vim vnoremap: E488 Trailing characters

Question

I'm using vim to program and I just want to make a shortcut for comment.

Here is how I set in .vimrc :

vnoremap <F7> :%s/^/\/\//g

I just want to add // in front of each selected line. However, when I press <F7> and press Enter in visual mode, I get an error:

E488 Trailing characters

Answer 1

Note that when you press F7 it just simulates pressing all the keys in the string. As soon as it presses : it gets into a state

:'<,'>

When it then types in all the rest of your command it gets into:

:'<,'>%s/^/\/\//g

Which is meaningless ( % after '<,'> doesn't make sense). If you just remove % from your command, it will already work. Even better, add <CR> at the end so that you don't need to press Enter:

vnoremap <F7> :s/^/\/\//g<CR>