Arrays - undefined offset warning

Can anyone tell me why the running the following code produces an 'undefined offset error'':

<?php
function emailsyntax_is_valid($email) {
// ERROR IS CAUSED BY THE FOLLOWING LINE:
list($local, $domain) = explode("@", $email);

$pattern_local = '^([0-9a-z]*([-|_]?[0-9a-z]+)*)(([-|_]?)\.([-|_]?)[0-9a-z]*([-|_]?[0-9a-z]+)+)*([-|_]?)$';
$pattern_domain = '^([0-9a-z]+([-]?[0-9a-z]+)*)(([-]?)\.([-]?)[0-9a-z]*([-]?[0-9a-z]+)+)*\.[a-z]{2,4}$';

$match_local = eregi($pattern_local, $local);
$match_domain = eregi($pattern_domain, $domain);

if ($match_local && $match_domain) {
return 1;
} else {
return 0;
}
}
?>