MachineName::validateMachineName

public static MachineName::validateMachineName(&$element, FormStateInterface $form_state, &$complete_form)

public static function validateMachineName(&$element, FormStateInterface $form_state, &$complete_form) {
  // Verify that the machine name not only consists of replacement tokens.
  if (preg_match('@^' . $element['#machine_name']['replace'] . '+$@', $element['#value'])) {
    $form_state->setError($element, t('The machine-readable name must contain unique characters.'));
  }

  // Verify that the machine name contains no disallowed characters.
  if (preg_match('@' . $element['#machine_name']['replace_pattern'] . '@', $element['#value'])) {
    if (!isset($element['#machine_name']['error'])) {
      // Since a hyphen is the most common alternative replacement character,
      // a corresponding validation error message is supported here.
      if ($element['#machine_name']['replace'] == '-') {
        $form_state->setError($element, t('The machine-readable name must contain only lowercase letters, numbers, and hyphens.'));
      }
      // Otherwise, we assume the default (underscore).
      else {
        $form_state->setError($element, t('The machine-readable name must contain only lowercase letters, numbers, and underscores.'));
      }
    }
    else {
      $form_state->setError($element, $element['#machine_name']['error']);
    }
  }

  // Verify that the machine name is unique.
  if ($element['#default_value'] !== $element['#value']) {
    $function = $element['#machine_name']['exists'];
    if (call_user_func($function, $element['#value'], $element, $form_state)) {
      $form_state->setError($element, t('The machine-readable name is already in use. It must be unique.'));
    }
  }
}