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:

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<?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(string $input)
    {
        $this->setInput($input);
    }

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

    /**
     * {@inheritdoc}
     */
    protected function getType(&$value): int
    {
        $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;
    }
}
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This is roughly what the DQL Parser looks like that uses the above Lexer implementation:

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<?php

class Parser
{
    private $lexer;

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

    // ...

    public function getAST(): AST\SelectStatement|AST\UpdateStatement|AST\DeleteStatement
    {
        // Parse & build AST
        $AST = $this->QueryLanguage();

        // ...

        return $AST;
    }

    public function QueryLanguage(): AST\SelectStatement|AST\UpdateStatement|AST\DeleteStatement
    {
        $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');
        }

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

        return $statement;
    }

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

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<?php

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

AST stands for 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.