->addItem(tr("< Less-than"));
+    m_operators_combo->addItem(tr("& And"));
+  }
+  m_operators_combo->addItem(tr("| Or"));
+  if (m_type == Type::Input)
+  {
+    m_operators_combo->addItem(tr(", Comma"));
+  }
+
+  m_functions_combo = new QComboBox();
+  m_functions_combo->addItem(tr("Functions"));
+  m_functions_combo->insertSeparator(1);
+  m_functions_combo->addItem(QStringLiteral("if"));
+  m_functions_combo->addItem(QStringLiteral("timer"));
+  m_functions_combo->addItem(QStringLiteral("toggle"));
+  m_functions_combo->addItem(QStringLiteral("deadzone"));
+  m_functions_combo->addItem(QStringLiteral("smooth"));
+  m_functions_combo->addItem(QStringLiteral("hold"));
+  m_functions_combo->addItem(QStringLiteral("tap"));
+  m_functions_combo->addItem(QStringLiteral("relative"));
+  m_functions_combo->addItem(QStringLiteral("pulse"));
+
   // Devices
   m_main_layout->addWidget(m_devices_combo);
 
@@ -77,16 +268,6 @@ void IOWindow::CreateMainLayout()
   m_range_spinbox->setMaximum(500);
   m_main_layout->addLayout(range_hbox);
 
-  // Options (Buttons, Outputs) and action buttons
-  // macOS style doesn't support expanding buttons
-#ifndef __APPLE__
-  for (QPushButton* button : {m_select_button, m_detect_button, m_or_button, m_and_button,
-                              m_add_button, m_not_button, m_test_button})
-  {
-    button->setSizePolicy(QSizePolicy::Expanding, QSizePolicy::Expanding);
-  }
-#endif
-
   auto* hbox = new QHBoxLayout();
   auto* button_vbox = new QVBoxLayout();
   hbox->addWidget(m_option_list, 8);
@@ -94,17 +275,15 @@ void IOWindow::CreateMainLayout()
 
   button_vbox->addWidget(m_select_button);
   button_vbox->addWidget(m_type == Type::Input ? m_detect_button : m_test_button);
-  button_vbox->addWidget(m_or_button);
-
+  button_vbox->addWidget(m_operators_combo);
   if (m_type == Type::Input)
   {
-    button_vbox->addWidget(m_and_button);
-    button_vbox->addWidget(m_add_button);
-    button_vbox->addWidget(m_not_button);
+    button_vbox->addWidget(m_functions_combo);
   }
 
   m_main_layout->addLayout(hbox, 2);
   m_main_layout->addWidget(m_expression_text, 1);
+  m_main_layout->addWidget(m_parse_text);
 
   // Button Box
   m_main_layout->addWidget(m_button_box);
@@ -132,11 +311,7 @@ void IOWindow::ConfigChanged()
 
 void IOWindow::ConnectWidgets()
 {
-  connect(m_select_button, &QPushButton::clicked, [this] { AppendSelectedOption(""); });
-  connect(m_add_button, &QPushButton::clicked, [this] { AppendSelectedOption(" + "); });
-  connect(m_and_button, &QPushButton::clicked, [this] { AppendSelectedOption(" & "); });
-  connect(m_or_button, &QPushButton::clicked, [this] { AppendSelectedOption(" | "); });
-  connect(m_not_button, &QPushButton::clicked, [this] { AppendSelectedOption("!"); });
+  connect(m_select_button, &QPushButton::clicked, [this] { AppendSelectedOption(); });
 
   connect(m_detect_button, &QPushButton::clicked, this, &IOWindow::OnDetectButtonPressed);
   connect(m_test_button, &QPushButton::clicked, this, &IOWindow::OnTestButtonPressed);
@@ -147,17 +322,38 @@ void IOWindow::ConnectWidgets()
           this, &IOWindow::OnRangeChanged);
   connect(m_range_slider, static_cast<void (QSlider::*)(int value)>(&QSlider::valueChanged), this,
           &IOWindow::OnRangeChanged);
+
+  connect(m_expression_text, &QPlainTextEdit::textChanged, [this] {
+    m_apply_button->setText(m_apply_button->text().remove(QStringLiteral("*")));
+    m_apply_button->setText(m_apply_button->text() + QStringLiteral("*"));
+  });
+
+  connect(m_operators_combo, QOverload<int>::of(&QComboBox::activated), [this](int index) {
+    if (0 == index)
+      return;
+
+    m_expression_text->insertPlainText(m_operators_combo->currentText().left(1));
+
+    m_operators_combo->setCurrentIndex(0);
+  });
+
+  connect(m_functions_combo, QOverload<int>::of(&QComboBox::activated), [this](int index) {
+    if (0 == index)
+      return;
+
+    m_expression_text->insertPlainText(m_functions_combo->currentText() + QStringLiteral("()"));
+
+    m_functions_combo->setCurrentIndex(0);
+  });
 }
 
-void IOWindow::AppendSelectedOption(const std::string& prefix)
+void IOWindow::AppendSelectedOption()
 {
   if (m_option_list->currentItem() == nullptr)
     return;
 
-  m_expression_text->insertPlainText(
-      QString::fromStdString(prefix) +
-      MappingCommon::GetExpressionForControl(m_option_list->currentItem()->text(), m_devq,
-                                             m_controller->GetDefaultDevice()));
+  m_expression_text->insertPlainText(MappingCommon::GetExpressionForControl(
+      m_option_list->currentItem()->text(), m_devq, m_controller->GetDefaultDevice()));
 }
 
 void IOWindow::OnDeviceChanged(const QString& device)
@@ -175,7 +371,19 @@ void IOWindow::OnDialogButtonPressed(QAbstractButton* button)
   }
 
   m_reference->SetExpression(m_expression_text->toPlainText().toStdString());
-  m_controller->UpdateReferences(g_controller_interface);
+  m_controller->UpdateSingleControlReference(g_controller_interface, m_reference);
+
+  m_apply_button->setText(m_apply_button->text().remove(QStringLiteral("*")));
+
+  if (ciface::ExpressionParser::ParseStatus::SyntaxError == m_reference->GetParseStatus())
+  {
+    QMessageBox error(this);
+    error.setIcon(QMessageBox::Critical);
+    error.setWindowTitle(tr("Error"));
+    error.setText(tr("The expression contains a syntax error."));
+    error.setWindowModality(Qt::WindowModal);
+    error.exec();
+  }
 
   if (button != m_apply_button)
     accept();
diff --git a/Source/Core/DolphinQt/Config/Mapping/IOWindow.h b/Source/Core/DolphinQt/Config/Mapping/IOWindow.h
index 5921b14424..1d8a3f4f2f 100644
--- a/Source/Core/DolphinQt/Config/Mapping/IOWindow.h
+++ b/Source/Core/DolphinQt/Config/Mapping/IOWindow.h
@@ -6,6 +6,7 @@
 
 #include <QDialog>
 #include <QString>
+#include <QSyntaxHighlighter>
 
 #include "Common/Flag.h"
 #include "InputCommon/ControllerInterface/Device.h"
@@ -14,6 +15,7 @@ class ControlReference;
 class QAbstractButton;
 class QComboBox;
 class QDialogButtonBox;
+class QLineEdit;
 class QListWidget;
 class QVBoxLayout;
 class QWidget;
@@ -27,6 +29,19 @@ namespace ControllerEmu
 class EmulatedController;
 }
 
+class ControlExpressionSyntaxHighlighter final : public QSyntaxHighlighter
+{
+  Q_OBJECT
+public:
+  ControlExpressionSyntaxHighlighter(QTextDocument* parent, QLineEdit* result);
+
+protected:
+  void highlightBlock(const QString& text) final override;
+
+private:
+  QLineEdit* const m_result_text;
+};
+
 class IOWindow final : public QDialog
 {
   Q_OBJECT
@@ -51,7 +66,7 @@ private:
   void OnTestButtonPressed();
   void OnRangeChanged(int range);
 
-  void AppendSelectedOption(const std::string& prefix);
+  void AppendSelectedOption();
   void UpdateOptionList();
   void UpdateDeviceList();
 
@@ -70,19 +85,18 @@ private:
 
   // Shared actions
   QPushButton* m_select_button;
-  QPushButton* m_or_button;
+  QComboBox* m_operators_combo;
 
   // Input actions
   QPushButton* m_detect_button;
-  QPushButton* m_and_button;
-  QPushButton* m_not_button;
-  QPushButton* m_add_button;
+  QComboBox* m_functions_combo;
 
   // Output actions
   QPushButton* m_test_button;
 
   // Textarea
   QPlainTextEdit* m_expression_text;
+  QLineEdit* m_parse_text;
 
   // Buttonbox
   QDialogButtonBox* m_button_box;
diff --git a/Source/Core/DolphinQt/Config/Mapping/MappingButton.cpp b/Source/Core/DolphinQt/Config/Mapping/MappingButton.cpp
index 435033c8c1..a9a142276f 100644
--- a/Source/Core/DolphinQt/Config/Mapping/MappingButton.cpp
+++ b/Source/Core/DolphinQt/Config/Mapping/MappingButton.cpp
@@ -100,7 +100,7 @@ void MappingButton::Detect()
     return;
 
   m_reference->SetExpression(expression.toStdString());
-  m_parent->GetController()->UpdateReferences(g_controller_interface);
+  m_parent->GetController()->UpdateSingleControlReference(g_controller_interface, m_reference);
 
