Lua: string interpolation (See related posts)

---
-- Interpolates values into a string.
--
-- Values are encoded in the form %(key,format)
-- where key is the index into the values table
-- and format is the string.format option without
-- the % character.
--
-- @param s the string into which values are inserted
-- @param tab the table of values for lookup
-- @return
function interp(s, tab)
  return (s:gsub('%%%((%a[^,]*),([-0-9%.]*[cdeEfgGiouxXsq])%)',
            function(k, fmt)
              local f = loadstring("return " .. k)
              setfenv(f, tab)
              value = f()
              return value and ("%"..fmt):format(value) or '%('..k..')'..fmt
            end))
end

-- Sets interpolation for the '%' operator on strings
getmetatable("").__mod = interp

-- Example
t = {key = "concentration", val = {56.2795, 1.2}}
print( "%(key,s) is %(val[2],7.2f)%" % t )