http://tex.stackexchange.com/q/126683/14066

charsperline.lua

-----------------------------------------------------------------------
--         FILE:  charsperline.lua
--        USAGE:  called from t-charsperline.mkvi
--  DESCRIPTION:  extract line contents and lengths
-- REQUIREMENTS:  ConTeXt MkIV
--       AUTHOR:  Philipp Gesang (Phg), <phg42.2a@gmail.com>
--      VERSION:  1.0
--      CREATED:  2013-08-04 20:01:04+0200
-----------------------------------------------------------------------
--
--- based on t-wordsperline:
--- http://www.ntg.nl/pipermail/ntg-context/2012/070668.html

thirddata                  = thirddata or { }
thirddata.chars_per_line   = thirddata.chars_per_line or { }

local stringformat         = string.format
local tableconcat          = table.concat
local utfchar              = utf.char

local traverse_nodetype    = node.traverse_id
local traverse_nodelist    = node.traverse

local nodecodes            = nodes.nodecodes
local listcodes            = nodes.listcodes
local skipcodes            = nodes.skipcodes

local hlist_t              = nodecodes.hlist
local vlist_t              = nodecodes.vlist
local glue_t               = nodecodes.glue
local glyph_t              = nodecodes.glyph
local line_t               = listcodes.line
local userskip_t           = skipcodes.userskip

local tasks                = nodes.tasks
local enableaction         = tasks.enableaction
local disableaction        = tasks.disableaction

local linedata             = { }

local resolve_ligatures
resolve_ligatures = function (lst, hd)
  for n in traverse_nodetype (glyph_t, hd) do
    local components = n.components
    if components then
      lst = resolve_ligatures (lst, components)
    else
      lst[#lst+1] = utfchar (n.char)
    end
  end
  return lst
end

local collect = function (hd, groupcode)
  if groupcode == "vbox" then
    return hd
  end

  for current in traverse_nodetype (hlist_t, hd) do

    if current.subtype == line_t then
      local chars, has_glyphs = { }, false

      for n in traverse_nodelist (current.list) do

        local ntype, nsubtype = n.id, n.subtype

        -- we care only for glyphs’n’glue
        if ntype == glyph_t then
          has_glyphs = true
          if n.components then
            chars = resolve_ligatures (chars, n.components)
          else
            chars[#chars+1] = utfchar (n.char)
          end
        elseif ntype == glue_t and nsubtype == userskip_t then
          chars[#chars+1] = " "
        end

      end

      if has_glyphs then
        linedata[#linedata+1] = chars
      end
    end
  end

  return hd
end

thirddata.chars_per_line.collect = collect

tasks.appendaction ("finalizers", "before",
                    "thirddata.chars_per_line.collect")
tasks.disableaction("finalizers",
                    "thirddata.chars_per_line.count_words")

local write_stats = function (...) texiowrite_nl(stringformat(...)) end

local datafile = "./linedata.txt"

write_linedata = function (filename)
  filename = filename or datafile

  local result = { }
  for i = 1, #linedata do local line = linedata[i]
    result[#result+1] = stringformat ("%q,%d",
                                      tableconcat (line),
                                      #line)
  end

  io.savedata (filename, result, "\n")
end

local active --- callback state

commands.start_chars_per_line = function ()
  if not active then
    enableaction("finalizers",
                 "thirddata.chars_per_line.count_words")
    active = true
  end
end

commands.stop_chars_per_line = function ()
  if active then
    disableaction("finalizers",
                  "thirddata.chars_per_line.count_words")
    active = false
  end
end

commands.write_linedata = write_linedata

t-charsperline.mkvi

\startmodule [charsperline]

\unprotect

\ctxloadluafile{charsperline}

\def\startdumplines{\ctxcommand{start_chars_per_line ()}}

\def\stopdumplines{\endgraf\ctxcommand{stop_chars_per_line ()}}

\prependtoks \charsperline_dump \to \everystoptext

\def\charsperline_dump{\ctxcommand{write_linedata ()}}

\protect

\stopmodule \endinput

test.tex

\usemodule[charsperline]
\setuplayout[width=5cm]

\starttext

\startdumplines
  \input knuth\par
\stopdumplines

\stoptext