   ConfigChanged();
   m_parent->SaveSettings();
@@ -111,7 +111,7 @@ void MappingButton::Clear()
   m_reference->range = 100.0 / SLIDER_TICK_COUNT;
 
   m_reference->SetExpression("");
-  m_parent->GetController()->UpdateReferences(g_controller_interface);
+  m_parent->GetController()->UpdateSingleControlReference(g_controller_interface, m_reference);
 
   m_parent->SaveSettings();
   ConfigChanged();
diff --git a/Source/Core/InputCommon/CMakeLists.txt b/Source/Core/InputCommon/CMakeLists.txt
index df8fa6423c..b47503a9ae 100644
--- a/Source/Core/InputCommon/CMakeLists.txt
+++ b/Source/Core/InputCommon/CMakeLists.txt
@@ -45,6 +45,8 @@ add_library(inputcommon
   ControlReference/ControlReference.h
   ControlReference/ExpressionParser.cpp
   ControlReference/ExpressionParser.h
+  ControlReference/FunctionExpression.cpp
+  ControlReference/FunctionExpression.h
 )
 
 target_link_libraries(inputcommon PUBLIC
diff --git a/Source/Core/InputCommon/ControlReference/ControlReference.cpp b/Source/Core/InputCommon/ControlReference/ControlReference.cpp
index cfc6fa658e..af7536547c 100644
--- a/Source/Core/InputCommon/ControlReference/ControlReference.cpp
+++ b/Source/Core/InputCommon/ControlReference/ControlReference.cpp
@@ -25,12 +25,12 @@ bool ControlReference::InputGateOn()
 // Updates a controlreference's binded devices/controls
 // need to call this to re-bind a control reference after changing its expression
 //
-void ControlReference::UpdateReference(const ciface::Core::DeviceContainer& devices,
-                                       const ciface::Core::DeviceQualifier& default_device)
+void ControlReference::UpdateReference(ciface::ExpressionParser::ControlEnvironment& env)
 {
-  ControlFinder finder(devices, default_device, IsInput());
   if (m_parsed_expression)
-    m_parsed_expression->UpdateReferences(finder);
+  {
+    m_parsed_expression->UpdateReferences(env);
+  }
 }
 
 int ControlReference::BoundCount() const
@@ -54,7 +54,9 @@ std::string ControlReference::GetExpression() const
 void ControlReference::SetExpression(std::string expr)
 {
   m_expression = std::move(expr);
-  std::tie(m_parse_status, m_parsed_expression) = ParseExpression(m_expression);
+  auto parse_result = ParseExpression(m_expression);
+  m_parse_status = parse_result.status;
+  m_parsed_expression = std::move(parse_result.expr);
 }
 
 ControlReference::ControlReference() : range(1), m_parsed_expression(nullptr)
diff --git a/Source/Core/InputCommon/ControlReference/ControlReference.h b/Source/Core/InputCommon/ControlReference/ControlReference.h
index 83fa288676..3f2c205501 100644
--- a/Source/Core/InputCommon/ControlReference/ControlReference.h
+++ b/Source/Core/InputCommon/ControlReference/ControlReference.h
@@ -30,8 +30,7 @@ public:
 
   int BoundCount() const;
   ciface::ExpressionParser::ParseStatus GetParseStatus() const;
-  void UpdateReference(const ciface::Core::DeviceContainer& devices,
-                       const ciface::Core::DeviceQualifier& default_device);
+  void UpdateReference(ciface::ExpressionParser::ControlEnvironment& env);
   std::string GetExpression() const;
   void SetExpression(std::string expr);
 
diff --git a/Source/Core/InputCommon/ControlReference/ExpressionParser.cpp b/Source/Core/InputCommon/ControlReference/ExpressionParser.cpp
index c864b22a20..00afc5e036 100644
--- a/Source/Core/InputCommon/ControlReference/ExpressionParser.cpp
+++ b/Source/Core/InputCommon/ControlReference/ExpressionParser.cpp
@@ -2,222 +2,201 @@
 // Licensed under GPLv2+
 // Refer to the license.txt file included.
 
-#include <algorithm>
 #include <cassert>
+#include <cmath>
 #include <iostream>
-#include <map>
 #include <memory>
+#include <regex>
 #include <string>
+#include <utility>
 #include <vector>
 
+#include "Common/Common.h"
 #include "Common/StringUtil.h"
+
 #include "InputCommon/ControlReference/ExpressionParser.h"
+#include "InputCommon/ControlReference/FunctionExpression.h"
 
 namespace ciface::ExpressionParser
 {
 using namespace ciface::Core;
 
-enum TokenType
+Token::Token(TokenType type_) : type(type_)
 {
-  TOK_DISCARD,
-  TOK_INVALID,
-  TOK_EOF,
-  TOK_LPAREN,
-  TOK_RPAREN,
-  TOK_AND,
-  TOK_OR,
-  TOK_NOT,
-  TOK_ADD,
-  TOK_CONTROL,
-};
+}
 
-inline std::string OpName(TokenType op)
+Token::Token(TokenType type_, std::string data_) : type(type_), data(std::move(data_))
 {
-  switch (op)
+}
+
+bool Token::IsBinaryOperator() const
+{
+  return type >= TOK_BINARY_OPS_BEGIN && type < TOK_BINARY_OPS_END;
+}
+
+Lexer::Lexer(std::string expr_) : expr(std::move(expr_))
+{
+  it = expr.begin();
+}
+
+std::string Lexer::FetchDelimString(char delim)
+{
+  const std::string result = FetchCharsWhile([delim](char c) { return c != delim; });
+  if (it != expr.end())
+    ++it;
+  return result;
+}
+
+std::string Lexer::FetchWordChars()
+{
+  // Valid word characters:
+  std::regex rx(R"([a-z\d_])", std::regex_constants::icase);
+
+  return FetchCharsWhile([&rx](char c) { return std::regex_match(std::string(1, c), rx); });
+}
+
+Token Lexer::GetDelimitedLiteral()
+{
+  return Token(TOK_LITERAL, FetchDelimString('\''));
+}
+
+Token Lexer::GetVariable()
+{
+  return Token(TOK_VARIABLE, FetchWordChars());
+}
+
+Token Lexer::GetFullyQualifiedControl()
+{
+  return Token(TOK_CONTROL, FetchDelimString('`'));
+}
+
+Token Lexer::GetBareword(char first_char)
+{
+  return Token(TOK_BAREWORD, first_char + FetchWordChars());
+}
+
+Token Lexer::GetRealLiteral(char first_char)
+{
+  std::string value;
+  value += first_char;
+  value += FetchCharsWhile([](char c) { return isdigit(c, std::locale::classic()) || ('.' == c); });
+
+  if (std::regex_match(value, std::regex(R"(\d+(\.\d+)?)")))
+    return Token(TOK_LITERAL, value);
+
+  return Token(TOK_INVALID);
+}
+
+Token Lexer::PeekToken()
+{
+  const auto old_it = it;
+  const auto tok = NextToken();
+  it = old_it;
+  return tok;
+}
+
+Token Lexer::NextToken()
+{
+  if (it == expr.end())
+    return Token(TOK_EOF);
+
+  char c = *it++;
+  switch (c)
   {
-  case TOK_AND:
-    return "And";
-  case TOK_OR:
-    return "Or";
-  case TOK_NOT:
-    return "Not";
-  case TOK_ADD:
-    return "Add";
+  case ' ':
+  case '\t':
+  case '\n':
+  case '\r':
+    return Token(TOK_WHITESPACE);
+  case '(':
+    return Token(TOK_LPAREN);
+  case ')':
+    return Token(TOK_RPAREN);
+  case '&':
+    return Token(TOK_AND);
+  case '|':
+    return Token(TOK_OR);
+  case '!':
+    return Token(TOK_NOT);
+  case '+':
+    return Token(TOK_ADD);
+  case '-':
+    return Token(TOK_SUB);
+  case '*':
+    return Token(TOK_MUL);
+  case '/':
+    return Token(TOK_DIV);
+  case '%':
+    return Token(TOK_MOD);
+  case '=':
+    return Token(TOK_ASSIGN);
+  case '<':
+    return Token(TOK_LTHAN);
+  case '>':
+    return Token(TOK_GTHAN);
+  case ',':
+    return Token(TOK_COMMA);
+  case '\'':
+    return GetDelimitedLiteral();
+  case '$':
+    return GetVariable();
+  case '`':
+    return GetFullyQualifiedControl();
   default:
-    assert(false);
-    return "";
+    if (isalpha(c, std::locale::classic()))
+      return GetBareword(c);
+    else if (isdigit(c, std::locale::classic()))
+      return GetRealLiteral(c);
+    else
+      return Token(TOK_INVALID);
   }
 }
 
