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294 changes: 294 additions & 0 deletions docs/en/dql-parser.rst
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DQL Lexer
=========

Here is a more complicated example from the Doctrine ORM project.
The ``Doctrine\ORM\Query\Lexer`` implementation for DQL looks something
like the following:

.. code-block:: php

use Doctrine\Common\Lexer\AbstractLexer;

class Lexer extends AbstractLexer
{
// All tokens that are not valid identifiers must be < 100
public const T_NONE = 1;
public const T_INTEGER = 2;
public const T_STRING = 3;
public const T_INPUT_PARAMETER = 4;
public const T_FLOAT = 5;
public const T_CLOSE_PARENTHESIS = 6;
public const T_OPEN_PARENTHESIS = 7;
public const T_COMMA = 8;
public const T_DIVIDE = 9;
public const T_DOT = 10;
public const T_EQUALS = 11;
public const T_GREATER_THAN = 12;
public const T_LOWER_THAN = 13;
public const T_MINUS = 14;
public const T_MULTIPLY = 15;
public const T_NEGATE = 16;
public const T_PLUS = 17;
public const T_OPEN_CURLY_BRACE = 18;
public const T_CLOSE_CURLY_BRACE = 19;

// All tokens that are identifiers or keywords that could be considered as identifiers should be >= 100
public const T_ALIASED_NAME = 100;
public const T_FULLY_QUALIFIED_NAME = 101;
public const T_IDENTIFIER = 102;

// All keyword tokens should be >= 200
public const T_ALL = 200;
public const T_AND = 201;
public const T_ANY = 202;
public const T_AS = 203;
public const T_ASC = 204;
public const T_AVG = 205;
public const T_BETWEEN = 206;
public const T_BOTH = 207;
public const T_BY = 208;
public const T_CASE = 209;
public const T_COALESCE = 210;
public const T_COUNT = 211;
public const T_DELETE = 212;
public const T_DESC = 213;
public const T_DISTINCT = 214;
public const T_ELSE = 215;
public const T_EMPTY = 216;
public const T_END = 217;
public const T_ESCAPE = 218;
public const T_EXISTS = 219;
public const T_FALSE = 220;
public const T_FROM = 221;
public const T_GROUP = 222;
public const T_HAVING = 223;
public const T_HIDDEN = 224;
public const T_IN = 225;
public const T_INDEX = 226;
public const T_INNER = 227;
public const T_INSTANCE = 228;
public const T_IS = 229;
public const T_JOIN = 230;
public const T_LEADING = 231;
public const T_LEFT = 232;
public const T_LIKE = 233;
public const T_MAX = 234;
public const T_MEMBER = 235;
public const T_MIN = 236;
public const T_NEW = 237;
public const T_NOT = 238;
public const T_NULL = 239;
public const T_NULLIF = 240;
public const T_OF = 241;
public const T_OR = 242;
public const T_ORDER = 243;
public const T_OUTER = 244;
public const T_PARTIAL = 245;
public const T_SELECT = 246;
public const T_SET = 247;
public const T_SOME = 248;
public const T_SUM = 249;
public const T_THEN = 250;
public const T_TRAILING = 251;
public const T_TRUE = 252;
public const T_UPDATE = 253;
public const T_WHEN = 254;
public const T_WHERE = 255;
public const T_WITH = 256;

/**
* Creates a new query scanner object.
*
* @param string $input A query string.
*/
public function __construct($input)
{
$this->setInput($input);
}

/**
* {@inheritdoc}
*/
protected function getCatchablePatterns()
{
return [
'[a-z_][a-z0-9_]*\:[a-z_][a-z0-9_]*(?:\\\[a-z_][a-z0-9_]*)*', // aliased name
'[a-z_\\\][a-z0-9_]*(?:\\\[a-z_][a-z0-9_]*)*', // identifier or qualified name
'(?:[0-9]+(?:[\.][0-9]+)*)(?:e[+-]?[0-9]+)?', // numbers
"'(?:[^']|'')*'", // quoted strings
'\?[0-9]*|:[a-z_][a-z0-9_]*', // parameters
];
}

