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Re: Parse::RecDescent: how does <matchrule:> work?

by 7stud (Deacon)
on Feb 07, 2013 at 16:58 UTC ( #1017698=note: print w/ replies, xml ) Need Help??


in reply to Parse::RecDescent: how does <matchrule:> work?

Question #1 is ... what do you want? (No, seriously.) What interpretation of these three strings do you want to be “correct?” Are they three distinct cases, or examples of the same one?

Sorry, I know the frustration. I see now that what I said has multiple interpretations, and that I didn't even follow what I tell others to do when posting questions like this. To clarify: I want to be able to apply my grammar to take this string:

     { hello }

...and find a match.. Then I want to be able to apply the same grammar to this string:

     {{ hello }}

...and also find a match. Then I want to be able to apply the same grammar to this string:

     {{{ hello }}}

and also find a match. By "match", I mean that my program should produce no errors, and these lines:

my $parser = Parse::RecDescent->new($grammar) or die "Bad grammar!\n"; defined $parser->startrule($text) or die "Can't match text";

...should not produce any output. So when I run my program three times on each of those strings, each time there should be no output at all. What I am trying to code is a backreference.

But now that I think about it, applying my desired grammar to this three line block of text:

    { hello }
    {{ hello }}
    {{{ hello }}}

...would only require a trivial adjustment to the grammar:

startrule: brace_block(s) #adjustment brace_block: lbrace(1..) 'hello' <something> lbrace: / [{] /xms rbrace: / [}] /xms

In any case, I figured out a solution. The grammar below allows me to parse 'hello', preceded by a variable number of braces, followed by the same number of braces that preceded 'hello':

startrule: <rulevar: $rbraces> startrule: lbrace(s) { my $lbraces = join '', @{$item[1]}; $rbraces = "}" x length $lbraces; } 'hello' "$rbraces" lbrace: / [{] /xms

Instead of using a rule for the right braces, I am using a literal. Is there a better way to produce a backreference?

Please provide a full representative example of the type of data you need to parse, and we’ll help you write the proper grammar-mojo. It does take practice.

Yes, this is part of a larger grammar I'm working on, but I don't think posting the whole thing would be helpful. I would love to have you take a look at the complete grammar when I'm done. My current problem: when I incorporate the solution above into one of my rules, I can't get the blasted rule to return the entire match to other rules without causing the error:

Can't use string ("{{") as an ARRAY ref while "strict refs" in use at +(eval 14) line 377
I find it frustrating that Damian didn't use Carp, so that I could pin down exactly which line in my code is causing that error--I could care less which line in his code is causing the error. Another thing I find frustrating is that a rule doesn't return its whole match to another rule. Why in the heck is the default to return only what matched the last term of a rule? I have no idea how the default behavior would ever be useful.


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