I wanna check $username to make sure that it only contains letters and numbers and a underscore, anythig else giver error();

error() unless $username =~ m< ^ \w+ \z >x;
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Is there in way in SQL to select a row by random, so if it havd 1000 rows it picked row 200.

There isn't a generic way to pick a random row in MySQL, but if you've got a primary key which is an integer and you know the amount of rows you've got then you can just pick a random number and SELECT from that e.g

my $sth = $dbh->prepare(
               'SELECT some,rows FROM your_table WHERE id = ?'
          );
$sth->execute( 
    int rand $dbh->selectrow_array('SELECT MAX(*) FROM your_table')
);
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HTH

_________
broquaint

update: changed COUNT to MAX (thanks to UnderMine for that one)