Barebones example of how to make a simple vim syntax file.

todo.vim

" Barebones example of how to make a simple vim syntax file.
" Language: todofile
" Maintainer: leihog

"if exists ("b:current_syntax")
"	finish
"endif
let b:current_syntax = "todo"

" v:folddashes  a string that contains dashes to represent the foldlevel.
" v:foldstart   line number the fold starts on
" v:foldend     line number the fold ends on
" v:foldlevel   fold level
set foldtext=MyFoldText()
function MyFoldText()
	let line = getline(v:foldstart)
	let output = substitute(line, '###', '', 'g')
	return output
endfunction

" Regions
syn region todoFold keepend start=/^###/ end=/^###/me=s-1,he=s-1,re=s-1 fold contains=todoHeader

syn match todoHeader /^###.*$/ contained contains=todoHeaderItem,todoHeaderStart
syn match todoHeaderStart "###" contained
syn match todoHeaderItem /[^###]*$/ contained

hi link todoHeaderStart Comment
hi todoHeaderItem ctermfg=red guifg=red

syn sync fromstart
set foldmethod=syntax