Bnf parser and story generator

bnf-story-parser.php

<?php

class BNFParser {

    private $unparsedString;

    private $defs = [];

    public function __construct(string $bnfString = null)
    {
        $this->unparsedString = $bnfString;
        if($this->unparsedString) {
            $this->parseString($bnfString);
        }
    }

    public function parseString($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;
        }

    }
    public function getRandomWeightedElement(array $weightedValues) {
        $rand = mt_rand(1, (int) array_sum($weightedValues));

        foreach ($weightedValues as $key => $value) {
            $rand -= $value;
            if ($rand <= 0) {
                return $key;
            }
        }
        return null;
    }

    public function generateString(string $startGeneratorName): string {

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

        $key = array_rand($this->defs[$startGeneratorName]);

        $phrase = $this->defs[$startGeneratorName][$key];
        $re = '/(?<!\\\\)<(.*?)(?<!\\\\)>/';
        $result = preg_replace_callback($re, function($match)  {
                return $this->generateString($match[1]);
        }, $phrase);
        $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 BNFParser();

$parser->parseString($bnfString);

$result = '';

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