DTD attribute enumeration

hi, have a little problem with attribute enumeration in DTD im hoping someone can help me with.

<!ELEMENT movie_classification EMPTY>
<!ATTLIST movie_classification
rating (General Exhibition|Parental Guidance|Mature|Mature Adult|Restricted) #REQUIRED>

simply will not validate. is there a restriction on how long attlist enumeration list can be? or a particular word that shouldn't be in there? i tried declaring it as entity:

<!ENTITY G "General Exhibition">
<!ENTITY PG "Parental Guide">
<!ENTITY M "Mature">
<!ENTITY MA "Mature Adult">
<!ENTITY R "Restricted">

then use:
<!ATTLIST movie_classification
rating (&G;|&PG;|&M;|&MA;|&R;) #REQUIRED>

that doesnt work either. not sure what to do. help please.

Spaces are not allowed in the attribute enumeration list. There is probably a way to do this, but you could just try something like what I have included below:

<?xml version="1.0"?>
<!DOCTYPE movie_classification[
<!ELEMENT movie_classification EMPTY>
<!ATTLIST movie_classification rating (General_Exhibition|Parental_Guidance|Mature|Mature_Adult|Restricted) #REQUIRED>]>
<movie_classification rating="Mature" />

http://www.w3schools.com/dtd/dtd_attributes.asp

thanks. that explains it.