BNF form parser, exampled with story generating bnf

bnf-story-parser-fixed.php

<?php

class BNFParserGenerator {

    private $defs = [];

    /**
     * This converts the BNF string into a set of generator definitions
     * that we can use later
     * @param string $string
     */
    public function parseString(string $string) {
        $string = str_replace("\\n", 'NEW_LINE', $string);
        $split = array_filter(explode("\n", $string));
        $exploded = array_map(function($gen) {
            $gen = str_replace("NEW_LINE", "\n", $gen);
            return explode("::=", trim($gen));
        }, $split);

        foreach ($exploded as $item) {
            $matches = [];
            preg_match('/<([A-z\-]+)>/', $item[0], $matches);
            $generators = explode("|", $item[1]);
            $generators = array_map(function($gen) {
                return trim($gen);
            }, $generators);
            $this->defs[$matches[1]] = $generators;
        }

    }

    /**
     * This creates whole new strings based on the grammar
     * @param string $startGeneratorName
     * @return string
     */
    public function generateString(string $startGeneratorName): string {

        if(!isset($this->defs[$startGeneratorName])) {
            return 'Generator Doesnt Exist';
        }

        //get a random key from the set of tokens that the generator can make
        $key = array_rand($this->defs[$startGeneratorName]);
        $phrase = $this->defs[$startGeneratorName][$key];

        //regex to potentially find replaceable elements in the selected token
        $re = '/(?<!\\\\)<(.*?)(?<!\\\\)>/';

        //Does a callback on all regex matches and replaces them with the generator
        //aka recurse recurse
        $result = preg_replace_callback($re, function($match)  {
                return $this->generateString($match[1]);
        }, $phrase);

        //dont need this and could fix grammar to not have '' but pfft
        $result = str_replace("'", "", $result);

        return $result;
    }
}


$bnfString = "
    <plot> ::= <plot-point>. <plot> | <plot-point>. <transition><plot-point>.<plot> | <plot-point>.
    <plot-point> ::= <char-action><necessities><character><about> | <char-action>
    <char-action> ::= <character><action><about>
    <necessities> ::= '-' | '->' | <about> | '+' | '*'
    <action> ::= '1' | '2' | '3' | '4' |'5' | ''
    <about> ::= [<character>] | [<character>+<character>]  | ''
    <character> ::= 'A'|'B'|'C'|'D'|'E'|'Z'
    <transition> ::= '\\n=\\n' | '\\n...\\n' | ''
";

$parser = new BNFParserGenerator();

$parser->parseString($bnfString);

$result = '';

echo $parser->generateString('plot')."\n";