Is there a simple way in Vim to extend the syntax highlighting for a language to allow for important comments to stand out? For instance, if a line that starts with // denotes a regular comment in a C file, I'd like to have lines starting with //!! be highlighted in a more prominent color.
// this is a regular comment - line color should be the default color for comments
//!! this is an important comment - highlight line in red
:syn match specialComment #//!!.*# | hi specialComment ctermfg=red guifg=red
As Ingo Karkat points out, you can have the commands executing after a .c file is loaded by placing them in ~/.vim/after/syntax/c.vim.
Another option, if you'd like to place everything in a single file, such as ~/.vimrc, could be to bind the commands to the buffer enter event:
au! BufEnter *.c syn match specialComment #//!!.*# " C files (*.c)
au! BufEnter *.py syn match specialComment /#!!.*/ " Python files (*.py)
...
hi specialComment ctermfg=red guifg=red