How do I use regular expressions in ternary statements / keymappings?

Question

I'd like to create a keymapping with a conditional statement, so that it behaves differently when a given line is blank.

This thread gave me the following snippet, which is great – except that it doesn't match lines that are blank-except-for-a-comment-marker. I'd like to capture those lines, too.

inoremap <expr> [KEY SEQUENCE] getline(line(".")+1) =~ "^$" ? "[COMMAND]" : "[COMMAND]"

So I set out to build a regular expression. '\v^(#|"|*)?$' is what I came up with. I tested it in vim's command line, and it looks like it should work:

:echo getline(line(".")) =~ '\v^(#|"|*)?$'

returns 1 on the target lines, and returns 0 on all others.

But then the same expression used in the following mapping throws an error:

nnoremap <expr> <Leader>o (getline(line(".")) =~ '\v^(#|"|*)?$') ? (":echo '1'") : (":echo '0'")

gives

E115: Missing quote: '\v^(#
E15: Invalid expression: (getline(line(".")) =~ '\v^(#`

Then I tried rewriting the regular expression without the \v flag: '^\(\#\|\"\|\*\)\?$'. It still works in command-line testing, and eliminates the error, but still doesn't match as expected. (It matches blank lines, but not those beginning with a comment marker.)

What am I missing? (Additionally, does Vimscript have a way to specify "current-filetype's-comment-marker", so I don't have to lump all three of these together as one?)

Answer 1

1. The parentheses are unnecessary.

2. The error is caused by the unescaped pipes in your pattern. Escape them to fix that error:

   nnoremap <expr> <key> getline(line(".")) =~ '\v^(#\|"\|*)?$' ? ":echo 1\<CR>" : ":echo 0\<CR>"
   

Additionally, does Vimscript have a way to specify "current-filetype's-comment-marker", so I don't have to lump all three of these together as one?

Yes, you can use &commentstring but it may or may not be correctly set for every filetype.