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[EDIT] FILE: FrenchInflector.php

<?php

/*
 * This file is part of the Symfony package.
 *
 * (c) Fabien Potencier <fabien@symfony.com>
 *
 * For the full copyright and license information, please view the LICENSE
 * file that was distributed with this source code.
 */

namespace Symfony\Component\String\Inflector;

/**
 * French inflector.
 *
 * This class does only inflect nouns; not adjectives nor composed words like "soixante-dix".
 */
final class FrenchInflector implements InflectorInterface
{
    /**
     * A list of all rules for pluralise.
     *
     * @see https://la-conjugaison.nouvelobs.com/regles/grammaire/le-pluriel-des-noms-121.php
     */
    private const PLURALIZE_REGEXP = [
        // First entry: regexp
        // Second entry: replacement

// Words finishing with "s", "x" or "z" are invariables
        // Les mots finissant par "s", "x" ou "z" sont invariables
        ['/(s|x|z)$/i', '\1'],

// Words finishing with "eau" are pluralized with a "x"
        // Les mots finissant par "eau" prennent tous un "x" au pluriel
        ['/(eau)$/i', '\1x'],

// Words finishing with "au" are pluralized with a "x" excepted "landau"
        // Les mots finissant par "au" prennent un "x" au pluriel sauf "landau"
        ['/^(landau)$/i', '\1s'],
        ['/(au)$/i', '\1x'],

// Words finishing with "eu" are pluralized with a "x" excepted "pneu", "bleu", "émeu"
        // Les mots finissant en "eu" prennent un "x" au pluriel sauf "pneu", "bleu", "émeu"
        ['/^(pneu|bleu|émeu)$/i', '\1s'],
        ['/(eu)$/i', '\1x'],

// Bijou, caillou, chou, genou, hibou, joujou et pou qui prennent un x au pluriel
        ['/^(bij|caill|ch|gen|hib|jouj|p)ou$/i', '\1oux'],

// Invariable words
        ['/^(cinquante|soixante|mille)$/i', '\1'],

/**
     * A list of all rules for singularize.
     */
    private const SINGULARIZE_REGEXP = [
        // First entry: regexp
        // Second entry: replacement

// Words finishing with "eau" are pluralized with a "x"
        // Les mots finissant par "eau" prennent tous un "x" au pluriel
        ['/(eau)x$/i', '\1'],

// Words finishing with "au" are pluralized with a "x" excepted "landau"
        // Les mots finissant par "au" prennent un "x" au pluriel sauf "landau"
        ['/(au)x$/i', '\1'],

// Words finishing with "eu" are pluralized with a "x" excepted "pneu", "bleu", "émeu"
        // Les mots finissant en "eu" prennent un "x" au pluriel sauf "pneu", "bleu", "émeu"
        ['/(eu)x$/i', '\1'],

//  Words finishing with "ou" are pluralized with a "s" excepted bijou, caillou, chou, genou, hibou, joujou, pou
        // Les mots finissant par "ou" prennent un "s" sauf bijou, caillou, chou, genou, hibou, joujou, pou
        ['/(bij|caill|ch|gen|hib|jouj|p)oux$/i', '\1ou'],

// French titles
        ['/^mes(dame|demoiselle)s$/', 'ma\1'],
        ['/^Mes(dame|demoiselle)s$/', 'Ma\1'],
        ['/^mes(sieur|seigneur)s$/', 'mon\1'],
        ['/^Mes(sieur|seigneur)s$/', 'Mon\1'],

// Default rule
        ['/s$/i', ''],
    ];

/**
     * A list of words which should not be inflected.
     * This list is only used by singularize.
     */
    private const UNINFLECTED = '/^(abcès|accès|abus|albatros|anchois|anglais|autobus|bois|brebis|carquois|cas|chas|colis|concours|corps|cours|cyprès|décès|devis|discours|dos|embarras|engrais|entrelacs|excès|fils|fois|gâchis|gars|glas|héros|intrus|jars|jus|kermès|lacis|legs|lilas|marais|mars|matelas|mépris|mets|mois|mors|obus|os|palais|paradis|parcours|pardessus|pays|plusieurs|poids|pois|pouls|printemps|processus|progrès|puits|pus|rabais|radis|recors|recours|refus|relais|remords|remous|rictus|rhinocéros|repas|rubis|sans|sas|secours|sens|souris|succès|talus|tapis|tas|taudis|temps|tiers|univers|velours|verglas|vernis|virus)$/i';

public function singularize(string $plural): array
    {
        if ($this->isInflectedWord($plural)) {
            return [$plural];
        }

foreach (self::SINGULARIZE_REGEXP as $rule) {
            [$regexp, $replace] = $rule;

if (1 === preg_match($regexp, $plural)) {
                return [preg_replace($regexp, $replace, $plural)];
            }
        }

return [$plural];
    }

public function pluralize(string $singular): array
    {
        if ($this->isInflectedWord($singular)) {
            return [$singular];
        }

foreach (self::PLURALIZE_REGEXP as $rule) {
            [$regexp, $replace] = $rule;

if (1 === preg_match($regexp, $singular)) {
                return [preg_replace($regexp, $replace, $singular)];
            }
        }

return [$singular.'s'];
    }

private function isInflectedWord(string $word): bool
    {
        return 1 === preg_match(self::UNINFLECTED, $word);
    }
}

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