diff --git a/python/gui/gui_auto.sip b/python/gui/gui_auto.sip
index 5bd30d7e244..000356b960b 100644
--- a/python/gui/gui_auto.sip
+++ b/python/gui/gui_auto.sip
@@ -79,6 +79,9 @@
 %If ( HAVE_QSCI_SIP )
 %Include auto_generated/qgscodeeditorsql.sip
 %End
+%If ( HAVE_QSCI_SIP )
+%Include auto_generated/qgscodeeditorexpression.sip
+%End
 %Include auto_generated/qgscollapsiblegroupbox.sip
 %Include auto_generated/qgscolorbrewercolorrampdialog.sip
 %Include auto_generated/qgscolorbutton.sip
diff --git a/src/gui/CMakeLists.txt b/src/gui/CMakeLists.txt
index 6fbe34d668d..7330c65b0fe 100755
--- a/src/gui/CMakeLists.txt
+++ b/src/gui/CMakeLists.txt
@@ -229,6 +229,7 @@ SET(QGIS_GUI_SRCS
   qgscodeeditorhtml.cpp
   qgscodeeditorpython.cpp
   qgscodeeditorsql.cpp
+  qgscodeeditorexpression.cpp
   qgscollapsiblegroupbox.cpp
   qgscolorbrewercolorrampdialog.cpp
   qgscolorbutton.cpp
@@ -406,6 +407,7 @@ SET(QGIS_GUI_MOC_HDRS
   qgscodeeditorhtml.h
   qgscodeeditorpython.h
   qgscodeeditorsql.h
+  qgscodeeditorexpression.h
   qgscollapsiblegroupbox.h
   qgscolorbrewercolorrampdialog.h
   qgscolorbutton.h
diff --git a/src/gui/qgscodeeditorexpression.cpp b/src/gui/qgscodeeditorexpression.cpp
new file mode 100644
index 00000000000..f6ed08a3d57
--- /dev/null
+++ b/src/gui/qgscodeeditorexpression.cpp
@@ -0,0 +1,163 @@
+/***************************************************************************
+    qgscodeeditorexpressoin.cpp - An expression editor based on QScintilla
+     --------------------------------------
+    Date                 : 8.9.2018
+    Copyright            : (C) 2018 by Matthias Kuhn
+    Email                : matthias@opengis.ch
+ ***************************************************************************
+ *                                                                         *
+ *   This program is free software; you can redistribute it and/or modify  *
+ *   it under the terms of the GNU General Public License as published by  *
+ *   the Free Software Foundation; either version 2 of the License, or     *
+ *   (at your option) any later version.                                   *
+ *                                                                         *
+ ***************************************************************************/
+
+#include "qgsapplication.h"
+#include "qgscodeeditorexpression.h"
+
+#include <QWidget>
+#include <QString>
+#include <QFont>
+#include <QLabel>
+
+
+QgsCodeEditorExpression::QgsCodeEditorExpression( QWidget *parent )
+  : QgsCodeEditor( parent )
+{
+  if ( !parent )
+  {
+    setTitle( tr( "Expression Editor" ) );
+  }
+  setMarginVisible( false );
+  setFoldingVisible( true );
+  setAutoCompletionCaseSensitivity( false );
+  initializeLexer();
+}
+
+void QgsCodeEditorExpression::setExpressionContext( const QgsExpressionContext &context )
+{
+  mVariables.clear();
+
+  const QStringList variableNames = context.filteredVariableNames();
+  for ( const QString &var : variableNames )
+  {
+    mVariables << '@' + var;
+  }
+
+  mContextFunctions = context.functionNames();
+
+  mFunctions.clear();
+
+  const int count = QgsExpression::functionCount();
+  for ( int i = 0; i < count; i++ )
+  {
+    QgsExpressionFunction *func = QgsExpression::Functions()[i];
+    if ( func->isDeprecated() ) // don't show deprecated functions
+      continue;
+    if ( func->isContextual() )
+    {
+      //don't show contextual functions by default - it's up the the QgsExpressionContext
+      //object to provide them if supported
