LPeg question about substitution captures with group captures
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- Subject: LPeg question about substitution captures with group captures
- From: Sean Conner <sean@...>
- Date: Sat, 8 Dec 2018 00:54:37 -0500
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.