Provided you have a good collection of bad usernames, this should do the trick:

$sth = $dbh->prepare ("SELECT count(*) FROM forbidden_usernames WHERE 
+f_user LIKE ?");
$sth->execute($str);
($bad) = $sth->fetchrow_array;
print "Bad username\n" if $bad;
[download]

Warning: untested code! It's too late to cobble a mySQL database together to test it and my "Programming the Perl DBI" is upstairs.

CountZero

"If you have four groups working on a compiler, you'll get a 4-pass compiler." - Conway's Law