Using gsub to strip multiple characters

Question

I have an object which contains the following string "XXXXyyyy!!!!zzzz"

XXXX - never the same, will always change, it may be a million characters long
yyyy - will never change, exact number of characters all the time
!!!! - this is the data I want to result with after I clean the string. This data will always change as well
zzzz - will never change, exact number of characters all the time

so if:

string = "XXXXyyyy!!!!zzzz"  
string.gsub("zzzz","")  
# => "XXXXyyyy!!!!"

What can I do next to get just "!!!!". How can I use 'yyyy', which is static, to erase "XXXXyyyy" and leave only "!!!!". I hope this explains my question. Sorry about the last one.