Re: LPeg back captures

[Andrew Starks <andrew.starks@...>]

On Wednesday, October 23, 2013, Sean Conner  wrote:
>   Okay, I may not fully understand back captures in LPeg.  Here's the
> problem:  I'm attempting to parse NAPTR DNS records.  Once I obtain a given
> record, I have a string in the form of: [1]
>
> !^.*$!pstndata:cnam/+15714344048;;charset=us-ascii;ds=local;score=98,gn=CRYSTA;sn=SPERBER!
>
> (this is the regexp portion of the NAPTR record).  RFC-3402 gives the
> following grammar for this field:
>
>         subst-expr   = delim-char  ere  delim-char  repl  delim-char  *flags
>         delim-char   = "/" / "!" / <Any octet not in 'POS-DIGIT' or 'flags'>
>                            ; All occurrences of a delim_char in a subst_expr
>                            ; must be the same character.>
>         ere          = <POSIX Extended Regular _expression_>
>         repl         = *(string / backref)
>         string       = *(anychar / escapeddelim)
>         anychar      = <any character other than delim-char>
>         escapeddelim = "\" delim-char
>         backref      = "\" POS-DIGIT
>         flags        = "i"
>         POS-DIGIT    = "1" / "2" / "3" / "4" / "5" / "6" / "7" / "8" / "9"
>
>   I have this translated into LPeg:
>
>         DIGIT        = R"09"
>         delim_char   = P"!" -- Cb("delim")
>         flags        = P"i"
>         backref      = P"\\" * DIGIT
>         escapeddelim = P"\\" * delim_char
>         anychar      = P(1) - delim_char
>         string       = (escapeddelim + anychar)^1
>         repl         = C((string + backref)^0)
>         ere          = C((P(1) - delim_char)^0)
>         idelim_char  = Cg(P"/" + P"!" + (P(1) - (DIGIT + flags)),"delim")
>         regexp       = Ct(
>                             idelim_char
>                             * Cg(ere,"re")
>                             * delim_char
>                             * Cg(repl,"replace")
>                             * delim_char
>                             * Cg(flags^0,"flags")
>                          )
>
> and it works, except for the hardcoded delimeter.  If I leave delim_char as
> is, I get the expected data:
>
>         regexp =
>         {
>           re = "^.*$",
>           flags = "",
>           replace = "pstndata:cnam/+15714344048;;charset=us-ascii;ds=local;score=98,gn=CRYSTA;sn=SPERBER"
>           delim = "!",
>         }
>
> But if I try to use a backreference (delim_char = Cb("delim")), it doesn't work:
>
>         regexp =
>         {
>           [1] = "!",
>           [2] = "!",
>           replace = "",
>           flags = "",
>           re = "",
>           delim = "!",
>         }
>
>   I'm wondering, am I using back references correctly?  The example in the
> LPeg documentation [2] is close to what I want, but I'm missing something.
> I know I can use Lua's builtin regular expressions to break this string
> apart, but I'd rather use LPeg, if only to figure out how to parse this type
> of data.
>
>   -spc (Who's really puzzled by this)
>
> [1]     All characters appearing in this work are fictitious. Any
>         resemblance to real persons, living or dead, is purely coincidental.
>
> [2]     http://www.inf.puc-rio.br/~roberto/lpeg/

Crap, sorry. That was wrong. I mixed up the other half of it, which is Cg. 

Okay, first, I believe that you need to have your "delim" defined in a Cg before the back capture, which I didn't see. So

delim = <some capture>

delim_pair = Cg (delim, "delim")

Somepat= P(term*delim_pair*otherterm*Cb("delim"))

In this context, Cg does not provide key=value. It names the chapter so that you can reuse it by name, like Cc, but with a value tied to a previous capture.  

I know that you probably know that. I say it in case I'm wrong and someone can correct me. :)

-Andrew