-class Token
+ParseStatus Lexer::Tokenize(std::vector<Token>& tokens)
 {
-public:
-  TokenType type;
-  ControlQualifier qualifier;
-
-  Token(TokenType type_) : type(type_) {}
-  Token(TokenType type_, ControlQualifier qualifier_) : type(type_), qualifier(qualifier_) {}
-  operator std::string() const
+  while (true)
   {
-    switch (type)
+    const std::size_t string_position = it - expr.begin();
+    Token tok = NextToken();
+
+    tok.string_position = string_position;
+    tok.string_length = it - expr.begin();
+
+    // Handle /* */ style comments.
+    if (tok.type == TOK_DIV && PeekToken().type == TOK_MUL)
     {
-    case TOK_DISCARD:
-      return "Discard";
-    case TOK_EOF:
-      return "EOF";
-    case TOK_LPAREN:
-      return "(";
-    case TOK_RPAREN:
-      return ")";
-    case TOK_AND:
-      return "&";
-    case TOK_OR:
-      return "|";
-    case TOK_NOT:
-      return "!";
-    case TOK_ADD:
-      return "+";
-    case TOK_CONTROL:
-      return "Device(" + (std::string)qualifier + ")";
-    case TOK_INVALID:
-      break;
-    }
+      const auto end_of_comment = expr.find("*/", it - expr.begin());
 
-    return "Invalid";
-  }
-};
-
-class Lexer
-{
-public:
-  std::string expr;
-  std::string::iterator it;
-
-  Lexer(const std::string& expr_) : expr(expr_) { it = expr.begin(); }
-  bool FetchBacktickString(std::string& value, char otherDelim = 0)
-  {
-    value = "";
-    while (it != expr.end())
-    {
-      char c = *it;
-      ++it;
-      if (c == '`')
-        return false;
-      if (c > 0 && c == otherDelim)
-        return true;
-      value += c;
-    }
-    return false;
-  }
-
-  Token GetFullyQualifiedControl()
-  {
-    ControlQualifier qualifier;
-    std::string value;
-
-    if (FetchBacktickString(value, ':'))
-    {
-      // Found colon, this is the device name
-      qualifier.has_device = true;
-      qualifier.device_qualifier.FromString(value);
-      FetchBacktickString(value);
-    }
-
-    qualifier.control_name = value;
-
-    return Token(TOK_CONTROL, qualifier);
-  }
-
-  Token GetBarewordsControl(char c)
-  {
-    std::string name;
-    name += c;
-
-    while (it != expr.end())
-    {
-      c = *it;
-      if (!isalpha(c))
-        break;
-      name += c;
-      ++it;
-    }
-
-    ControlQualifier qualifier;
-    qualifier.control_name = name;
-    return Token(TOK_CONTROL, qualifier);
-  }
-
-  Token NextToken()
-  {
-    if (it == expr.end())
-      return Token(TOK_EOF);
-
-    char c = *it++;
-    switch (c)
-    {
-    case ' ':
-    case '\t':
-    case '\n':
-    case '\r':
-      return Token(TOK_DISCARD);
-    case '(':
-      return Token(TOK_LPAREN);
-    case ')':
-      return Token(TOK_RPAREN);
-    case '&':
-      return Token(TOK_AND);
-    case '|':
-      return Token(TOK_OR);
-    case '!':
-      return Token(TOK_NOT);
-    case '+':
-      return Token(TOK_ADD);
-    case '`':
-      return GetFullyQualifiedControl();
-    default:
-      if (isalpha(c))
-        return GetBarewordsControl(c);
-      else
-        return Token(TOK_INVALID);
-    }
-  }
-
-  ParseStatus Tokenize(std::vector<Token>& tokens)
-  {
-    while (true)
-    {
-      Token tok = NextToken();
-
-      if (tok.type == TOK_DISCARD)
-        continue;
-
-      if (tok.type == TOK_INVALID)
-      {
-        tokens.clear();
+      if (end_of_comment == std::string::npos)
         return ParseStatus::SyntaxError;
-      }
 
-      tokens.push_back(tok);
+      tok.type = TOK_COMMENT;
+      tok.string_length = end_of_comment + 4;
 
-      if (tok.type == TOK_EOF)
-        break;
+      it = expr.begin() + end_of_comment + 2;
     }
-    return ParseStatus::Successful;
+
+    tokens.push_back(tok);
+
+    if (tok.type == TOK_INVALID)
+      return ParseStatus::SyntaxError;
+
+    if (tok.type == TOK_EOF)
+      break;
   }
-};
+  return ParseStatus::Successful;
+}
 
 class ControlExpression : public Expression
 {
 public:
-  ControlQualifier qualifier;
-  Device::Control* control = nullptr;
-  // Keep a shared_ptr to the device so the control pointer doesn't become invalid
+  // Keep a shared_ptr to the device so the control pointer doesn't become invalid.
   std::shared_ptr<Device> m_device;
 
   explicit ControlExpression(ControlQualifier qualifier_) : qualifier(qualifier_) {}
   ControlState GetValue() const override
   {
-    if (!control)
+    if (!input)
       return 0.0;
 
     // Note: Inputs may return negative values in situations where opposing directions are
@@ -226,20 +205,25 @@ public:
     // FYI: Clamping values greater than 1.0 is purposely not done to support unbounded values in
     // the future. (e.g. raw accelerometer/gyro data)
 
-    return std::max(0.0, control->ToInput()->GetState());
+    return std::max(0.0, input->GetState());
   }
   void SetValue(ControlState value) override
   {
-    if (control)
-      control->ToOutput()->SetState(value);
+    if (output)
+      output->SetState(value);
   }
-  int CountNumControls() const override { return control ? 1 : 0; }
-  void UpdateReferences(ControlFinder& finder) override
+  int CountNumControls() const override { return (input || output) ? 1 : 0; }
+  void UpdateReferences(ControlEnvironment& env) override
   {
-    m_device = finder.FindDevice(qualifier);
-    control = finder.FindControl(qualifier);
+    m_device = env.FindDevice(qualifier);
+    input = env.FindInput(qualifier);
+    output = env.FindOutput(qualifier);
   }
-  operator std::string() const override { return "`" + static_cast<std::string>(qualifier) + "`"; }
+
+private:
+  ControlQualifier qualifier;
+  Device::Input* input = nullptr;
+  Device::Output* output = nullptr;
 };
 
 class BinaryExpression : public Expression
@@ -257,16 +241,43 @@ public:
 
   ControlState GetValue() const override
   {
-    ControlState lhsValue = lhs->GetValue();
-    ControlState rhsValue = rhs->GetValue();
     switch (op)
     {
     case TOK_AND:
-      return std::min(lhsValue, rhsValue);
+      return std::min(lhs->GetValue(), rhs->GetValue());
     case TOK_OR:
-      return std::max(lhsValue, rhsValue);
+      return std::max(lhs->GetValue(), rhs->GetValue());
     case TOK_ADD:
-      return std::min(lhsValue + rhsValue, 1.0);
+      return lhs->GetValue() + rhs->GetValue();
+    case TOK_SUB:
+      return lhs->GetValue() - rhs->GetValue();
+    case TOK_MUL:
+      return lhs->GetValue() * rhs->GetValue();
+    case TOK_DIV:
+    {
+      const ControlState result = lhs->GetValue() / rhs->GetValue();
+      return std::isinf(result) ? 0.0 : result;
+    }
+    case TOK_MOD:
+    {
+      const ControlState result = std::fmod(lhs->GetValue(), rhs->GetValue());
+      return std::isnan(result) ? 0.0 : result;
+    }
+    case TOK_ASSIGN:
+    {
+      lhs->SetValue(rhs->GetValue());
+      return lhs->GetValue();
+    }
+    case TOK_LTHAN:
+      return lhs->GetValue() < rhs->GetValue();
+    case TOK_GTHAN:
+      return lhs->GetValue() > rhs->GetValue();
+    case TOK_COMMA:
+    {
+      // Eval and discard lhs:
+      lhs->GetValue();
+      return rhs->GetValue();
+    }
     default:
       assert(false);
       return 0;
@@ -286,57 +297,73 @@ public:
     return lhs->CountNumControls() + rhs->CountNumControls();
   }
 
-  void UpdateReferences(ControlFinder& finder) override
+  void UpdateReferences(ControlEnvironment& env) override
   {
-    lhs->UpdateReferences(finder);
-    rhs->UpdateReferences(finder);
-  }
-
-  operator std::string() const override
-  {
-    return OpName(op) + "(" + (std::string)(*lhs) + ", " + (std::string)(*rhs) + ")";
+    lhs->UpdateReferences(env);
+    rhs->UpdateReferences(env);
   }
 };
 
-class UnaryExpression : public Expression
+class LiteralExpression : public Expression
 {
 public:
-  TokenType op;
-  std::unique_ptr<Expression> inner;
-
-  UnaryExpression(TokenType op_, std::unique_ptr<Expression>&& inner_)
-      : op(op_), inner(std::move(inner_))
+  void SetValue(ControlState) override
   {
-  }
-  ControlState GetValue() const override
-  {
-    ControlState value = inner->GetValue();
-    switch (op)
-    {
-    case TOK_NOT:
-      return 1.0 - value;
-    default:
-      assert(false);
-      return 0;
-    }
+    // Do nothing.
   }
 
-  void SetValue(ControlState value) override
-  {
-    switch (op)
-    {
-    case TOK_NOT:
-      inner->SetValue(1.0 - value);
-      break;
+  int CountNumControls() const override { return 1; }
 
-    default:
-      assert(false);
-    }
+  void UpdateReferences(ControlEnvironment&) override
+  {
+    // Nothing needed.
   }
 
