priorities of ( )

Simple question

If the Header ABC exists then I want to use it, if it doesn't exist then I  
want to use XYZ but I don't ever want to use XYZ if ABC exists in the same  
message

Right now I have two recipes, which are identical except the first one  
checks for ABC and the second for XYZ

Will

* ^(ABC|XYZ)

always prefer ABC and only use XYZ if ABC doesn't exist, or will it use XYZ  
if it comes first (higher in the headers)?

Thanks

TjL