For some reason this ereg won't accept like dd in the username. Can some one explain to me what this does allow?

PHP Code:

if(ereg("^[a-zA-Z0-9[:space:] [.[.] [.].] \_ \{ \} -]{3,12}$", $username)) 
{ 


$username must be between 3 and 12 characters in length alphanumeric, with or without a space, hyphen, underscore...