+      continue;
+    }
+
+    QString signature = func->name();
+    if ( !signature.startsWith( '$' ) )
+    {
+      signature += '(';
+
+      QStringList paramNames;
+      const auto &parameters = func->parameters();
+      for ( const auto &param : parameters )
+      {
+        paramNames << param.name();
+      }
+
+      // No named parameters but there should be parameteres? Show an ellipsis at least
+      if ( parameters.isEmpty() && func->params() )
+        signature += QChar( 0x2026 );
+
+      signature += paramNames.join( ", " );
+
+      signature += ')';
+    }
+    mFunctions << signature;
+  }
+
+  updateApis();
+}
+
+void QgsCodeEditorExpression::setFields( const QgsFields &fields )
+{
+  mFieldNames.clear();
+
+  for ( const QgsField &field : fields )
+  {
+    mFieldNames << field.name();
+  }
+
+  updateApis();
+}
+
+
+void QgsCodeEditorExpression::initializeLexer()
+{
+  QFont font = getMonospaceFont();
+#ifdef Q_OS_MAC
+  // The font size gotten from getMonospaceFont() is too small on Mac
+  font.setPointSize( QLabel().font().pointSize() );
+#endif
+  mSqlLexer = new QgsCaseInsensitiveLexerExpression( this );
+  mSqlLexer->setDefaultFont( font );
+  mSqlLexer->setFont( font, -1 );
+  font.setBold( true );
+  mSqlLexer->setFont( font, QsciLexerSQL::Keyword );
+  mSqlLexer->setColor( Qt::darkYellow, QsciLexerSQL::DoubleQuotedString ); // fields
+
+  setLexer( mSqlLexer );
+}
+
+void QgsCodeEditorExpression::updateApis()
+{
+  mApis = new QsciAPIs( mSqlLexer );
+
+  for ( const QString &var : qgis::as_const( mVariables ) )
+  {
+    mApis->add( var );
+  }
+
+  for ( const QString &function : qgis::as_const( mContextFunctions ) )
+  {
+    mApis->add( function );
+  }
+
+  for ( const QString &function : qgis::as_const( mFunctions ) )
+  {
+    mApis->add( function );
+  }
+
+  for ( const QString &fieldName : qgis::as_const( mFieldNames ) )
+  {
+    mApis->add( fieldName );
+  }
+
+  mApis->prepare();
+  mSqlLexer->setAPIs( mApis );
+}
+
+bool QgsCaseInsensitiveLexerExpression::caseSensitive() const
+{
+  return false;
+}
+#if 0
+const char *QgsCaseInsensitiveLexerExpression::wordCharacters() const
+{
+  static QString wordChars;
+
+  wordChars = QsciLexerSQL::wordCharacters();
+  wordChars += '@';
+  return wordChars.toUtf8().constData();
+}
+
+#endif
diff --git a/src/gui/qgscodeeditorexpression.h b/src/gui/qgscodeeditorexpression.h
new file mode 100644
index 00000000000..8593de290cd
--- /dev/null
+++ b/src/gui/qgscodeeditorexpression.h
@@ -0,0 +1,97 @@
+/***************************************************************************
+    qgscodeeditorsql.h - A SQL editor based on QScintilla
+     --------------------------------------
+    Date                 : 06-Oct-2013
+    Copyright            : (C) 2013 by Salvatore Larosa
+    Email                : lrssvtml (at) gmail (dot) com
+ ***************************************************************************
+ *                                                                         *
+ *   This program is free software; you can redistribute it and/or modify  *
+ *   it under the terms of the GNU General Public License as published by  *
+ *   the Free Software Foundation; either version 2 of the License, or     *
+ *   (at your option) any later version.                                   *
+ *                                                                         *
+ ***************************************************************************/
+
