Regex in hive

in your databases have many fields but one filed have data something like this: a1955ce0b318391b contain only a-z0-9 but some errors occurs while collect and store to database somehow that field contains null values or some dummy charracter how to filter which entry errors use this regex:
> select idvisitor from log where day = '2013-11-29' and idvisitor not rlike '^[a-z0-9]{16}$';
what does it means in hive?
rlike mean: regular expression like to use regex in hive query.
^ : from start of line
[a-z0-9] only contain a-z or 0-9 without order.
{16} contains exact 16 characters.
$ end of line.
It save my day.
:)