/**
* {@inheritdoc}
*/
protected function getNonCatchablePatterns()
{
return ['\s+', '(.)'];
}

/**
* {@inheritdoc}
*/
protected function getType(&$value)
{
$type = self::T_NONE;

switch (true) {
// Recognize numeric values
case (is_numeric($value)):
if (strpos($value, '.') !== false || stripos($value, 'e') !== false) {
return self::T_FLOAT;
}

return self::T_INTEGER;

// Recognize quoted strings
case ($value[0] === "'"):
$value = str_replace("''", "'", substr($value, 1, strlen($value) - 2));

return self::T_STRING;

// Recognize identifiers, aliased or qualified names
case (ctype_alpha($value[0]) || $value[0] === '_' || $value[0] === '\\'):
$name = 'Doctrine\ORM\Query\Lexer::T_' . strtoupper($value);

if (defined($name)) {
$type = constant($name);

if ($type > 100) {
return $type;
}
}

if (strpos($value, ':') !== false) {
return self::T_ALIASED_NAME;
}

if (strpos($value, '\\') !== false) {
return self::T_FULLY_QUALIFIED_NAME;
}

return self::T_IDENTIFIER;

// Recognize input parameters
case ($value[0] === '?' || $value[0] === ':'):
return self::T_INPUT_PARAMETER;

// Recognize symbols
case ($value === '.'):
return self::T_DOT;
case ($value === ','):
return self::T_COMMA;
case ($value === '('):
return self::T_OPEN_PARENTHESIS;
case ($value === ')'):
return self::T_CLOSE_PARENTHESIS;
case ($value === '='):
return self::T_EQUALS;
case ($value === '>'):
return self::T_GREATER_THAN;
case ($value === '<'):
return self::T_LOWER_THAN;
case ($value === '+'):
return self::T_PLUS;
case ($value === '-'):
return self::T_MINUS;
case ($value === '*'):
return self::T_MULTIPLY;
case ($value === '/'):
return self::T_DIVIDE;
case ($value === '!'):
return self::T_NEGATE;
case ($value === '{'):
return self::T_OPEN_CURLY_BRACE;
case ($value === '}'):
return self::T_CLOSE_CURLY_BRACE;

// Default
default:
// Do nothing
}

return $type;
}
}

This is roughly what the DQL Parser looks like that uses the above
Lexer implementation:

.. note::

You can see the full implementation `here <https://github.com/doctrine/doctrine2/blob/master/lib/Doctrine/ORM/Query/Parser.php>`_.

.. code-block:: php

class Parser
{
private $lexer;

public function __construct($dql)
{
$this->lexer = new Lexer();
$this->lexer->setInput($dql);
}

// ...

public function getAST()
{
// Parse & build AST
$AST = $this->QueryLanguage();

// ...

return $AST;
}

public function QueryLanguage()
{
$this->lexer->moveNext();

switch ($this->lexer->lookahead['type']) {
case Lexer::T_SELECT:
$statement = $this->SelectStatement();
break;
case Lexer::T_UPDATE:
$statement = $this->UpdateStatement();
break;
case Lexer::T_DELETE:
$statement = $this->DeleteStatement();
break;
default:
$this->syntaxError('SELECT, UPDATE or DELETE');
break;
}

// Check for end of string
if ($this->lexer->lookahead !== null) {
$this->syntaxError('end of string');
}

return $statement;
}

// ...
}

Now the AST is used to transform the DQL query in to portable SQL for whatever relational
database you are using!

.. code-block:: php

$parser = new Parser('SELECT u FROM User u');
$AST = $parser->getAST(); // returns \Doctrine\ORM\Query\AST\SelectStatement

What is an AST?
===============

AST stands for `Abstract syntax tree <http://en.wikipedia.org/wiki/Abstract_syntax_tree>`_.
In computer science, an abstract syntax tree (AST), or just syntax tree, is a
tree representation of the abstract syntactic structure of source code written
in a programming language. Each node of the tree denotes a construct occurring in
the source code.