Add method to search variables in text with expressions

python/core/expression/qgsexpression.sip.in

@@ -369,6 +369,15 @@ Additional substitutions can be passed through the substitutionMap parameter
 and area conversion
 
 .. versionadded:: 2.12
+%End
+
+    static QSet<QString> referencedVariables( const QString &text );
+%Docstring
+This function returns variables in each expression between [% and %].
+
+:param text: The source string in which variables should be searched.
+
+.. versionadded:: 3.2
 %End
 
     static double evaluateToDouble( const QString &text, const double fallbackValue );

src/core/expression/qgsexpression.cpp

@@ -477,6 +477,29 @@ QString QgsExpression::replaceExpressionText( const QString &action, const QgsEx
   return expr_action;
 }
 
+QSet<QString> QgsExpression::referencedVariables( const QString &text )
+{
+  QSet<QString> variables;
+  int index = 0;
+  while ( index < text.size() )
+  {
+    QRegExp rx = QRegExp( "\\[%([^\\]]+)%\\]" );
+
+    int pos = rx.indexIn( text, index );
+    if ( pos < 0 )
+      break;
+
+    int start = index;
+    index = pos + rx.matchedLength();
+    QString to_replace = rx.cap( 1 ).trimmed();
+
+    QgsExpression exp( to_replace );
+    variables.unite( exp.referencedVariables() );
+  }
+
+  return variables;
+}
+
 double QgsExpression::evaluateToDouble( const QString &text, const double fallbackValue )
 {
   bool ok;

src/core/expression/qgsexpression.h

@@ -400,6 +400,15 @@ class CORE_EXPORT QgsExpression
     static QString replaceExpressionText( const QString &action, const QgsExpressionContext *context,
                                           const QgsDistanceArea *distanceArea = nullptr );
 
+    /**
+     * This function returns variables in each expression between [% and %].
+     *
+     * \param text The source string in which variables should be searched.
+     *
+     * \since QGIS 3.2
+     */
+    static QSet<QString> referencedVariables( const QString &text );
+
     /**
      * Attempts to evaluate a text string as an expression to a resultant double
      * value.