LPeg question about substitution captures with group captures

[Sean Conner <sean@...>]

  I'm working on a personal project [1] and for some media types, I'm using
mailcap files to specify external programs to view media types not directly
supported by the program I'm writing.  So I have a mailcap file:

application/x-foo; foo -t %t %s
application/x-bar; bar -t %t

  This, I can parse [2].  The first field is the MIME type, followed by the
command to run, but there are substitutions that need to happen before the
command is run.  The '%t' is replaced by the MIME type, and the '%s' is
replaced by the file; if '%s' is *NOT* specified, then the data is piped in
via stdin. This is where I'm having an issue.  I would like to have LPeg do
the substitutions but the part I'm having trouble with is indicating if '%s'
was indeed, part of the command.  While I could check to see if '%s' exists
in the string before I do the substition, I'd prefer if I didn't have to.

  My current attempt:

lpeg = require "lpeg"

char = lpeg.P"%s" * lpeg.Carg(1) / "%1" * lpeg.Cg(lpeg.Cc(false),'redirect')
     + lpeg.P"%t" * lpeg.Carg(2) / "%1"
     + lpeg.R" ~"
cmd  = lpeg.Cg(lpeg.Cc(true),'redirect')
     * lpeg.Cs(char^1)
     * lpeg.Cb'redirect'

print(cmd:match("foo -t %t %s",1,"/tmp/bar.foo","application/x-foo"))
print(cmd:match("bar -t %t",   1,"/tmp/foo.bar","application/x-bar"))

  This outputs:

foo -t application/x-foo /tmp/bar.foo   true
bar -t application/x-bar        true

I'd like the output to be:

foo -t application/x-foo /tmp/bar.foo   false
bar -t application/x-bar        true

Now, lpeg.Cg() states:

	An anonymous group serves to join values from several captures into
	a single capture. A named group has a different behavior. In most
	situations, a named group returns no values at all. Its values are
	only relevant for a following back capture or when used inside a
	table capture.

and lpeg.Cs():

	Creates a substitution capture, which captures the substring of the
	subject that matches patt, with substitutions. For any capture
	inside patt with a value, the substring that matched the capture is
	replaced by the capture value (which should be a string). The final
	captured value is the string resulting from all replacements.

  I'm using a named group to track if I need redirection or not, and since a
named group does not return a value, it shouldn't affect the substitution
capture (and it doesn't).  But the group capture in the char expression
seems to be ignored.

  What's going on here?  Am I misunderstanding the documentation?

  -spc

[1]	A gopher client for those curious.

[2]	There's more to the format but I don't want to bog down the issue
	more than I have to, and as I said, parsing the mailcap file isn't
	the issue.