-  int CountNumControls() const override { return inner->CountNumControls(); }
-  void UpdateReferences(ControlFinder& finder) override { inner->UpdateReferences(finder); }
-  operator std::string() const override { return OpName(op) + "(" + (std::string)(*inner) + ")"; }
+protected:
+  virtual std::string GetName() const = 0;
+};
+
+class LiteralReal : public LiteralExpression
+{
+public:
+  LiteralReal(ControlState value) : m_value(value) {}
+
+  ControlState GetValue() const override { return m_value; }
+
+  std::string GetName() const override { return ValueToString(m_value); }
+
+private:
+  const ControlState m_value{};
+};
+
+ParseResult MakeLiteralExpression(Token token)
+{
+  ControlState val{};
+  if (TryParse(token.data, &val))
+    return ParseResult::MakeSuccessfulResult(std::make_unique<LiteralReal>(val));
+  else
+    return ParseResult::MakeErrorResult(token, _trans("Invalid literal."));
+}
+
+class VariableExpression : public Expression
+{
+public:
+  VariableExpression(std::string name) : m_name(name) {}
+
+  ControlState GetValue() const override { return *m_value_ptr; }
+
+  void SetValue(ControlState value) override { *m_value_ptr = value; }
+
+  int CountNumControls() const override { return 1; }
+
+  void UpdateReferences(ControlEnvironment& env) override
+  {
+    m_value_ptr = env.GetVariablePtr(m_name);
+  }
+
+protected:
+  const std::string m_name;
+  ControlState* m_value_ptr{};
 };
 
 // This class proxies all methods to its either left-hand child if it has bound controls, or its
@@ -353,16 +380,10 @@ public:
   void SetValue(ControlState value) override { GetActiveChild()->SetValue(value); }
 
   int CountNumControls() const override { return GetActiveChild()->CountNumControls(); }
-  operator std::string() const override
+  void UpdateReferences(ControlEnvironment& env) override
   {
-    return "Coalesce(" + static_cast<std::string>(*m_lhs) + ", " +
-           static_cast<std::string>(*m_rhs) + ')';
-  }
-
-  void UpdateReferences(ControlFinder& finder) override
-  {
-    m_lhs->UpdateReferences(finder);
-    m_rhs->UpdateReferences(finder);
+    m_lhs->UpdateReferences(env);
+    m_rhs->UpdateReferences(env);
   }
 
 private:
@@ -375,7 +396,7 @@ private:
   std::unique_ptr<Expression> m_rhs;
 };
 
-std::shared_ptr<Device> ControlFinder::FindDevice(ControlQualifier qualifier) const
+std::shared_ptr<Device> ControlEnvironment::FindDevice(ControlQualifier qualifier) const
 {
   if (qualifier.has_device)
     return container.FindDevice(qualifier.device_qualifier);
@@ -383,150 +404,303 @@ std::shared_ptr<Device> ControlFinder::FindDevice(ControlQualifier qualifier) co
     return container.FindDevice(default_device);
 }
 
-Device::Control* ControlFinder::FindControl(ControlQualifier qualifier) const
+Device::Input* ControlEnvironment::FindInput(ControlQualifier qualifier) const
 {
   const std::shared_ptr<Device> device = FindDevice(qualifier);
   if (!device)
     return nullptr;
 
-  if (is_input)
-    return device->FindInput(qualifier.control_name);
-  else
-    return device->FindOutput(qualifier.control_name);
+  return device->FindInput(qualifier.control_name);
 }
 
-struct ParseResult
+Device::Output* ControlEnvironment::FindOutput(ControlQualifier qualifier) const
 {
-  ParseResult(ParseStatus status_, std::unique_ptr<Expression>&& expr_ = {})
-      : status(status_), expr(std::move(expr_))
-  {
-  }
+  const std::shared_ptr<Device> device = FindDevice(qualifier);
+  if (!device)
+    return nullptr;
 
-  ParseStatus status;
-  std::unique_ptr<Expression> expr;
-};
+  return device->FindOutput(qualifier.control_name);
+}
+
+ControlState* ControlEnvironment::GetVariablePtr(const std::string& name)
+{
+  return &m_variables[name];
+}
+
+ParseResult ParseResult::MakeEmptyResult()
+{
+  ParseResult result;
+  result.status = ParseStatus::EmptyExpression;
+  return result;
+}
+
+ParseResult ParseResult::MakeSuccessfulResult(std::unique_ptr<Expression>&& expr)
+{
+  ParseResult result;
+  result.status = ParseStatus::Successful;
+  result.expr = std::move(expr);
+  return result;
+}
+
+ParseResult ParseResult::MakeErrorResult(Token token, std::string description)
+{
+  ParseResult result;
+  result.status = ParseStatus::SyntaxError;
+  result.token = std::move(token);
+  result.description = std::move(description);
+  return result;
+}
 
 class Parser
 {
 public:
-  explicit Parser(std::vector<Token> tokens_) : tokens(tokens_) { m_it = tokens.begin(); }
-  ParseResult Parse() { return Toplevel(); }
+  explicit Parser(const std::vector<Token>& tokens_) : tokens(tokens_) { m_it = tokens.begin(); }
+  ParseResult Parse()
+  {
+    ParseResult result = ParseToplevel();
+
+    if (ParseStatus::Successful != result.status)
+      return result;
+
+    if (Peek().type == TOK_EOF)
+      return result;
+
+    return ParseResult::MakeErrorResult(Peek(), _trans("Expected EOF."));
+  }
 
 private:
-  std::vector<Token> tokens;
-  std::vector<Token>::iterator m_it;
+  const std::vector<Token>& tokens;
+  std::vector<Token>::const_iterator m_it;
+
+  Token Chew()
+  {
+    const Token tok = Peek();
+    if (TOK_EOF != tok.type)
+      ++m_it;
+    return tok;
+  }
 
-  Token Chew() { return *m_it++; }
   Token Peek() { return *m_it; }
+
   bool Expects(TokenType type)
   {
     Token tok = Chew();
     return tok.type == type;
   }
 
-  ParseResult Atom()
+  ParseResult ParseFunctionArguments(const std::string_view& func_name,
+                                     std::unique_ptr<FunctionExpression>&& func,
+                                     const Token& func_tok)
+  {
+    std::vector<std::unique_ptr<Expression>> args;
+
+    if (TOK_LPAREN != Peek().type)
+    {
+      // Single argument with no parens (useful for unary ! function)
+      const auto tok = Chew();
+      auto arg = ParseAtom(tok);
+      if (ParseStatus::Successful != arg.status)
+        return arg;
+
+      args.emplace_back(std::move(arg.expr));
+    }
+    else
+    {
+      // Chew the L-Paren
+      Chew();
+
+      // Check for empty argument list:
+      if (TOK_RPAREN == Peek().type)
+      {
+        Chew();
+      }
+      else
+      {
+        while (true)
+        {
+          // Read one argument.
+          // Grab an expression, but stop at comma.
+          auto arg = ParseBinary(BinaryOperatorPrecedence(TOK_COMMA));
+          if (ParseStatus::Successful != arg.status)
+            return arg;
+
+          args.emplace_back(std::move(arg.expr));
+
+          // Right paren is the end of our arguments.
+          const Token tok = Chew();
+          if (TOK_RPAREN == tok.type)
+            break;
+
+          // Comma before the next argument.
+          if (TOK_COMMA != tok.type)
+            return ParseResult::MakeErrorResult(tok, _trans("Expected comma."));
+        };
+      }
+    }
+
+    const auto argument_validation = func->SetArguments(std::move(args));
+
+    if (std::holds_alternative<FunctionExpression::ExpectedArguments>(argument_validation))
+    {
+      const auto text = std::string(func_name) + '(' +
+                        std::get<FunctionExpression::ExpectedArguments>(argument_validation).text +
+                        ')';
+
+      return ParseResult::MakeErrorResult(func_tok, _trans("Expected arguments: " + text));
+    }
+
+    return ParseResult::MakeSuccessfulResult(std::move(func));
+  }
+
+  ParseResult ParseAtom(const Token& tok)
   {
-    Token tok = Chew();
     switch (tok.type)
     {
-    case TOK_CONTROL:
-      return {ParseStatus::Successful, std::make_unique<ControlExpression>(tok.qualifier)};
-    case TOK_LPAREN:
-      return Paren();
-    default:
-      return {ParseStatus::SyntaxError};
-    }
-  }
-
-  bool IsUnaryExpression(TokenType type)
-  {
-    switch (type)
+    case TOK_BAREWORD:
     {
-    case TOK_NOT:
-      return true;
-    default:
-      return false;
-    }
-  }
+      auto func = MakeFunctionExpression(tok.data);
 
-  ParseResult Unary()
-  {
-    if (IsUnaryExpression(Peek().type))
-    {
-      Token tok = Chew();
-      ParseResult result = Atom();
-      if (result.status == ParseStatus::SyntaxError)
-        return result;
-      return {ParseStatus::Successful,
-              std::make_unique<UnaryExpression>(tok.type, std::move(result.expr))};
-    }
-
-    return Atom();
-  }
-
-  bool IsBinaryToken(TokenType type)
-  {
-    switch (type)
-    {
-    case TOK_AND:
-    case TOK_OR:
-    case TOK_ADD:
-      return true;
-    default:
-      return false;
-    }
-  }
-
-  ParseResult Binary()
-  {
-    ParseResult result = Unary();
-    if (result.status == ParseStatus::SyntaxError)
-      return result;
-
-    std::unique_ptr<Expression> expr = std::move(result.expr);
-    while (IsBinaryToken(Peek().type))
-    {
-      Token tok = Chew();
-      ParseResult unary_result = Unary();
-      if (unary_result.status == ParseStatus::SyntaxError)
+      if (!func)
       {
-        return unary_result;
+        // Invalid function, interpret this as a bareword control.
+        Token control_tok(tok);
+        control_tok.type = TOK_CONTROL;
+        return ParseAtom(control_tok);
       }
 
