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+Twig Internals
+==============
+
+Twig is very extensible and you can easily hack it. Keep in mind that you
+should probably try to create an extension before hacking the core, as most
+features and enhancements can be handled with extensions. This chapter is also
+useful for people who want to understand how Twig works under the hood.
+
+How does Twig work?
+-------------------
+
+The rendering of a Twig template can be summarized into four key steps:
+
+* **Load** the template: If the template is already compiled, load it and go
+  to the *evaluation* step, otherwise:
+
+  * First, the **lexer** tokenizes the template source code into small pieces
+    for easier processing;
+  * Then, the **parser** converts the token stream into a meaningful tree
+    of nodes (the Abstract Syntax Tree);
+  * Eventually, the *compiler* transforms the AST into PHP code.
+
+* **Evaluate** the template: It basically means calling the ``display()``
+  method of the compiled template and passing it the context.
+
+The Lexer
+---------
+
+The lexer tokenizes a template source code into a token stream (each token is
+an instance of ``Twig_Token``, and the stream is an instance of
+``Twig_TokenStream``). The default lexer recognizes 13 different token types:
+
+* ``Twig_Token::BLOCK_START_TYPE``, ``Twig_Token::BLOCK_END_TYPE``: Delimiters for blocks (``{% %}``)
+* ``Twig_Token::VAR_START_TYPE``, ``Twig_Token::VAR_END_TYPE``: Delimiters for variables (``{{ }}``)
+* ``Twig_Token::TEXT_TYPE``: A text outside an expression;
+* ``Twig_Token::NAME_TYPE``: A name in an expression;
+* ``Twig_Token::NUMBER_TYPE``: A number in an expression;
+* ``Twig_Token::STRING_TYPE``: A string in an expression;
+* ``Twig_Token::OPERATOR_TYPE``: An operator;
+* ``Twig_Token::PUNCTUATION_TYPE``: A punctuation sign;
+* ``Twig_Token::INTERPOLATION_START_TYPE``, ``Twig_Token::INTERPOLATION_END_TYPE`` (as of Twig 1.5): Delimiters for string interpolation;
+* ``Twig_Token::EOF_TYPE``: Ends of template.
+
+You can manually convert a source code into a token stream by calling the
+``tokenize()`` method of an environment::
+
+    $stream = $twig->tokenize(new Twig_Source($source, $identifier));
+
+.. versionadded:: 1.27
+    ``Twig_Source`` was introduced in version 1.27, pass the source and the
+    identifier directly on previous versions.
+
+As the stream has a ``__toString()`` method, you can have a textual
+representation of it by echoing the object::
+
+    echo $stream."\n";
+
+Here is the output for the ``Hello {{ name }}`` template:
+
+.. code-block:: text
+
+    TEXT_TYPE(Hello )
+    VAR_START_TYPE()
+    NAME_TYPE(name)
+    VAR_END_TYPE()
+    EOF_TYPE()
+
+.. note::
+
+    The default lexer (``Twig_Lexer``) can be changed by calling
+    the ``setLexer()`` method::
+
+        $twig->setLexer($lexer);
+
+The Parser
+----------
+
+The parser converts the token stream into an AST (Abstract Syntax Tree), or a
+node tree (an instance of ``Twig_Node_Module``). The core extension defines
+the basic nodes like: ``for``, ``if``, ... and the expression nodes.
+
+You can manually convert a token stream into a node tree by calling the
+``parse()`` method of an environment::
+
+    $nodes = $twig->parse($stream);
+
+Echoing the node object gives you a nice representation of the tree::
+
+    echo $nodes."\n";
+
+Here is the output for the ``Hello {{ name }}`` template:
+
+.. code-block:: text
+
+    Twig_Node_Module(
+      Twig_Node_Text(Hello )
+      Twig_Node_Print(
+        Twig_Node_Expression_Name(name)
+      )
+    )
+
+.. note::
+
+    The default parser (``Twig_TokenParser``) can be changed by calling the
+    ``setParser()`` method::
+
+        $twig->setParser($parser);
+
+The Compiler
+------------
+
+The last step is done by the compiler. It takes a node tree as an input and
+generates PHP code usable for runtime execution of the template.
+
+You can manually compile a node tree to PHP code with the ``compile()`` method
+of an environment::
+
+    $php = $twig->compile($nodes);
+
+The generated template for a ``Hello {{ name }}`` template reads as follows
+(the actual output can differ depending on the version of Twig you are
+using)::
+
+    /* Hello {{ name }} */
+    class __TwigTemplate_1121b6f109fe93ebe8c6e22e3712bceb extends Twig_Template
+    {
+        protected function doDisplay(array $context, array $blocks = array())
+        {
+            // line 1
+            echo "Hello ";
+            echo twig_escape_filter($this->env, isset($context["name"]) ? $context["name"] : null), "html", null, true);
+        }
+
+        // some more code
+    }
+
+.. note::
+
+    The default compiler (``Twig_Compiler``) can be changed by calling the
+    ``setCompiler()`` method::
+
+        $twig->setCompiler($compiler);