These are my tests:

# echo cold.case.S01E12.qweqwe |perl -pe 's/^(?:(.*)\.(\d+)x(\d+).*)|(?:(.*)\.S(\d+)E(\d+)).*$/\1 \2 \3 \4 \5 \6 \7/'
cold.case    01 12
#echo cold_case.1x12 | perl -pe 's/^(?:(.*)\.(\d+)x(\d+).*)|(?:(.*)\.S(\d+)E(\d+)).*$/\1 \2 \3 \4 \5 \6 \7/'
cold_case 1 12

Notice the three spaces in front of cold.case? It's because I can't get perl to do an alternate match. A friend came up with the following line:
perl -pe 's/(?:^(?<a>.*)\.(?<b>[0-9])+x(?<c>[0-9]+).*$)|(?:^(?<a>.*)\.S(?<b>[0-9]+)E(?<c>[0-9]+).*$)/$+{a} $+{b} $+{c}/'
Which works just fine, if you're running Perl 5.10, and I'm not! Or, it's not in the Gentoo repository yet. So I'll have to wait. Crap!

function checkname($string, $allowed) {
$allowed = "/[$allowed]/s";
$string = preg_replace($allowed, "", $string);
if ( $string )
return FALSE;
else
return TRUE;
}

So what you'll do is remove all allowed characters, and if there are any left in the string the user has chosen an invalid username. Ofcourse it's regexp that does the trick. Using the code:

if ( ! checkname($input, "A-Za-z0-9_\.") )
echo "Invalid username!";
else
echo "Ok, lets go!

We're allowing A-Z, a-z, 0-9, underscore and dots in the username.