The J4 Rest API includes the ability to apply filters when GETing existing users. However, the documentation does not list any examples of how to do so.

By posting on the Joomla forum on StackExchange, I was able to get an example which I tested and it worked:

/api/index.php/v1/users?filter[search]=montgomery-scott&filter[active]=1

It would be great if this could be added to the API documentation. The code shows the wide variety of filtering syntax available:

`public function displayList()
{
$apiFilterInfo = $this->input->get('filter', [], 'array');
$filter = InputFilter::getInstance();

    if (\array_key_exists('state', $apiFilterInfo)) {
        $this->modelState->set('filter.state', $filter->clean($apiFilterInfo['state'], 'INT'));
    }

    if (\array_key_exists('active', $apiFilterInfo)) {
        $this->modelState->set('filter.active', $filter->clean($apiFilterInfo['active'], 'INT'));
    }

    if (\array_key_exists('groupid', $apiFilterInfo)) {
        $this->modelState->set('filter.group_id', $filter->clean($apiFilterInfo['groupid'], 'INT'));
    }

    if (\array_key_exists('search', $apiFilterInfo)) {
        $this->modelState->set('filter.search', $filter->clean($apiFilterInfo['search'], 'STRING'));
    }

    if (\array_key_exists('registrationDateStart', $apiFilterInfo)) {
        $registrationStartInput = $filter->clean($apiFilterInfo['registrationDateStart'], 'STRING');
        $registrationStartDate  = Date::createFromFormat(\DateTimeInterface::RFC3339, $registrationStartInput);

        if (!$registrationStartDate) {
            // Send the error response
            $error = Text::sprintf('JLIB_FORM_VALIDATE_FIELD_INVALID', 'registrationDateStart');

            throw new InvalidParameterException($error, 400, null, 'registrationDateStart');
        }

        $this->modelState->set('filter.registrationDateStart', $registrationStartDate);
    }

    if (\array_key_exists('registrationDateEnd', $apiFilterInfo)) {
        $registrationEndInput = $filter->clean($apiFilterInfo['registrationDateEnd'], 'STRING');
        $registrationEndDate  = Date::createFromFormat(\DateTimeInterface::RFC3339, $registrationEndInput);

        if (!$registrationEndDate) {
            // Send the error response
            $error = Text::sprintf('JLIB_FORM_VALIDATE_FIELD_INVALID', 'registrationDateEnd');
            throw new InvalidParameterException($error, 400, null, 'registrationDateEnd');
        }

        $this->modelState->set('filter.registrationDateEnd', $registrationEndDate);
    } elseif (
        \array_key_exists('registrationDateStart', $apiFilterInfo)
        && !\array_key_exists('registrationDateEnd', $apiFilterInfo)
    ) {
        // If no end date specified the end date is now
        $this->modelState->set('filter.registrationDateEnd', new Date());
    }

    if (\array_key_exists('lastVisitDateStart', $apiFilterInfo)) {
        $lastVisitStartInput = $filter->clean($apiFilterInfo['lastVisitDateStart'], 'STRING');
        $lastVisitStartDate  = Date::createFromFormat(\DateTimeInterface::RFC3339, $lastVisitStartInput);

        if (!$lastVisitStartDate) {
            // Send the error response
            $error = Text::sprintf('JLIB_FORM_VALIDATE_FIELD_INVALID', 'lastVisitDateStart');
            throw new InvalidParameterException($error, 400, null, 'lastVisitDateStart');
        }

        $this->modelState->set('filter.lastVisitStart', $lastVisitStartDate);
    }

    if (\array_key_exists('lastVisitDateEnd', $apiFilterInfo)) {
        $lastVisitEndInput = $filter->clean($apiFilterInfo['lastVisitDateEnd'], 'STRING');
        $lastVisitEndDate  = Date::createFromFormat(\DateTimeInterface::RFC3339, $lastVisitEndInput);

        if (!$lastVisitEndDate) {
            // Send the error response
            $error = Text::sprintf('JLIB_FORM_VALIDATE_FIELD_INVALID', 'lastVisitDateEnd');

            throw new InvalidParameterException($error, 400, null, 'lastVisitDateEnd');
        }

        $this->modelState->set('filter.lastVisitEnd', $lastVisitEndDate);
    } elseif (
        \array_key_exists('lastVisitDateStart', $apiFilterInfo)
        && !\array_key_exists('lastVisitDateEnd', $apiFilterInfo)
    ) {
        // If no end date specified the end date is now
        $this->modelState->set('filter.lastVisitEnd', new Date());
    }

    return parent::displayList();
}`