FormElement::validatePattern

public static FormElement::validatePattern(&$element, FormStateInterface $form_state, &$complete_form)

public static function validatePattern(&$element, FormStateInterface $form_state, &$complete_form) {
  if ($element['#value'] !== '') {
    // The pattern must match the entire string and should have the same
    // behavior as the RegExp object in ECMA 262.
    // - Use bracket-style delimiters to avoid introducing a special delimiter
    //   character like '/' that would have to be escaped.
    // - Put in brackets so that the pattern can't interfere with what's
    //   prepended and appended.
    $pattern = '{^(?:' . $element['#pattern'] . ')$}';

    if (!preg_match($pattern, $element['#value'])) {
      $form_state->setError($element, t('%name field is not in the right format.', array('%name' => $element['#title'])));
    }
  }
}