A Todo list syntax in Vim, with an actual checkbox

todo.vim

" Vim syntax file
" Language: Todo
" Maintainer: Huy Tran
" Latest Revision: 14 June 2020

if exists("b:current_syntax")
  finish
endif

" Custom conceal
syntax match todoCheckbox "\[\ \]" conceal cchar=
syntax match todoCheckbox "\[x\]" conceal cchar=

let b:current_syntax = "todo"

hi def link todoCheckbox Todo
hi Conceal guibg=NONE

setlocal cole=1

@Melkster nice you found the issue :)
Check I also added support for VimWiki ToDo's state using some symbols I found:

syntax match VimwikiListTodo '\v(\s+)?(-|\*)\s\[\s\]'hs=e-4 conceal cchar=
syntax match VimwikiListTodo '\v(\s+)?(-|\*)\s\[X\]'hs=e-4 conceal cchar=
syntax match VimwikiListTodo '\v(\s+)?(-|\*)\s\[-\]'hs=e-4 conceal cchar=☒
syntax match VimwikiListTodo '\v(\s+)?(-|\*)\s\[\.\]'hs=e-4 conceal cchar=⊡
syntax match VimwikiListTodo '\v(\s+)?(-|\*)\s\[o\]'hs=e-4 conceal cchar=⬕

For users working in neovim, you can put this in your ftplugin/markdown.lua.

function _G.pcall_markdown_sugar()
  vim.cmd([[
    augroup markdown
      autocmd!
      au BufEnter *.md :syn match todoCheckbox '\v(\s+)?(-|\*)\s\[\s\]'hs=e-4 conceal cchar=
      au BufEnter *.md :syn match todoCheckbox '\v(\s+)?(-|\*)\s\[x\]'hs=e-4 conceal cchar=
      au BufEnter *.md :syn match todoCheckbox '\v(\s+)?(-|\*)\s\[-\]'hs=e-4 conceal cchar=󰅘
      au BufEnter *.md :syn match todoCheckbox '\v(\s+)?(-|\*)\s\[\.\]'hs=e-4 conceal cchar=⊡
      au BufEnter *.md :syn match todoCheckbox '\v(\s+)?(-|\*)\s\[o\]'hs=e-4 conceal cchar=⬕
      :hi def link todoCheckbox Todo
      :highlight Conceal guibg=NONE guifg=#00cf37
      :setlocal conceallevel=1
    augroup END
  ]])
end

pcall_markdown_sugar()

@rohit-kumar-j I took the liberty of improving your code to use the available Neovim Lua APIs:

local function markdown_sugar()
  local augroup = vim.api.nvim_create_augroup('markdown', {})
  vim.api.nvim_create_autocmd('BufEnter', {
    pattern = '*.md',
    group = augroup,
    callback = function()
      vim.api.nvim_set_hl(0, 'Conceal', { bg = 'NONE', fg = '#00cf37' })
      vim.api.nvim_set_hl(0, 'todoCheckbox', { link = 'Todo' })
      vim.bo.conceallevel = 1

      vim.cmd [[
        syn match todoCheckbox '\v(\s+)?(-|\*)\s\[\s\]'hs=e-4 conceal cchar=
        syn match todoCheckbox '\v(\s+)?(-|\*)\s\[x\]'hs=e-4 conceal cchar=
        syn match todoCheckbox '\v(\s+)?(-|\*)\s\[-\]'hs=e-4 conceal cchar=󰅘
        syn match todoCheckbox '\v(\s+)?(-|\*)\s\[\.\]'hs=e-4 conceal cchar=⊡
        syn match todoCheckbox '\v(\s+)?(-|\*)\s\[o\]'hs=e-4 conceal cchar=⬕
      ]]
    end
  })
end

markdown_sugar()