Online PHP editor | refs for iS8Ih

<?php function checkOperation(string $operation): ?string { if (!preg_match('/^\d+([a-zA-Z]+)\+\d+([a-zA-Z]+)$/', $operation, $matches)) { return null; } if (array_count_values(str_split($matches[1])) != array_count_values(str_split($matches[2]))) { return null; } return $operation; } var_dump(checkOperation('5p+3p')); echo PHP_EOL; var_dump(checkOperation('5AB+3AB')); echo PHP_EOL; var_dump(checkOperation('5AB+3BA')); echo PHP_EOL; var_dump(checkOperation('5p+3q'));