I want to copy something to a register or more ideally the system clipboard and then run a search / replace regex on it the way that I do in the buffer...is that possible?
Assume you copied the following line to the register
Copied to clipboard.
Now you want to replace "clipboard" with "register" inside the register
:let @+=substitute(@+, 'clipboard', 'register', 'g')
@+ represents the register
@@ is the unnamed register,
@a is register
a,... . See
substitute(...) is processing a string (here from reg
:s/clipboard/register/g would process the line in the file.