eregi Woes

[influx]

Hi there all,

When users sign up for my web page, I only allow their usernames to contain letters and numbers (no spaces)..however, I want them to be able to have underscores (_) as well. How can I modify my eregi statement to allow underscores as well?

if(!eregi("^[a-z0-9]+$",$user)) //check if username is valid, numbers and letters only, if not redirect and give error code 3
{
	header('Location: register.php?code=3');
	exit;
}

Thanks in advance.
-influx

[Installer]

$allowed = array_merge(range('a', 'z'), range('A', 'Z'), array('_'));
for ($i = 0; $i < strlen($str); $i++) {
    if (!in_array($str{$i}, $allowed)) {
        header('Location: phpinfo.php'); 
    }
}

[mtmosier]

if(!eregi("^[a-z0-9_]+$",$user))