diff --git a/components/expression_language/extending.rst b/components/expression_language/extending.rst
new file mode 100644
index 00000000000..ddcc44d476f
--- /dev/null
+++ b/components/expression_language/extending.rst
@@ -0,0 +1,87 @@
+.. index::
+    single: Extending; ExpressionLanguage
+
+Extending the ExpressionLanguage
+================================
+
+The ExpressionLanguage can be extended by adding custom functions. For
+instance, in the framework, the security has custom functions to check the
+user's role.
+
+.. note::
+
+    If you want to learn how to use functions in an expression, read
+    ":ref:`component-expression-functions`".
+
+Register Functions
+------------------
+
+Functions will be registered on the current ``ExpressionLanguage`` instance.
+That means the functions can be used in any expression executed by that
+instance.
+
+To register a function, use
+:method:`Symfony\\Component\\ExpressionLanguage\\ExpressionLanguage::register``.
+This method has 3 arguments:
+
+* **name** - The name of the function in an expression;
+* **compiler** - A function executed when compiling an expression using the
+  function;
+* **evaluator** - A function executed when the expression is evaluated.
+
+.. code-block:: php
+
+    use Symfony\Component\ExpressionLanguage\ExpressionLanguage;
+
+    $language = new ExpressionLanguage();
+    $language->register('lowercase', function ($str) {
+        if (!is_string($str)) {
+            return $str;
+        }
+
+        return sprintf('strtolower(%s)', $str);
+    }, function ($str) {
+        if (!is_string($str)) {
+            return $str;
+        }
+
+        return strtolower($str);
+    });
+
+    echo $language->evaluate('lowercase("HELLO")');
+
+This will print ``hello``.
+
+Creating a new ExpressionLanguage class
+---------------------------------------
+
+When you use the ``ExpressionLanguage`` class in your library, it's recommend
+to create a new ``ExpressionLanguage`` class and register the functions there.
+The class will execute ``registerFunctions`` to register the default
+functions, you can override this to also add your own functions::
+
+    namespace Acme\AwesomeLib\ExpressionLanguage;
+
+    use Symfony\Component\ExpressionLanguage\ExpressionLanguage as BaseExpressionLanguage;
+
+    class ExpressionLanguage extends BaseExpressionLanguage
+    {
+        protected function registerFunctions()
+        {
+            parent::registerFunctions(); // do not forget to also register core functions
+
+            $this->register('lowercase', function ($str) {
+                if (!is_string($str)) {
+                    return $str;
+                }
+
+                return sprintf('strtolower(%s)', $str);
+            }, function ($str) {
+                if (!is_string($str)) {
+                    return $str;
+                }
+
+                return strtolower($str);
+            });
+        }
+    }
diff --git a/components/expression_language/syntax.rst b/components/expression_language/syntax.rst
index 3552cad5b7d..e105e82bdb8 100644
--- a/components/expression_language/syntax.rst
+++ b/components/expression_language/syntax.rst
@@ -81,6 +81,29 @@ JavaScript::
 
 This will print ``Hi Hi Hi!``.
 
+.. _component-expression-functions:
+
+Working with Functions
+----------------------
+
+You can also use registered functions in the expression by using the same
+syntax as PHP and JavaScript. The ExpressionLanguage component comes with one
+function by default: ``constant()`` Which will return the value of the PHP
+constant::
+
+    define('DB_USER', 'root');
+
+    echo $language->evaluate(
+        'constant("DB_USER")'
+    );
+
+This will print ``root``.
+
+.. tip::
+
+    To read how to register your own function to use in an expression, see
+    ":doc:`/components/expression_language/extending`".
+
 .. _component-expression-arrays:
 
 Working with Arrays
@@ -128,11 +151,6 @@ For example::
 
 This will print out ``42``.
 
-Assignment Operators
-~~~~~~~~~~~~~~~~~~~~
-
-* ``=``
-
 Bitwise Operators
 ~~~~~~~~~~~~~~~~~
 
@@ -249,6 +267,26 @@ Numeric Operators
 
 * ``..`` (range)
 
+For example::
+
+    class User
+    {
+        public $age;
+    }
+
+    $user = new User();
+    $user->age = 34;
+
+    $language->evaluate(
+        'user.age in 18..45',
+        array(
+            'user' => $user,
+        )
+    );
+
+This will evaluate to ``true``, because ``user.age`` is in the range from
+``18`` till ``45``
+
 Ternary Operators
 ~~~~~~~~~~~~~~~~~