-- Return the first word, after skipping Lua comments.
  if not buf:match('^%s*%-%-') then
    return buf:match('^%s*(%S+)')  -- avoid unnecessary allocations

  return first_noncomment_word(buf)
end
-- return the first word (separated by whitespace) that's not in a Lua comment
-- or empty string if there's nothing
-- ignore strings; we don't expect them to be the first word in a program
function first_noncomment_word(str)
  local pos = 1
  while pos <= #str do  -- not Unicode-aware; hopefully it doesn't need to be
    if str:sub(pos,pos) == '-' then
      -- skip any comments
      if str:sub(pos+1,pos+1) == '-' then
        -- definitely start of a comment
        local long_comment_header = str:match('^%[=*%[', pos+2)
        if long_comment_header then
          -- long comment
          local long_comment_trailer = long_comment_header:gsub('%[', ']')
          pos = str:find(long_comment_trailer, pos, --[[plain]]true)
          if pos == nil then return '' end  -- incomplete comment; no first word
          pos = pos + #long_comment_trailer
        else
          -- line comment
          pos = str:find('\n', pos)
          if pos == nil then return '' end  -- incomplete comment; no first word
        end
      end
    end
    -- any non-whitespace that's not a comment is the first word
    if str:sub(pos,pos):match('%s') then
      pos = pos+1
    else
      return str:match('^%S*', pos)
    end

  -- wastefully strip out comments everywhere, not just at the start
  buf = buf:gsub('%-%-[^\n]*', '')  -- line comments
  return buf:match('^%s*(%S+)')

  return ''
end
function test_first_noncomment_word()
  check_eq(first_noncomment_word(''), '', 'empty string')
  check_eq(first_noncomment_word('abc'), 'abc', 'single word')
  check_eq(first_noncomment_word('abc def'), 'abc', 'stop at space')
  check_eq(first_noncomment_word('abc\tdef'), 'abc', 'stop at tab')
  check_eq(first_noncomment_word('abc\ndef'), 'abc', 'stop at newline')
  check_eq(first_noncomment_word('-- abc\ndef'), 'def', 'ignore line comment')
  check_eq(first_noncomment_word('--[[abc]] def'), 'def', 'ignore block comment')
  check_eq(first_noncomment_word('--[[abc\n]] def'), 'def', 'ignore multi-line block comment')
  check_eq(first_noncomment_word('--[[abc\n--]] def'), 'def', 'ignore comment leader before block comment trailer')
  check_eq(first_noncomment_word('--[=[abc]=] def'), 'def', 'ignore long comment')
  check_eq(first_noncomment_word('--[=[abc]] def ]=] ghi'), 'ghi', 'ignore long comment containing block comment trailer')
  check_eq(first_noncomment_word('--[===[abc\n\ndef ghi\njkl]===]mno\npqr'), 'mno', 'ignore long comment containing block comment trailer')
  check_eq(first_noncomment_word('-'), '-', 'incomplete comment token')
  check_eq(first_noncomment_word('--abc'), '', 'incomplete line comment')
  check_eq(first_noncomment_word('--abc\n'), '', 'just a line comment')
  check_eq(first_noncomment_word('--abc\n  '), '', 'just a line comment 2')
  check_eq(first_noncomment_word('--[ab\n'), '', 'incomplete block comment token is a line comment')
  check_eq(first_noncomment_word('--[[ab'), '', 'incomplete block comment')
  check_eq(first_noncomment_word('--[[ab\n]'), '', 'incomplete block comment 2')
  check_eq(first_noncomment_word('--[=[ab\n]] ]='), '', 'incomplete block comment 3')
  check_eq(first_noncomment_word('--[=[ab\n]] ]=]'), '', 'just a block comment')
  check_eq(first_noncomment_word('--[=[ab\n]] ]=]  \n  \n '), '', 'just a block comment 2')