Replacing a blank space between two capital letters with an underscore

I have characters in a string that look like this:

" A E 222;CMPSC 201 orCMPSC 202" 

What I want to do is make is so that it looks like this:

" A_E_222;CMPSC_201 orCMPSC_202" 

So far I tried the following code but it only puts an underscore in front or in back so I'm not sure what else to try.

str_replace_all(x, "([A-Z][:blank:][A-Z])", "\\1_")

str_replace_all(x, "([A-Z][:blank:][:digit:])", "([A-Z][:digit:])")