-- wrap utf8.sub(str,head_index, tail_index)
-- wrap string.split(str,sp,sp1)
--      string.utf8_len = utf8.len
--      string.utf8_offset= utf8.offset
--      string.utf8_sub= utf8.sub
function string.split( str, sp,sp1)
  sp =type(sp) == "string"  and sp or " "    
  if #sp == 0 then
    sp= "([%z\1-\127\194-\244][\128-\191]*)"
  elseif #sp == 1 then 
    sp= "[^" ..  (sp=="%" and "%%" or sp) .. "]*"  
  else 
    sp1= sp1 or "^"
    str=str:gsub(sp,sp1)
    sp=  "[^".. sp1 .. "]*"
  end 

  local tab= {}
  for v in str:gmatch(sp) do
    table.insert(tab,v)
  end 
  return tab 
end

function utf8.gsub(str,si,ei)
  local function index(ustr,i)
    return i>=0 and ( ustr:utf8_offset(i) or ustr:len() +1 )
    or ( ustr:utf8_offset(i) or 1 )
  end

  local u_si= index(str,si)
  ei = ei or str:utf8_len()
  ei = ei >=0 and ei +1 or ei
  local u_ei= index(str, ei ) -1
  return str:sub(u_si,u_ei)
end
string.utf8_len= utf8.len
string.utf8_offset=utf8.offset
string.utf8_sub= utf8.gsub
return true