- preg_match: documentation ( source)
<?php
$usernames = array('asdasd123', 'asdas_asdasd123', 'asdsad--asdsa123123', 'asdasd<><><>');
foreach($usernames as $u)
{
if(preg_match('([A-Za-z0-9\-\_]+)', $u))
{
echo $u.' is totally valid';
}
else
{
echo $u. 'is not valid';
}
}