109 lines
3.4 KiB
ReStructuredText
109 lines
3.4 KiB
ReStructuredText
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Parser Library
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==============
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This library allows you to easily implement recursive-descent parsers.
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Installation
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------------
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You can install this library through composer:
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.. code-block :: bash
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composer require jms/parser-lib
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or add it to your ``composer.json`` file directly.
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Example
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-------
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Let's assume that you would like to write a parser for a calculator. For simplicity
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sake, we will assume that the parser would already return the result of the
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calculation. Inputs could look like this ``1 + 1`` and we would expect ``2`` as
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a result.
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The first step, is to create a lexer which breaks the input string up into
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individual tokens which can then be consumed by the parser. This library provides
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a convenient class for simple problems which we will use::
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$lexer = new \JMS\Parser\SimpleLexer(
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'/
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# Numbers
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([0-9]+)
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# Do not surround with () because whitespace is not meaningful for
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# our purposes.
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|\s+
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# Operators; we support only + and -
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|(+)|(-)
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/x', // The x modifier tells PCRE to ignore whitespace in the regex above.
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// This maps token types to a human readable name.
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array(0 => 'T_UNKNOWN', 1 => 'T_INT', 2 => 'T_PLUS', 3 => 'T_MINUS'),
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// This function tells the lexer which type a token has. The first element is
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// an integer from the map above, the second element the normalized value.
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function($value) {
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if ('+' === $value) {
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return array(2, '+');
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}
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if ('-' === $value) {
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return array(3, '-');
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}
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if (is_numeric($value)) {
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return array(1, (integer) $value);
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}
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return array(0, $value);
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}
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);
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Now the second step, is to create the parser which can consume the tokens once
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the lexer has split them::
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class MyParser extends \JMS\Parser\AbstractParser
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{
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const T_UNKNOWN = 0;
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const T_INT = 1;
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const T_PLUS = 2;
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const T_MINUS = 3;
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public function parseInternal()
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{
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$result = $this->match(self::T_INT);
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while ($this->lexer->isNextAny(array(self::T_PLUS, self::T_MINUS))) {
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if ($this->lexer->isNext(self::T_PLUS)) {
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$this->lexer->moveNext();
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$result += $this->match(self::T_INT);
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} else if ($this->lexer->isNext(self::T_MINUS)) {
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$this->lexer->moveNext();
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$result -= $this->match(self::T_INT);
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} else {
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throw new \LogicException('Previous ifs were exhaustive.');
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}
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}
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return $result;
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}
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}
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$parser = new MyParser($lexer);
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$parser->parse('1 + 1'); // int(2)
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$parser->parse('5 + 10 - 4'); // int(11)
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That's it. Now you can perform basic operations already. If you like you can now
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also replace the hard-coded integers in the lexer with the class constants of the
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parser.
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License
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-------
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The code is released under the business-friendly `Apache2 license`_.
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Documentation is subject to the `Attribution-NonCommercial-NoDerivs 3.0 Unported
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license`_.
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.. _Apache2 license: http://www.apache.org/licenses/LICENSE-2.0.html
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.. _Attribution-NonCommercial-NoDerivs 3.0 Unported license: http://creativecommons.org/licenses/by-nc-nd/3.0/
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