public static function username($x)
	{
		// Does it contain invalid characters?
		if (!preg_match("/^[0-9a-z]+[_0-9a-z]*$/", $x))
		{
			return false;
		}

		// Is it a positive integer?
		if (self::nonNegativeInteger($x))
		{
			return false;
		}

		// Is it a reserved username?
		if (self::reserved('username', $x))
		{
			return false;
		}

		return true;
	}