Toggle casing

.vimrc

@@ -586,6 +586,21 @@ fun! TrimWhitespace()
 endfun
 command! TrimWhitespace call TrimWhitespace()
 
+" Visually select text then press ~ to convert the text to UPPER CASE, then to lower case, then to Title Case.
+" Keep pressing ~ until you get the case you want.
+" https://vim.fandom.com/wiki/Switching_case_of_characters
+function! TwiddleCase(str)
+  if a:str ==# toupper(a:str)
+    let result = tolower(a:str)
+  elseif a:str ==# tolower(a:str)
+    let result = substitute(a:str,'\(\<\w\+\>\)', '\u\1', 'g')
+  else
+    let result = toupper(a:str)
+  endif
+  return result
+endfunction
+vnoremap ~ y:call setreg('', TwiddleCase(@"), getregtype(''))<CR>gv""Pgv
+
 " enable words completion
 set dictionary+=/usr/share/dict/words
 " use ctrl-n ctrl-n instead of ctrl-x ctrl-k