I wish to use the content of a register as search pattern in Vim.
I would like to do this from the command line so I cannot use the <c-r> syntax since that assumes an interactive session.
It is possible to use a register as a replace pattern, like this
:%s/foo/\=@a/g
However using this syntax as a search pattern does not work
:%s/\=@a/foo/g
it outputs
E64: \= follows nothing
E476: Invalid command