-      expr = std::make_unique<BinaryExpression>(tok.type, std::move(expr),
-                                                std::move(unary_result.expr));
+      return ParseFunctionArguments(tok.data, std::move(func), tok);
+    }
+    case TOK_CONTROL:
+    {
+      ControlQualifier cq;
+      cq.FromString(tok.data);
+      return ParseResult::MakeSuccessfulResult(std::make_unique<ControlExpression>(cq));
+    }
+    case TOK_NOT:
+    {
+      return ParseFunctionArguments("not", MakeFunctionExpression("not"), tok);
+    }
+    case TOK_LITERAL:
+    {
+      return MakeLiteralExpression(tok);
+    }
+    case TOK_VARIABLE:
+    {
+      return ParseResult::MakeSuccessfulResult(std::make_unique<VariableExpression>(tok.data));
+    }
+    case TOK_LPAREN:
+    {
+      return ParseParens();
+    }
+    case TOK_SUB:
+    {
+      // An atom was expected but we got a subtraction symbol.
+      // Interpret it as a unary minus function.
+      return ParseFunctionArguments("minus", MakeFunctionExpression("minus"), tok);
+    }
+    default:
+    {
+      return ParseResult::MakeErrorResult(tok, _trans("Expected start of expression."));
+    }
     }
-
-    return {ParseStatus::Successful, std::move(expr)};
   }
 
-  ParseResult Paren()
+  static int BinaryOperatorPrecedence(TokenType type)
+  {
+    switch (type)
+    {
+    case TOK_MUL:
+    case TOK_DIV:
+    case TOK_MOD:
+      return 1;
+    case TOK_ADD:
+    case TOK_SUB:
+      return 2;
+    case TOK_GTHAN:
+    case TOK_LTHAN:
+      return 3;
+    case TOK_AND:
+      return 4;
+    case TOK_OR:
+      return 5;
+    case TOK_ASSIGN:
+      return 6;
+    case TOK_COMMA:
+      return 7;
+    default:
+      assert(false);
+      return 0;
+    }
+  }
+
+  ParseResult ParseBinary(int precedence = 999)
+  {
+    ParseResult lhs = ParseAtom(Chew());
+
+    if (lhs.status == ParseStatus::SyntaxError)
+      return lhs;
+
+    std::unique_ptr<Expression> expr = std::move(lhs.expr);
+
+    // TODO: handle LTR/RTL associativity?
+    while (Peek().IsBinaryOperator() && BinaryOperatorPrecedence(Peek().type) < precedence)
+    {
+      const Token tok = Chew();
+      ParseResult rhs = ParseBinary(BinaryOperatorPrecedence(tok.type));
+      if (rhs.status == ParseStatus::SyntaxError)
+      {
+        return rhs;
+      }
+
+      expr = std::make_unique<BinaryExpression>(tok.type, std::move(expr), std::move(rhs.expr));
+    }
+
+    return ParseResult::MakeSuccessfulResult(std::move(expr));
+  }
+
+  ParseResult ParseParens()
   {
     // lparen already chewed
-    ParseResult result = Toplevel();
+    ParseResult result = ParseToplevel();
     if (result.status != ParseStatus::Successful)
       return result;
 
-    if (!Expects(TOK_RPAREN))
+    const auto rparen = Chew();
+    if (rparen.type != TOK_RPAREN)
     {
-      return {ParseStatus::SyntaxError};
+      return ParseResult::MakeErrorResult(rparen, _trans("Expected closing paren."));
     }
 
     return result;
   }
 
-  ParseResult Toplevel() { return Binary(); }
-};
+  ParseResult ParseToplevel() { return ParseBinary(); }
+};  // namespace ExpressionParser
+
+ParseResult ParseTokens(const std::vector<Token>& tokens)
+{
+  return Parser(tokens).Parse();
+}
 
 static ParseResult ParseComplexExpression(const std::string& str)
 {
   Lexer l(str);
   std::vector<Token> tokens;
-  ParseStatus tokenize_status = l.Tokenize(tokens);
+  const ParseStatus tokenize_status = l.Tokenize(tokens);
   if (tokenize_status != ParseStatus::Successful)
-    return {tokenize_status};
+    return ParseResult::MakeErrorResult(Token(TOK_INVALID), _trans("Tokenizing failed."));
 
-  return Parser(std::move(tokens)).Parse();
+  RemoveInertTokens(&tokens);
+  return ParseTokens(tokens);
+}
+
+void RemoveInertTokens(std::vector<Token>* tokens)
+{
+  tokens->erase(std::remove_if(tokens->begin(), tokens->end(),
+                               [](const Token& tok) {
+                                 return tok.type == TOK_COMMENT || tok.type == TOK_WHITESPACE;
+                               }),
+                tokens->end());
 }
 
 static std::unique_ptr<Expression> ParseBarewordExpression(const std::string& str)
@@ -538,21 +712,24 @@ static std::unique_ptr<Expression> ParseBarewordExpression(const std::string& st
   return std::make_unique<ControlExpression>(qualifier);
 }
 
-std::pair<ParseStatus, std::unique_ptr<Expression>> ParseExpression(const std::string& str)
+ParseResult ParseExpression(const std::string& str)
 {
   if (StripSpaces(str).empty())
-    return std::make_pair(ParseStatus::EmptyExpression, nullptr);
+    return ParseResult::MakeEmptyResult();
 
   auto bareword_expr = ParseBarewordExpression(str);
   ParseResult complex_result = ParseComplexExpression(str);
 
   if (complex_result.status != ParseStatus::Successful)
   {
-    return std::make_pair(complex_result.status, std::move(bareword_expr));
+    // This is a bit odd.
+    // Return the error status of the complex expression with the fallback barewords expression.
+    complex_result.expr = std::move(bareword_expr);
+    return complex_result;
   }
 
-  auto combined_expr = std::make_unique<CoalesceExpression>(std::move(bareword_expr),
-                                                            std::move(complex_result.expr));
-  return std::make_pair(complex_result.status, std::move(combined_expr));
+  complex_result.expr = std::make_unique<CoalesceExpression>(std::move(bareword_expr),
+                                                             std::move(complex_result.expr));
+  return complex_result;
 }
 }  // namespace ciface::ExpressionParser
diff --git a/Source/Core/InputCommon/ControlReference/ExpressionParser.h b/Source/Core/InputCommon/ControlReference/ExpressionParser.h
index 56c0340f49..cab6096c0e 100644
--- a/Source/Core/InputCommon/ControlReference/ExpressionParser.h
+++ b/Source/Core/InputCommon/ControlReference/ExpressionParser.h
@@ -4,56 +4,58 @@
 
 #pragma once
 
+#include <map>
 #include <memory>
+#include <optional>
 #include <string>
-#include <utility>
+
 #include "InputCommon/ControllerInterface/Device.h"
 
 namespace ciface::ExpressionParser
 {
-class ControlQualifier
+enum TokenType
 {
-public:
-  bool has_device;
-  Core::DeviceQualifier device_qualifier;
-  std::string control_name;
-
-  ControlQualifier() : has_device(false) {}
-  operator std::string() const
-  {
-    if (has_device)
-      return device_qualifier.ToString() + ":" + control_name;
-    else
-      return control_name;
-  }
+  TOK_WHITESPACE,
+  TOK_INVALID,
+  TOK_EOF,
+  TOK_LPAREN,
+  TOK_RPAREN,
+  TOK_NOT,
+  TOK_CONTROL,
+  TOK_LITERAL,
+  TOK_VARIABLE,
+  TOK_BAREWORD,
+  TOK_COMMENT,
+  // Binary Ops:
+  TOK_BINARY_OPS_BEGIN,
+  TOK_AND = TOK_BINARY_OPS_BEGIN,
+  TOK_OR,
+  TOK_ADD,
+  TOK_SUB,
+  TOK_MUL,
+  TOK_DIV,
+  TOK_MOD,
+  TOK_ASSIGN,
+  TOK_LTHAN,
+  TOK_GTHAN,
+  TOK_COMMA,
+  TOK_BINARY_OPS_END,
 };
 
-class ControlFinder
+class Token
 {
 public:
-  ControlFinder(const Core::DeviceContainer& container_, const Core::DeviceQualifier& default_,
-                const bool is_input_)
-      : container(container_), default_device(default_), is_input(is_input_)
-  {
-  }
-  std::shared_ptr<Core::Device> FindDevice(ControlQualifier qualifier) const;
-  Core::Device::Control* FindControl(ControlQualifier qualifier) const;
+  TokenType type;
+  std::string data;
 
-private:
-  const Core::DeviceContainer& container;
-  const Core::DeviceQualifier& default_device;
-  bool is_input;
-};
+  // Position in the input string:
+  std::size_t string_position = 0;
+  std::size_t string_length = 0;
 
-class Expression
-{
-public:
-  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;
+  explicit Token(TokenType type_);
+  Token(TokenType type_, std::string data_);
+
+  bool IsBinaryOperator() const;
 };
 
 enum class ParseStatus
@@ -63,5 +65,129 @@ enum class ParseStatus
   EmptyExpression,
 };
 