+#ifndef QGSCODEEDITOREXPRESSION_H
+#define QGSCODEEDITOREXPRESSION_H
+
+#include "qgis_sip.h"
+#include "qgis_gui.h"
+#include "qgscodeeditor.h"
+#include "qgsexpressioncontext.h"
+
+#include <Qsci/qscilexersql.h>
+
+SIP_IF_MODULE( HAVE_QSCI_SIP )
+
+/**
+ * \ingroup gui
+ *
+ * A QGIS expression editor based on QScintilla2. Adds syntax highlighting and
+ * code autocompletion.
+ *
+ * \since QGIS 3.4
+ */
+class GUI_EXPORT QgsCodeEditorExpression : public QgsCodeEditor
+{
+    Q_OBJECT
+
+  public:
+    //! Constructor for QgsCodeEditorExpression
+    QgsCodeEditorExpression( QWidget *parent SIP_TRANSFERTHIS = nullptr );
+
+    /**
+     * Variables and functions from this expression context will be added to
+     * the API.
+     * Will also reload all globally registered functions.
+     */
+    void setExpressionContext( const QgsExpressionContext &context );
+
+    /**
+     * Field names will be added to the API.
+     */
+    void setFields( const QgsFields &fields );
+
+  private:
+    void initializeLexer();
+    void updateApis();
+    QsciAPIs *mApis;
+    QsciLexerSQL *mSqlLexer;
+
+    QStringList mVariables;
+    QStringList mContextFunctions;
+    QStringList mFunctions;
+    QStringList mFieldNames;
+};
+
+#ifndef SIP_RUN
+///@cond PRIVATE
+
+/**
+ * Internal use.
+
+   setAutoCompletionCaseSensitivity( false ) is not sufficient when installing
+   a lexer, since its caseSensitive() method is actually used, and defaults
+   to true.
+   \note not available in Python bindings
+   \ingroup gui
+*/
+class QgsCaseInsensitiveLexerExpression : public QsciLexerSQL
+{
+    Q_OBJECT
+
+  public:
+    //! constructor
+    explicit QgsCaseInsensitiveLexerExpression( QObject *parent = nullptr ) : QsciLexerSQL( parent ) {}
+
+    bool caseSensitive() const override;
+
+#if 0
+    const char *wordCharacters() const override;
+#endif
+};
+///@endcond
+#endif
+
+#endif
diff --git a/src/gui/qgsexpressionbuilderwidget.cpp b/src/gui/qgsexpressionbuilderwidget.cpp
index a2d5601bd4e..4ff261bccdc 100644
--- a/src/gui/qgsexpressionbuilderwidget.cpp
+++ b/src/gui/qgsexpressionbuilderwidget.cpp
@@ -49,7 +49,7 @@ QgsExpressionBuilderWidget::QgsExpressionBuilderWidget( QWidget *parent )
   connect( btnNewFile, &QToolButton::pressed, this, &QgsExpressionBuilderWidget::btnNewFile_pressed );
   connect( cmbFileNames, &QListWidget::currentItemChanged, this, &QgsExpressionBuilderWidget::cmbFileNames_currentItemChanged );
   connect( expressionTree, &QTreeView::doubleClicked, this, &QgsExpressionBuilderWidget::expressionTree_doubleClicked );
-  connect( txtExpressionString, &QgsCodeEditorSQL::textChanged, this, &QgsExpressionBuilderWidget::txtExpressionString_textChanged );
+  connect( txtExpressionString, &QgsCodeEditorExpression::textChanged, this, &QgsExpressionBuilderWidget::txtExpressionString_textChanged );
   connect( txtPython, &QgsCodeEditorPython::textChanged, this, &QgsExpressionBuilderWidget::txtPython_textChanged );
   connect( txtSearchEditValues, &QgsFilterLineEdit::textChanged, this, &QgsExpressionBuilderWidget::txtSearchEditValues_textChanged );
   connect( txtSearchEdit, &QgsFilterLineEdit::textChanged, this, &QgsExpressionBuilderWidget::txtSearchEdit_textChanged );
@@ -142,7 +142,10 @@ QgsExpressionBuilderWidget::QgsExpressionBuilderWidget( QWidget *parent )
   txtExpressionString->setIndicatorHoverForegroundColor( QColor( Qt::blue ), FUNCTION_MARKER_ID );
   txtExpressionString->setIndicatorHoverStyle( QgsCodeEditor::DotsIndicator, FUNCTION_MARKER_ID );
 
