syntax coloring does not work for flex file

Question

I'm using nord theme, it works normally with other kind of files but not flex file I have syntax on and filetype on I tried switching colorthing from gruvbox to nord but it still behaving the same. this is my vimrc file

set guifont=Fira\ Code\ 18
nnoremap <A-j> :m .+1<CR>==
nnoremap <A-k> :m .-2<CR>==
inoremap <A-j> <Esc>:m .+1<CR>==gi
inoremap <A-k> <Esc>:m .-2<CR>==gi
vnoremap <A-j> :m '>+1<CR>gv=gv
vnoremap <A-k> :m '<-2<CR>gv=gv

" line number
set number

syntax on

filetype on                  " required

set nocompatible              " be iMproved, required

" set the runtime path to include Vundle and initialize
set rtp+=~/.vim/bundle/Vundle.vim
syntax oncall vundle#begin()
" alternatively, pass a path where Vundle should install plugins
"call vundle#begin('~/some/path/here')

" let Vundle manage Vundle, required
Plugin 'VundleVim/Vundle.vim'

Plugin 'Valloric/YouCompleteMe'

" All of your Plugins must be added before the following line
call vundle#end()            " required
filetype plugin indent on    " required


call plug#begin(has('nvim') ? stdpath('data') . '/plugged' : '~/.vim/plugged')

    " Any valid git URL is allowed
    Plug 'https://github.com/junegunn/vim-github-dashboard.git'

    " Using a non-default branch
    Plug 'rdnetto/YCM-Generator', { 'branch': 'stable' }

    " Using a tagged release; wildcard allowed (requires git 1.9.2 or above)
    Plug 'fatih/vim-go', { 'tag': '*' }

    Plug 'vim-airline/vim-airline'
    Plug 'vim-airline/vim-airline-themes'

    Plug 'arcticicestudio/nord-vim'

    Plug 'preservim/nerdtree'

    "Plug 'morhetz/gruvbox'
call plug#end()
"let g:gruvbox_gruvbox_contrast_dark = 'soft'
"colorscheme gruvbox
"set background=dark

colorscheme nord

" custom key bindings
vmap <C-c> "+yi
vmap <C-x> "+c
vmap <C-v> c<ESC>"+p
imap <C-v> <C-r><C-o>+

" set compatibility to vim only
set nocompatibible

" status bar
set laststatus=2

"auto wrap text that extents beyond the sreen length
set wrap

" Encoding
set encoding=utf-8

filetype plugin indent on
" show existing tab with 4 spaces width
set tabstop=4
" when indenting with '>', use 4 spaces width
set shiftwidth=4
" On pressing tab, insert 4 spaces
set expandtab

set smartindent

" Auto save
let g:auto_save = 1
autocmd TextChanged, TextChangedI <buffer> silent write 
autocmd BufRead, BufNewFile * start 

" Airline
let g:airline_theme = 'base16_adwaita'
let g:airline_powerline_fonts = 1

set tags+=./tags;,tags

let g:zettelkasten = "/home/danhpc/opt/note_linked/"

" Make Ctrlp use ripgrep
if executable('rg')
    let g:ctrlp_user_command = 'rg %s --files --color=never --glob ""'
    let g:ctrlp_user_caching = 0
endif

" Make :grep use ripgrep
if executable('rg')
    set grepprg=rg\ --color=never\ --vimgrep
endif

" My own version, only searches markdown as well using ripgrep
" Thus depends on grepprg being set to rg
command! -nargs=1 Ngrep grep "<args>" -g "*.md" $NOTES_DIR
nnoremap <leader>nn :Ngrep<space>

command! Vlist botright vertical copen | vertical resize 50
nnoremap <leader>v :Vlist<CR>

" Generate ctags
nnoremap <leader>tt :silent !ctags -R . <CR>:redraw!<CR>

" Go to tag file
nnoremap <leader>nt :e $NOTES_DIR/tags<CR>:cd $NOTES_DIR<CR>

" Go to the note dir
nnoremap <leader>nd :ex $NOTES_DIR<CR>:cd $NOTES_DIR<CR>

" Change directory to directory of current file
nnoremap <leader>cd :cd %:h<CR>

" Binding for searching tags ("search tag")
nnoremap <leader>st :CtrlPTag<CR>

" 1) pre-fills all the stuff we don’t want to type, namely the timestamp and the extension (always markdown)
" 2) that prompts you for the name of your note
command! -nargs=1 NewZettel :execute ":e" zettelkasten . strftime("%Y%m%d%H%M%S") . "-<args>.md"

nnoremap <leader>nz :NewZettel 

" CtrlP function for inserting a markdown link with Ctrl-X
function! CtrlPOpenFunc(action, line)
   if a:action =~ '^h$'    
      " Get the filename
      let filename = fnameescape(fnamemodify(a:line, ':t'))
      let l:filename_wo_timestamp = fnameescape(fnamemodify(a:line, ':t:s/\(^\d\+-\)\?\(.*\)\..\{1,3\}/\2/'))
      let l:filename_wo_timestamp = substitute(l:filename_wo_timestamp, "_", " ", "g")

      " Close CtrlP
      call ctrlp#exit()
      call ctrlp#mrufiles#add(filename)

      " Insert the markdown link to the file in the current buffer
      let mdlink = "[".filename_wo_timestamp."]( ".filename." )"
      put=mdlink
  else    
      " Use CtrlP's default file opening function
      call call('ctrlp#acceptfile', [a:action, a:line])
   endif
endfunction

let g:ctrlp_open_func = { 
         \ 'files': 'CtrlPOpenFunc',
         \ 'mru files': 'CtrlPOpenFunc' 
         \ }

Edit 1: a part of my flex file

%x NESTED_COMMENT INLINE_COMMENT
int nested_level = 0;

%}

DARROW  =>

%%

// Nested comment

<INITIAL,NESTED_COMMENT>"(*"    {BEGIN NESTED_COMMENT;
                    ++nested_level;}
<NESTED_COMMENT>"*"+)   {if(nested_level) --nested_level;
            else BEGIN INITIAL;}
<NESTED_COMMENT>[^*)\n]+    { }
<NESTED_COMMENT>"*"+    { }
<NESTED_COMMENT>\n  { }

// inline comment
<INITIAL> 

Edit 2 a picture of uncolored flex file

Answer 1

Out of the box, Vim doesn't know anything about "flex": it doesn't detect it and doesn't provide specific syntax highlighting for it.

As a quick workaround, you can set the syntax to lex for flex files, which is supported. You can do it manually:

:set syntax=lex

or tell Vim to do it automatically for you:

" in ftdetect/flex.vim
" adjust the file pattern to taste
autocmd BufRead,BufNewFile *.flex setfiletype lex

Alternatively, you could use the .l file extension, which is automatically recognized by Vim as lex:

But I don't know if it will cover everything as there certainly are differences between flex and lex.

FWIW, there is a plugin that appears to provide updated syntax highlighting for lex/flex. You should probably give it a try.