-std::pair<ParseStatus, std::unique_ptr<Expression>> ParseExpression(const std::string& expr);
+class Lexer
+{
+public:
+  std::string expr;
+  std::string::iterator it;
+
+  explicit Lexer(std::string expr_);
+
+  ParseStatus Tokenize(std::vector<Token>& tokens);
+
+private:
+  template <typename F>
+  std::string FetchCharsWhile(F&& func)
+  {
+    std::string value;
+    while (it != expr.end() && func(*it))
+    {
+      value += *it;
+      ++it;
+    }
+    return value;
+  }
+
+  std::string FetchDelimString(char delim);
+  std::string FetchWordChars();
+  Token GetDelimitedLiteral();
+  Token GetVariable();
+  Token GetFullyQualifiedControl();
+  Token GetBareword(char c);
+  Token GetRealLiteral(char c);
+
+  Token PeekToken();
+  Token NextToken();
+};
+
+class ControlQualifier
+{
+public:
+  bool has_device;
+  Core::DeviceQualifier device_qualifier;
+  std::string control_name;
+
+  ControlQualifier() : has_device(false) {}
+
+  operator std::string() const
+  {
+    if (has_device)
+      return device_qualifier.ToString() + ":" + control_name;
+    else
+      return control_name;
+  }
+
+  void FromString(const std::string& str)
+  {
+    const auto col_pos = str.find_last_of(':');
+
+    has_device = (str.npos != col_pos);
+    if (has_device)
+    {
+      device_qualifier.FromString(str.substr(0, col_pos));
+      control_name = str.substr(col_pos + 1);
+    }
+    else
+    {
+      device_qualifier.FromString("");
+      control_name = str;
+    }
+  }
+};
+
+class ControlEnvironment
+{
+public:
+  using VariableContainer = std::map<std::string, ControlState>;
+
+  ControlEnvironment(const Core::DeviceContainer& container_, const Core::DeviceQualifier& default_,
+                     VariableContainer& vars)
+      : m_variables(vars), container(container_), default_device(default_)
+  {
+  }
+
+  std::shared_ptr<Core::Device> FindDevice(ControlQualifier qualifier) const;
+  Core::Device::Input* FindInput(ControlQualifier qualifier) const;
+  Core::Device::Output* FindOutput(ControlQualifier qualifier) const;
+  ControlState* GetVariablePtr(const std::string& name);
+
+private:
+  VariableContainer& m_variables;
+  const Core::DeviceContainer& container;
+  const Core::DeviceQualifier& default_device;
+};
+
+class Expression
+{
+public:
+  virtual ~Expression() = default;
+  virtual ControlState GetValue() const = 0;
+  virtual void SetValue(ControlState state) = 0;
+  virtual int CountNumControls() const = 0;
+  virtual void UpdateReferences(ControlEnvironment& finder) = 0;
+};
+
+class ParseResult
+{
+public:
+  static ParseResult MakeEmptyResult();
+  static ParseResult MakeSuccessfulResult(std::unique_ptr<Expression>&& expr);
+  static ParseResult MakeErrorResult(Token token, std::string description);
+
+  ParseStatus status;
+  std::unique_ptr<Expression> expr;
+
+  // Used for parse errors:
+  // TODO: This should probably be moved elsewhere:
+  std::optional<Token> token;
+  std::optional<std::string> description;
+
+private:
+  ParseResult() = default;
+};
+
+ParseResult ParseExpression(const std::string& expr);
+ParseResult ParseTokens(const std::vector<Token>& tokens);
+void RemoveInertTokens(std::vector<Token>* tokens);
+
 }  // namespace ciface::ExpressionParser
diff --git a/Source/Core/InputCommon/ControlReference/FunctionExpression.cpp b/Source/Core/InputCommon/ControlReference/FunctionExpression.cpp
new file mode 100644
index 0000000000..18b98c5e19
--- /dev/null
+++ b/Source/Core/InputCommon/ControlReference/FunctionExpression.cpp
@@ -0,0 +1,526 @@
+// Copyright 2019 Dolphin Emulator Project
+// Licensed under GPLv2+
+// Refer to the license.txt file included.
+
+#include <chrono>
+#include <cmath>
+
+#include "InputCommon/ControlReference/FunctionExpression.h"
+
+namespace ciface
+{
+namespace ExpressionParser
+{
+constexpr int LOOP_MAX_REPS = 10000;
+constexpr ControlState CONDITION_THRESHOLD = 0.5;
+
+using Clock = std::chrono::steady_clock;
+using FSec = std::chrono::duration<ControlState>;
+
+// usage: toggle(toggle_state_input, [clear_state_input])
+class ToggleExpression : public FunctionExpression
+{
+private:
+  ArgumentValidation
+  ValidateArguments(const std::vector<std::unique_ptr<Expression>>& args) override
+  {
+    // Optional 2nd argument for clearing state:
+    if (1 == args.size() || 2 == args.size())
+      return ArgumentsAreValid{};
+    else
+      return ExpectedArguments{"toggle_state_input, [clear_state_input]"};
+  }
+
+  ControlState GetValue() const override
+  {
+    const ControlState inner_value = GetArg(0).GetValue();
+
+    if (inner_value < CONDITION_THRESHOLD)
+    {
+      m_released = true;
+    }
+    else if (m_released && inner_value > CONDITION_THRESHOLD)
+    {
+      m_released = false;
+      m_state ^= true;
+    }
+
+    if (2 == GetArgCount() && GetArg(1).GetValue() > CONDITION_THRESHOLD)
+    {
+      m_state = false;
+    }
+
+    return m_state;
+  }
+
+  mutable bool m_released{};
+  mutable bool m_state{};
+};
+
+// usage: not(expression)
+class NotExpression : public FunctionExpression
+{
+private:
+  ArgumentValidation
+  ValidateArguments(const std::vector<std::unique_ptr<Expression>>& args) override
+  {
+    if (1 == args.size())
+      return ArgumentsAreValid{};
+    else
+      return ExpectedArguments{"expression"};
+  }
+
+  ControlState GetValue() const override { return 1.0 - GetArg(0).GetValue(); }
+  void SetValue(ControlState value) override { GetArg(0).SetValue(1.0 - value); }
+};
+
+// usage: sin(expression)
+class SinExpression : public FunctionExpression
+{
+private:
+  ArgumentValidation
+  ValidateArguments(const std::vector<std::unique_ptr<Expression>>& args) override
+  {
+    if (1 == args.size())
+      return ArgumentsAreValid{};
+    else
+      return ExpectedArguments{"expression"};
+  }
+
+  ControlState GetValue() const override { return std::sin(GetArg(0).GetValue()); }
+};
+
+// usage: timer(seconds)
+class TimerExpression : public FunctionExpression
+{
+private:
+  ArgumentValidation
+  ValidateArguments(const std::vector<std::unique_ptr<Expression>>& args) override
+  {
+    if (1 == args.size())
+      return ArgumentsAreValid{};
+    else
+      return ExpectedArguments{"seconds"};
+  }
+
+  ControlState GetValue() const override
+  {
+    const auto now = Clock::now();
+    const auto elapsed = now - m_start_time;
+
+    const ControlState val = GetArg(0).GetValue();
+
+    ControlState progress = std::chrono::duration_cast<FSec>(elapsed).count() / val;
+
+    if (std::isinf(progress) || progress < 0.0)
+    {
+      // User configured a non-positive timer. Reset the timer and return 0.0.
+      progress = 0.0;
+      m_start_time = now;
+    }
+    else if (progress >= 1.0)
+    {
+      const ControlState reset_count = std::floor(progress);
+
+      m_start_time += std::chrono::duration_cast<Clock::duration>(FSec(val * reset_count));
+      progress -= reset_count;
+    }
+
+    return progress;
+  }
+
+private:
+  mutable Clock::time_point m_start_time = Clock::now();
+};
+
+// usage: if(condition, true_expression, false_expression)
+class IfExpression : public FunctionExpression
+{
+private:
+  ArgumentValidation
+  ValidateArguments(const std::vector<std::unique_ptr<Expression>>& args) override
+  {
+    if (3 == args.size())
+      return ArgumentsAreValid{};
+    else
+      return ExpectedArguments{"condition, true_expression, false_expression"};
+  }
+
+  ControlState GetValue() const override
+  {
+    return (GetArg(0).GetValue() > CONDITION_THRESHOLD) ? GetArg(1).GetValue() :
+                                                          GetArg(2).GetValue();
+  }
+};
+
+// usage: minus(expression)
+class UnaryMinusExpression : public FunctionExpression
+{
+private:
+  ArgumentValidation
+  ValidateArguments(const std::vector<std::unique_ptr<Expression>>& args) override
+  {
+    if (1 == args.size())
+      return ArgumentsAreValid{};
+    else
+      return ExpectedArguments{"expression"};
+  }
+
+  ControlState GetValue() const override
+  {
+    // Subtraction for clarity:
+    return 0.0 - GetArg(0).GetValue();
+  }
+};
+
+// usage: deadzone(input, amount)
+class DeadzoneExpression : public FunctionExpression
+{
+  ArgumentValidation
+  ValidateArguments(const std::vector<std::unique_ptr<Expression>>& args) override
+  {
+    if (2 == args.size())
+      return ArgumentsAreValid{};
+    else
+      return ExpectedArguments{"input, amount"};
+  }
+
+  ControlState GetValue() const override
+  {
+    const ControlState val = GetArg(0).GetValue();
+    const ControlState deadzone = GetArg(1).GetValue();
+    return std::copysign(std::max(0.0, std::abs(val) - deadzone) / (1.0 - deadzone), val);
+  }
+};
+
+// usage: smooth(input, seconds_up, seconds_down = seconds_up)
+// seconds is seconds to change from 0.0 to 1.0
+class SmoothExpression : public FunctionExpression
+{
+  ArgumentValidation
+  ValidateArguments(const std::vector<std::unique_ptr<Expression>>& args) override
+  {
+    if (2 == args.size() || 3 == args.size())
+      return ArgumentsAreValid{};
+    else
+      return ExpectedArguments{"input, seconds_up, seconds_down = seconds_up"};
+  }
+
+  ControlState GetValue() const override
+  {
+    const auto now = Clock::now();
+    const auto elapsed = now - m_last_update;
+    m_last_update = now;
+
+    const ControlState desired_value = GetArg(0).GetValue();
+
+    const ControlState smooth_up = GetArg(1).GetValue();
+    const ControlState smooth_down = (3 == GetArgCount() ? GetArg(2).GetValue() : smooth_up);
+
+    const ControlState smooth = (desired_value < m_value) ? smooth_down : smooth_up;
+    const ControlState max_move = std::chrono::duration_cast<FSec>(elapsed).count() / smooth;
+
+    if (std::isinf(max_move))
+    {
+      m_value = desired_value;
+    }
+    else
+    {
+      const ControlState diff = desired_value - m_value;
