JSON in LPEG re

require "re" -- no shit sherlock

-- this is the table that holds all the \substitutions
-- it could easily be hardcoded into descape() but why bother
descapetable = {
  ['\\\\'] = '\\',
  ['\\"'] = '"', ['\\/'] = '/',
  ['\\t'] = '\9', ['\\n'] = '\10',
  ['\\f'] = '\12', ['\\r'] = '\13',
}

-- this function does the unescaping in strings
-- you could do this within the PEG using substitution captures
-- (at least, for everything besides the unicode)
-- but i just said fuck it and moved on with the bot
function descape (str)
  local tab = {}

  -- unicode support
  -- str = str:gsub('\\u(%x+)', function (num) end)

  -- other escapes
  for c in str:gmatch'\\?.' do
    if c == '\\b' then
      tab[#tab] = nil
    else
      tab[1+#tab] = descapetable[c] or c
    end
  end

  return table.concat(tab)
end

-- json:match(str) decodes str from json into a table
-- note: both {objects} and [arrays] are translated to lua tables
json = re.compile([[
  value <- %s* ( object / array / string / {number} / ('true' / 'false') -> bool / 'null' -> null ) %s*
  key <- %s* string %s*
  object <- '{' ( key ':' value ','? )* -> {} -> object '}'
  array <- '[' ( value ','? )* -> {} ']'
  string <- '"' { ( [^"\] / {escape} )* } -> unesc '"'
  escape <- '\' ( ["\/bfnrt] / 'u' %x^-4 )
  number <- '-'? ( '0' / [1-9] %d* ) ( '.' %d+ )? ( [eE] [+-]? %d+ )?
]], {
  bool = function (s) return s == 'true' end,
  null = function () return nil end,
  unesc = descape,
  object = function (tab)
    -- this is probably a little hackish
    for i=1,#tab,2 do
      tab[tab[i]], tab[i], tab[i+1] = tab[i+1]
    end
    return tab
  end,
})