Dear Monks

Suppose I want to match text like this:

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

I started simply by writing a grammar to match the two brace string:

use strict; 
use warnings; 
use 5.012;

use Parse::RecDescent;

$::RD_ERRORS = 1; #Parser dies when it encounters an error
$::RD_WARN   = 1; #Enable warnings - warn on unused rules &c.
$::RD_HINT   = 1; #Give out hints to help fix problems.
#$::RD_TRACE  = 1; #Trace parsers' behaviour

my $text = <<'END_OF_TEXT';
{{ hello }}
END_OF_TEXT

my $grammar = <<'END_OF_GRAMMAR';
    {
        use 5.012;
        use Data::Dumper;
    }

    startrule: lbrace(2) 
               'hello' 
               rbrace(2) 

    lbrace: / [{] /xms
    rbrace: / [}] /xms
                  
END_OF_GRAMMAR

my $parser = Parse::RecDescent->new($grammar) 
    or die "Bad grammar!\n";

defined $parser->startrule($text) 
    or die "Can't match text";
[download]

Then I tried to adjust the grammar to make the rbrace subrule dynamic. My idea was to use <rulevar:> to declare a local variable called $lbrace_count:

    startrule: <rulevar: $lbrace_count>
[download]

Then use an action to assign $lbrace_count the number of lbraces that matched:

    startrule: lbrace(2)  { $lbrace = length $item[1] }
               'hello' 
               rbrace(2)
[download]

The grammar above still matches.

Next I used <matchrule:> to create the string 'rbrace(2)'. According to the docs, you are supposed to put an unquoted series of characters after the colon in <matchrule:> and then P::RD will put the series of characters inside a qq{} to produce the name of a subrule:

    startrule: <rulevar: $lbrace_count>

    startrule: lbrace(2)  { $lbrace_count = length $item[1] }
               'hello' 
               <matchrule: rbrace($lbrace_count)>

    lbrace: / [{] /xms
    rbrace: / [}] /xms
[download]

But that produces a weird error that I can't sort out:

Unknown starting rule (Parse::RecDescent::namespace000001::rbrace(18))
+ called
 at prd1.pl line 39.
[download]

In fact, I can't even get the following simplification to work:

    startrule: lbrace(2)  
               'hello' 
               <matchrule: rbrace(2)>

    lbrace: / [{] /xms
    rbrace: / [}] /xms


--output:--
Unknown starting rule (Parse::RecDescent::namespace000001::rbrace(2)) 
+called
 at prd1.pl line 39.
[download]

Here's the complete program:

use strict; 
use warnings; 
use 5.012;

use Parse::RecDescent;

$::RD_ERRORS = 1; #Parser dies when it encounters an error
$::RD_WARN   = 1; #Enable warnings - warn on unused rules &c.
$::RD_HINT   = 1; #Give out hints to help fix problems.
$::RD_TRACE  = 1; #Trace parsers' behaviour

my $text = <<'END_OF_TEXT';
{{ hello }}
END_OF_TEXT

my $grammar = <<'END_OF_GRAMMAR';
    {
        use 5.012;
        use Data::Dumper;
    }

    startrule: <rulevar: $lbrace_count>
    startrule: lbrace(2)  { $lbrace_count = length $item[1] }
               'hello' 
               <matchrule: rbrace($lbrace_count)>

    lbrace: / [{] /xms
    rbrace: / [}] /xms

                  
END_OF_GRAMMAR



my $parser = Parse::RecDescent->new($grammar) 
    or die "Bad grammar!\n";

#ERRROR ON NEXT LINE *****************************

defined $parser->startrule($text) 
    or die "Can't match text";
[download]

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