+      m_value += std::copysign(std::min(max_move, std::abs(diff)), diff);
+    }
+
+    return m_value;
+  }
+
+private:
+  mutable ControlState m_value = 0.0;
+  mutable Clock::time_point m_last_update = Clock::now();
+};
+
+// usage: hold(input, seconds)
+class HoldExpression : public FunctionExpression
+{
+  ArgumentValidation
+  ValidateArguments(const std::vector<std::unique_ptr<Expression>>& args) override
+  {
+    if (2 == args.size())
+      return ArgumentsAreValid{};
+    else
+      return ExpectedArguments{"input, seconds"};
+  }
+
+  ControlState GetValue() const override
+  {
+    const auto now = Clock::now();
+
+    const ControlState input = GetArg(0).GetValue();
+
+    if (input < CONDITION_THRESHOLD)
+    {
+      m_state = false;
+      m_start_time = Clock::now();
+    }
+    else if (!m_state)
+    {
+      const auto hold_time = now - m_start_time;
+
+      if (std::chrono::duration_cast<FSec>(hold_time).count() >= GetArg(1).GetValue())
+        m_state = true;
+    }
+
+    return m_state;
+  }
+
+private:
+  mutable bool m_state = false;
+  mutable Clock::time_point m_start_time = Clock::now();
+};
+
+// usage: tap(input, seconds, taps=2)
+class TapExpression : public FunctionExpression
+{
+  ArgumentValidation
+  ValidateArguments(const std::vector<std::unique_ptr<Expression>>& args) override
+  {
+    if (2 == args.size() || 3 == args.size())
+      return ArgumentsAreValid{};
+    else
+      return ExpectedArguments{"input, seconds, taps = 2"};
+  }
+
+  ControlState GetValue() const override
+  {
+    const auto now = Clock::now();
+
+    const auto elapsed = std::chrono::duration_cast<FSec>(now - m_start_time).count();
+
+    const ControlState input = GetArg(0).GetValue();
+    const ControlState seconds = GetArg(1).GetValue();
+
+    const bool is_time_up = elapsed > seconds;
+
+    const u32 desired_taps = (3 == GetArgCount()) ? u32(GetArg(2).GetValue() + 0.5) : 2;
+
+    if (input < CONDITION_THRESHOLD)
+    {
+      m_released = true;
+
+      if (m_taps > 0 && is_time_up)
+      {
+        m_taps = 0;
+      }
+    }
+    else
+    {
+      if (m_released)
+      {
+        if (!m_taps)
+        {
+          m_start_time = now;
+        }
+
+        ++m_taps;
+        m_released = false;
+      }
+
+      return desired_taps == m_taps;
+    }
+
+    return 0.0;
+  }
+
+private:
+  mutable bool m_released = true;
+  mutable u32 m_taps = 0;
+  mutable Clock::time_point m_start_time = Clock::now();
+};
+
+// usage: relative(input, speed, [max_abs_value, [shared_state]])
+// speed is max movement per second
+class RelativeExpression : public FunctionExpression
+{
+  ArgumentValidation
+  ValidateArguments(const std::vector<std::unique_ptr<Expression>>& args) override
+  {
+    if (args.size() >= 2 && args.size() <= 4)
+      return ArgumentsAreValid{};
+    else
+      return ExpectedArguments{"input, speed, [max_abs_value, [shared_state]]"};
+  }
+
+  ControlState GetValue() const override
+  {
+    // There is a lot of funky math in this function but it allows for a variety of uses:
+    //
+    // e.g. A single mapping with a relatively adjusted value between 0.0 and 1.0
+    // Potentially useful for a trigger input
+    //  relative(`Up` - `Down`, 2.0)
+    //
+    // e.g. A value with two mappings (such as analog stick Up/Down)
+    // The shared state allows the two mappings to work together.
+    // This mapping (for up) returns a value clamped between 0.0 and 1.0
+    //  relative(`Up`, 2.0, 1.0, $y)
+    // This mapping (for down) returns the negative value clamped between 0.0 and 1.0
+    // (Adjustments created by `Down` are applied negatively to the shared state)
+    //  relative(`Down`, 2.0, -1.0, $y)
+
+    const auto now = Clock::now();
+
+    if (GetArgCount() >= 4)
+      m_state = GetArg(3).GetValue();
+
+    const auto elapsed = std::chrono::duration_cast<FSec>(now - m_last_update).count();
+    m_last_update = now;
+
+    const ControlState input = GetArg(0).GetValue();
+    const ControlState speed = GetArg(1).GetValue();
+
+    const ControlState max_abs_value = (GetArgCount() >= 3) ? GetArg(2).GetValue() : 1.0;
+
+    const ControlState max_move = input * elapsed * speed;
+    const ControlState diff_from_zero = std::abs(0.0 - m_state);
+    const ControlState diff_from_max = std::abs(max_abs_value - m_state);
+
+    m_state += std::min(std::max(max_move, -diff_from_zero), diff_from_max) *
+               std::copysign(1.0, max_abs_value);
+
+    if (GetArgCount() >= 4)
+      const_cast<Expression&>(GetArg(3)).SetValue(m_state);
+
+    return std::max(0.0, m_state * std::copysign(1.0, max_abs_value));
+  }
+
+private:
+  mutable ControlState m_state = 0.0;
+  mutable Clock::time_point m_last_update = Clock::now();
+};
+
+// usage: pulse(input, seconds)
+class PulseExpression : public FunctionExpression
+{
+  ArgumentValidation
+  ValidateArguments(const std::vector<std::unique_ptr<Expression>>& args) override
+  {
+    if (2 == args.size())
+      return ArgumentsAreValid{};
+    else
+      return ExpectedArguments{"input, seconds"};
+  }
+
+  ControlState GetValue() const override
+  {
+    const auto now = Clock::now();
+
+    const ControlState input = GetArg(0).GetValue();
+
+    if (input < CONDITION_THRESHOLD)
+    {
+      m_released = true;
+    }
+    else if (m_released)
+    {
+      m_released = false;
+
+      const auto seconds = std::chrono::duration_cast<Clock::duration>(FSec(GetArg(1).GetValue()));
+
+      if (m_state)
+      {
+        m_release_time += seconds;
+      }
+      else
+      {
+        m_state = true;
+        m_release_time = now + seconds;
+      }
+    }
+
+    if (m_state && now >= m_release_time)
+    {
+      m_state = false;
+    }
+
+    return m_state;
+  }
+
+private:
+  mutable bool m_released = false;
+  mutable bool m_state = false;
+  mutable Clock::time_point m_release_time = Clock::now();
+};
+
+std::unique_ptr<FunctionExpression> MakeFunctionExpression(std::string name)
+{
+  if ("not" == name)
+    return std::make_unique<NotExpression>();
+  else if ("if" == name)
+    return std::make_unique<IfExpression>();
+  else if ("sin" == name)
+    return std::make_unique<SinExpression>();
+  else if ("timer" == name)
+    return std::make_unique<TimerExpression>();
+  else if ("toggle" == name)
+    return std::make_unique<ToggleExpression>();
+  else if ("minus" == name)
+    return std::make_unique<UnaryMinusExpression>();
+  else if ("deadzone" == name)
+    return std::make_unique<DeadzoneExpression>();
+  else if ("smooth" == name)
+    return std::make_unique<SmoothExpression>();
+  else if ("hold" == name)
+    return std::make_unique<HoldExpression>();
+  else if ("tap" == name)
+    return std::make_unique<TapExpression>();
+  else if ("relative" == name)
+    return std::make_unique<RelativeExpression>();
+  else if ("pulse" == name)
+    return std::make_unique<PulseExpression>();
+  else
+    return nullptr;
+}
+
+int FunctionExpression::CountNumControls() const
+{
+  int result = 0;
+
+  for (auto& arg : m_args)
+    result += arg->CountNumControls();
+
+  return result;
+}
+
+void FunctionExpression::UpdateReferences(ControlEnvironment& env)
+{
+  for (auto& arg : m_args)
+    arg->UpdateReferences(env);
+}
+
+FunctionExpression::ArgumentValidation
+FunctionExpression::SetArguments(std::vector<std::unique_ptr<Expression>>&& args)
+{
+  m_args = std::move(args);
+
+  return ValidateArguments(m_args);
+}
+
+Expression& FunctionExpression::GetArg(u32 number)
+{
+  return *m_args[number];
+}
+
+const Expression& FunctionExpression::GetArg(u32 number) const
+{
+  return *m_args[number];
+}
+
+u32 FunctionExpression::GetArgCount() const
+{
+  return u32(m_args.size());
+}
+
+void FunctionExpression::SetValue(ControlState)
+{
+}
+
+}  // namespace ExpressionParser
+}  // namespace ciface
diff --git a/Source/Core/InputCommon/ControlReference/FunctionExpression.h b/Source/Core/InputCommon/ControlReference/FunctionExpression.h
new file mode 100644
index 0000000000..f5332a0d25
--- /dev/null
+++ b/Source/Core/InputCommon/ControlReference/FunctionExpression.h
@@ -0,0 +1,55 @@
+// Copyright 2019 Dolphin Emulator Project
+// Licensed under GPLv2+
+// Refer to the license.txt file included.
+
+#pragma once
+
+#include <memory>
+#include <string>
+#include <variant>
+#include <vector>
+
+#include "InputCommon/ControlReference/ExpressionParser.h"
+#include "InputCommon/ControlReference/FunctionExpression.h"
+
+namespace ciface
+{
+namespace ExpressionParser
+{
+class FunctionExpression : public Expression
+{
+public:
+  struct ArgumentsAreValid
+  {
+  };
+
+  struct ExpectedArguments
+  {
+    std::string text;
+  };
+
+  using ArgumentValidation = std::variant<ArgumentsAreValid, ExpectedArguments>;
+
+  int CountNumControls() const override;
+  void UpdateReferences(ControlEnvironment& env) override;
+
+  ArgumentValidation SetArguments(std::vector<std::unique_ptr<Expression>>&& args);
+
+  void SetValue(ControlState value) override;
+
+protected:
+  virtual ArgumentValidation
+  ValidateArguments(const std::vector<std::unique_ptr<Expression>>& args) = 0;
+
+  Expression& GetArg(u32 number);
+  const Expression& GetArg(u32 number) const;
+  u32 GetArgCount() const;
+
+private:
+  std::vector<std::unique_ptr<Expression>> m_args;
+};
+
+std::unique_ptr<FunctionExpression> MakeFunctionExpression(std::string name);
+
+}  // namespace ExpressionParser
+}  // namespace ciface
diff --git a/Source/Core/InputCommon/ControllerEmu/ControllerEmu.cpp b/Source/Core/InputCommon/ControllerEmu/ControllerEmu.cpp
index de0579a2b2..c5ff0705a2 100644
--- a/Source/Core/InputCommon/ControllerEmu/ControllerEmu.cpp
+++ b/Source/Core/InputCommon/ControllerEmu/ControllerEmu.cpp
@@ -38,23 +38,38 @@ std::unique_lock<std::recursive_mutex> EmulatedController::GetStateLock()
 
