RegEx for Twitter username not working

Question

The following regular expression function for validating a Twitter username is not working because a twitter name can be a minimum of 1 character and a maximum of 20. However, when I tested this, it allows usernames greater than 20 characters. Where did I go wrong?

public function val_username($subject)
{
    return (bool)preg_match('/[a-zA-Z0-9_]{1,20}/', $subject);      
}

Answer 1

You forgot the $ and ^

/^[a-zA-Z0-9_]{1,20}$/ should work

public function val_username($subject)
{
    return (bool)preg_match('/^[a-zA-Z0-9_]{1,20}$/', $subject);      
}