LPeg and function capture

[Philippe Lhoste <PhiLho@...>]

Posting in the same thread, as it is related to my experiments.
I am puzzled by a result I got.
This works as expected:

local cap = m.R'AZ'
local findCap = (1 - cap) + m.C(cap)
local toMatch = "Stairway To Heaven"
local patt = findCap^0 / function (a, b, c)
  return string.lower(a .. b .. c)
end
print(m.match(patt, toMatch))

But the following doesn't, the function gets all chars, not only the 
capital ones (those supposed to be captured):

local patt = (findCap / function (a) return string.lower(a) end)^0
print(m.match(patt, toMatch))

Why the behavior of the pattern is changed by the associated function?

--
Philippe Lhoste
--  (near) Paris -- France
--  http://Phi.Lho.free.fr
--  http://PhiLho.deviantART.com
--  --  --  --  --  --  --  --  --  --  --  --  --  --