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function validate_oib_field($event_id) {
    // Fetch the event data
    $event_data = get_post_meta($event_id, 'mec', true);

    // Check if OIB field exists in the event data
    if (isset($event_data['oib'])) {
        $oib = $event_data['oib'];

        // Validate the OIB length
        if (strlen($oib) !== 11) {
            // Prevent the event from being saved and return an error
            wp_die(__('OIB must be exactly 11 characters.'));
        }
    }
    return true;
}

add_action('mec_before_event_save', 'validate_oib_field');