PathFormBase::validateForm

public PathFormBase::validateForm(array &$form, FormStateInterface $form_state)

public function validateForm(array &$form, FormStateInterface $form_state) {
  $source = &$form_state->getValue('source');
  $source = $this->aliasManager->getPathByAlias($source);
  $alias = &$form_state->getValue('alias');

  // Trim the submitted value of whitespace and slashes. Ensure to not trim
  // the slash on the left side.
  $alias = rtrim(trim(trim($alias), ''), "\\/");

  if ($source[0] !== '/') {
    $form_state->setErrorByName('source', 'The source path has to start with a slash.');
  }
  if ($alias[0] !== '/') {
    $form_state->setErrorByName('alias', 'The alias path has to start with a slash.');
  }

  // Language is only set if language.module is enabled, otherwise save for all
  // languages.
  $langcode = $form_state->getValue('langcode', LanguageInterface::LANGCODE_NOT_SPECIFIED);

  if ($this->aliasStorage->aliasExists($alias, $langcode, $this->path['source'])) {
    $stored_alias = $this->aliasStorage->load(['alias' => $alias, 'langcode' => $langcode]);
    if ($stored_alias['alias'] !== $alias) {
      // The alias already exists with different capitalization as the default
      // implementation of AliasStorageInterface::aliasExists is
      // case-insensitive.
      $form_state->setErrorByName('alias', t('The alias %alias could not be added because it is already in use in this language with different capitalization: %stored_alias.', [
        '%alias' => $alias,
        '%stored_alias' => $stored_alias['alias'],
      ]));
    }
    else {
      $form_state->setErrorByName('alias', t('The alias %alias is already in use in this language.', ['%alias' => $alias]));
    }
  }


  if (!$this->pathValidator->isValid(trim($source, '/'))) {
    $form_state->setErrorByName('source', t("The path '@link_path' is either invalid or you do not have access to it.", array('@link_path' => $source)));
  }
}