 void EmulatedController::UpdateReferences(const ControllerInterface& devi)
 {
-  const auto lock = GetStateLock();
   m_default_device_is_connected = devi.HasConnectedDevice(m_default_device);
 
+  ciface::ExpressionParser::ControlEnvironment env(devi, GetDefaultDevice(), m_expression_vars);
+
+  UpdateReferences(env);
+}
+
+void EmulatedController::UpdateReferences(ciface::ExpressionParser::ControlEnvironment& env)
+{
+  const auto lock = GetStateLock();
+
   for (auto& ctrlGroup : groups)
   {
     for (auto& control : ctrlGroup->controls)
-      control->control_ref.get()->UpdateReference(devi, GetDefaultDevice());
+      control->control_ref->UpdateReference(env);
 
     // Attachments:
     if (ctrlGroup->type == GroupType::Attachments)
     {
       for (auto& attachment : static_cast<Attachments*>(ctrlGroup.get())->GetAttachmentList())
-        attachment->UpdateReferences(devi);
+        attachment->UpdateReferences(env);
     }
   }
 }
 
+void EmulatedController::UpdateSingleControlReference(const ControllerInterface& devi,
+                                                      ControlReference* ref)
+{
+  ciface::ExpressionParser::ControlEnvironment env(devi, GetDefaultDevice(), m_expression_vars);
+  ref->UpdateReference(env);
+}
+
 bool EmulatedController::IsDefaultDeviceConnected() const
 {
   return m_default_device_is_connected;
diff --git a/Source/Core/InputCommon/ControllerEmu/ControllerEmu.h b/Source/Core/InputCommon/ControllerEmu/ControllerEmu.h
index bf2ff666fa..2611880ee9 100644
--- a/Source/Core/InputCommon/ControllerEmu/ControllerEmu.h
+++ b/Source/Core/InputCommon/ControllerEmu/ControllerEmu.h
@@ -13,6 +13,7 @@
 
 #include "Common/Common.h"
 #include "Common/IniFile.h"
+#include "InputCommon/ControlReference/ExpressionParser.h"
 #include "InputCommon/ControllerInterface/Device.h"
 
 class ControllerInterface;
@@ -20,6 +21,8 @@ class ControllerInterface;
 const char* const named_directions[] = {_trans("Up"), _trans("Down"), _trans("Left"),
                                         _trans("Right")};
 
+class ControlReference;
+
 namespace ControllerEmu
 {
 class ControlGroup;
@@ -43,6 +46,7 @@ public:
   void SetDefaultDevice(ciface::Core::DeviceQualifier devq);
 
   void UpdateReferences(const ControllerInterface& devi);
+  void UpdateSingleControlReference(const ControllerInterface& devi, ControlReference* ref);
 
   // This returns a lock that should be held before calling State() on any control
   // references and GetState(), by extension. This prevents a race condition
@@ -75,6 +79,12 @@ public:
       return T(std::lround((zero_value - neg_1_value) * input_value + zero_value));
   }
 
+protected:
+  // TODO: Wiimote attachment has its own member that isn't being used..
+  ciface::ExpressionParser::ControlEnvironment::VariableContainer m_expression_vars;
+
+  void UpdateReferences(ciface::ExpressionParser::ControlEnvironment& env);
+
 private:
   ciface::Core::DeviceQualifier m_default_device;
   bool m_default_device_is_connected{false};
diff --git a/Source/Core/InputCommon/InputCommon.vcxproj b/Source/Core/InputCommon/InputCommon.vcxproj
index 8d0b0b3759..a22ad21394 100644
--- a/Source/Core/InputCommon/InputCommon.vcxproj
+++ b/Source/Core/InputCommon/InputCommon.vcxproj
@@ -64,6 +64,7 @@
     <ClCompile Include="ControllerInterface\ForceFeedback\ForceFeedbackDevice.cpp" />
     <ClCompile Include="ControllerInterface\Win32\Win32.cpp" />
     <ClCompile Include="ControllerInterface\XInput\XInput.cpp" />
+    <ClCompile Include="ControlReference\FunctionExpression.cpp" />
     <ClCompile Include="GCAdapter.cpp">
       <!--
       Disable "nonstandard extension used : zero-sized array in struct/union" warning,
@@ -100,6 +101,7 @@
     <ClInclude Include="ControllerInterface\DInput\DInputKeyboardMouse.h" />
     <ClInclude Include="ControllerInterface\DInput\XInputFilter.h" />
     <ClInclude Include="ControlReference\ControlReference.h" />
+    <ClInclude Include="ControlReference\FunctionExpression.h" />
     <ClInclude Include="ControlReference\ExpressionParser.h" />
     <ClInclude Include="ControllerInterface\ForceFeedback\ForceFeedbackDevice.h" />
     <ClInclude Include="ControllerInterface\Win32\Win32.h" />
diff --git a/Source/Core/InputCommon/InputCommon.vcxproj.filters b/Source/Core/InputCommon/InputCommon.vcxproj.filters
index 76195f5627..c1f26d6141 100644
--- a/Source/Core/InputCommon/InputCommon.vcxproj.filters
+++ b/Source/Core/InputCommon/InputCommon.vcxproj.filters
@@ -113,6 +113,9 @@
     <ClCompile Include="ControlReference\ControlReference.cpp">
       <Filter>ControllerInterface</Filter>
     </ClCompile>
+    <ClCompile Include="ControlReference\FunctionExpression.cpp">
+      <Filter>ControllerInterface</Filter>
+    </ClCompile>
     <ClCompile Include="InputProfile.cpp" />
     <ClCompile Include="ControllerEmu\ControlGroup\Attachments.cpp">
       <Filter>ControllerEmu\ControlGroup</Filter>
@@ -206,6 +209,9 @@
     <ClInclude Include="ControlReference\ControlReference.h">
       <Filter>ControllerInterface</Filter>
     </ClInclude>
+    <ClCompile Include="ControlReference\FunctionExpression.h">
+      <Filter>ControllerInterface</Filter>
+    </ClCompile>
     <ClInclude Include="InputProfile.h" />
     <ClInclude Include="ControllerEmu\ControlGroup\Attachments.h">
       <Filter>ControllerEmu\ControlGroup</Filter>