Regular expression functions like preg_match or eregi (the former being preferred).
$string = "abc1";
if( preg_match( '/\d/', $string ))
{
$errors['username'] = 'The username field must not contain any numbers';
}
Have a quick search around this forum for email validation using regular expression - there are lots of ways to do it.
Curses laserlight..curses! :xbones: