restricting input? [a-z] and [A-Z] and [0-9]

[bilis_money]

i'm planning to restrict user input.
I want only from character a-z, A-Z, and 0-9.

let say i have  a TEXT BOX with a name='topic',
and $topic_tmp = $_POST['topic'];

now i want $topic to accept the allowed character only how?
How could i do this on REGEX properly?


thanks PHP geeks in advance.

[corbin]

Lolz most people dont take the term geeks in a nice sense....  Anyways, I'm no regex expert (just starting messin with it today) but you could use something like
[code=php:0]
if (!preg_match("/[a-z0-9]$/i", $topic_tmp))
{
    die ("incorrect name.");
}
[/code]

[effigy]

[tt]/^[a-zA-Z0-9]+$/[/tt]