Help with regex on a valid username

[cooldude832]

I'm trying to do a regex that will check to make sure a user name is >5 characters and only contains numbers/alphas/underscode.  Any ideas?

[wildteen88]

A simple bit of regex:

<?php

$username = 'c00l_Dude'; // valid
//$username = 'cooldude832'; // valid
//$username = 'c00l'; // Invalid
//$username = 'Cool-Dude'; // Invalid

if(preg_match('/^([A-Z0-9_]){5,}$/i', $username))
{
    echo $username . ' -  Valid';
}
else
{
    echo $username . ' -  Invalid';
}

?>