-  connect( txtExpressionString, &QgsCodeEditorSQL::indicatorClicked, this, &QgsExpressionBuilderWidget::indicatorClicked );
+  connect( txtExpressionString, &QgsCodeEditorExpression::indicatorClicked, this, &QgsExpressionBuilderWidget::indicatorClicked );
+  txtExpressionString->setAutoCompletionCaseSensitivity( true );
+  txtExpressionString->setAutoCompletionSource( QsciScintilla::AcsAPIs );
+  txtExpressionString->setCallTipsVisible( 0 );
 
   setExpectedOutputFormat( QString() );
 }
@@ -341,6 +344,8 @@ void QgsExpressionBuilderWidget::loadFieldNames( const QgsFields &fields )
   if ( fields.isEmpty() )
     return;
 
+  txtExpressionString->setFields( fields );
+
   QStringList fieldNames;
   //Q_FOREACH ( const QgsField& field, fields )
   fieldNames.reserve( fields.count() );
@@ -696,6 +701,7 @@ void QgsExpressionBuilderWidget::txtExpressionString_textChanged()
 
 void QgsExpressionBuilderWidget::loadExpressionContext()
 {
+  txtExpressionString->setExpressionContext( mExpressionContext );
   QStringList variableNames = mExpressionContext.filteredVariableNames();
   Q_FOREACH ( const QString &variable, variableNames )
   {
diff --git a/src/gui/qgsexpressionlineedit.cpp b/src/gui/qgsexpressionlineedit.cpp
index a92c75cf00a..fd29bd29598 100644
--- a/src/gui/qgsexpressionlineedit.cpp
+++ b/src/gui/qgsexpressionlineedit.cpp
@@ -55,7 +55,7 @@ void QgsExpressionLineEdit::setMultiLine( bool multiLine )
 
   if ( multiLine && !mCodeEditor )
   {
-    mCodeEditor = new QgsCodeEditorSQL();
+    mCodeEditor = new QgsCodeEditorExpression();
     mCodeEditor->setSizePolicy( QSizePolicy::Expanding, QSizePolicy::Expanding );
     delete mLineEdit;
     mLineEdit = nullptr;
diff --git a/src/gui/qgsexpressionlineedit.h b/src/gui/qgsexpressionlineedit.h
index ec438219bc5..bf92ce248f6 100644
--- a/src/gui/qgsexpressionlineedit.h
+++ b/src/gui/qgsexpressionlineedit.h
@@ -27,7 +27,7 @@ class QgsFilterLineEdit;
 class QToolButton;
 class QgsDistanceArea;
 class QgsExpressionContextGenerator;
-class QgsCodeEditorSQL;
+class QgsCodeEditorExpression;
 
 /**
  * \ingroup gui
@@ -170,7 +170,7 @@ class GUI_EXPORT QgsExpressionLineEdit : public QWidget
 
   private:
     QgsFilterLineEdit *mLineEdit = nullptr;
-    QgsCodeEditorSQL *mCodeEditor = nullptr;
+    QgsCodeEditorExpression *mCodeEditor = nullptr;
     QToolButton *mButton = nullptr;
     QString mExpressionDialogTitle;
     std::unique_ptr<QgsDistanceArea> mDa;
diff --git a/src/ui/qgsexpressionbuilder.ui b/src/ui/qgsexpressionbuilder.ui
index d3a734476c1..deeb184c23b 100644
--- a/src/ui/qgsexpressionbuilder.ui
+++ b/src/ui/qgsexpressionbuilder.ui
@@ -273,7 +273,7 @@
                </widget>
               </item>
               <item>
-               <widget class="QgsCodeEditorSQL" name="txtExpressionString" native="true"/>
+               <widget class="QgsCodeEditorExpression" name="txtExpressionString" native="true"/>
               </item>
               <item>
                <layout class="QGridLayout" name="gridLayout">
@@ -768,9 +768,9 @@ Saved scripts are auto loaded on QGIS startup.</string>
    <header>qgsfilterlineedit.h</header>
   </customwidget>
   <customwidget>
-   <class>QgsCodeEditorSQL</class>
+   <class>QgsCodeEditorExpression</class>
    <extends>QWidget</extends>
-   <header>qgscodeeditorsql.h</header>
+   <header>qgscodeeditorexpression.h</header>
    <container>1</container>
   </customwidget>
   <customwidget>