Callback alpha with characters

[eluser]someone [/eluser]
Hello,

I'm looking for callback function which accept characters (čćžšđ). If I use default CI's set_rules function I got error message, so I would like to create some callback function with preg_match.

This is piece of default code for alpha function in CI:

Code:

... return ( ! preg_match("/^([a-z])+$/i", $str)) ...

What I must add to get characters čćžšđ?

Thanks! :-)