Error: Cannot update / add item in 1.8.4

[error404]

hi all.
I've installed zabbix ver 1.8.4.
When I want to add new item to the existing or new template I get next error:

preg_match(): Compilation failed: unknown option bit(s) set at offset 0[/srv/http/zabbix/include/items.inc.php:316]
Incorrect key format 'key_name[param1,param2,...]'

I input the next parameters for new item:
type: snmp v2 agent
SNMP OID: interfaces.ifTable.ifEntry.ifInOctets.1
key: ifInOctets.1

I get the same error (only string number changes) if I just open the existing item in any template and press save button without any changes.

preg_match(): Compilation failed: unknown option bit(s) set at offset 0[/srv/http/zabbix/include/items.inc.php:528]
Incorrect key format 'key_name[param1,param2,...]'

items.inc.php, string 316:
function add_item($item){
...
if(!preg_match('/^'.ZBX_PREG_ITEM_KEY_FORMAT.'$/u', $item['key_']) ){

items.inc.php, string 528:
// Update Item status
......
if( !preg_match('/^'.ZBX_PREG_ITEM_KEY_FORMAT.'$/u', $item['key_']) ){


Have you got any suggestions what to check?