class IssnValidator

IssnValidatorTest.php in drupal/core/vendor/symfony/validator/Symfony/Component/Validator/Tests/Constraints/IssnValidatorTest.php

class IssnValidator extends ConstraintValidator {

  /**
   * {@inheritDoc}
   */
  public function validate($value, Constraint $constraint) {
    if (null === $value || '' === $value) {
      return;
    }
    if (!is_scalar($value) && !(is_object($value) && method_exists($value, '__toString'))) {
      throw new UnexpectedTypeException($value, 'string');
    }

    // Compose regex pattern
    $digitsPattern = $constraint->requireHyphen ? '\\d{4}-\\d{3}' : '\\d{4}-?\\d{3}';
    $checksumPattern = $constraint->caseSensitive ? '[\\d|X]' : '[\\d|X|x]';
    $pattern = "/^" . $digitsPattern . $checksumPattern . "\$/";
    if (!preg_match($pattern, $value)) {
      $this->context
        ->addViolation($constraint->message);
    }
    else {
      $digits = str_split(strtoupper(str_replace('-', '', $value)));
      $sum = 0;
      for ($i = 8; $i > 1; $i--) {
        $sum += $i * (int) array_shift($digits);
      }
      $checksum = 'X' == reset($digits) ? 10 : (int) reset($digits);
      if (0 != ($sum + $checksum) % 11) {
        $this->context
          ->addViolation($constraint->message);
      }
    }
  }

}