0FGVYqxHQZ has asked for the wisdom of the Perl Monks concerning the following question:

I get the following error when I try to use <ALIAS=\IDENT> in Regexp::Grammars. I can't see what is wrong with my code (str_delim is not misspelled). Could anybody let me know what is wrong? 
~/linux/test/perl/library/Regexp/Grammars/</=/\$ cat main.pl 
#!/usr/bin/env perl

use strict;
use warnings;

use Data::Dumper;

my $grammar = do {
  use Regexp::Grammars;
  qr{
    (?#<logfile:->)
    <delimited_string>

    <token: delimited_string>
      <ldelim=str_delim> .*? <rdelim=\ldelim>

    <token: str_delim>
      ["'`]
  }xms;
};

my $string = "''";
if ($string =~ $grammar) {
  print Dumper \%/;
}

~/linux/test/perl/library/Regexp/Grammars/</=/\$ ./main.pl
 error | Found call to <ldelim=str_delim>, but no <rule: str_delim> or
       | <token: str_delim> was defined in the grammar
       | (Did you misspell the rule name or forget to define the rule?
+)
       | 
 error | Found call to <______0_88_str_delim>, but no <rule: ______0_8
+8_str_delim> or
       | <token: ______0_88_str_delim> was defined in the grammar
       | (Did you misspell the rule name or forget to define the rule?
+)
       | 
 error | Found call to <str_delim>, but no <rule: str_delim> or
       | <token: str_delim> was defined in the grammar
       | (Did you misspell the rule name or forget to define the rule?
+)
       | 
  warn | Possible invalid subrule call:
       |     <ldelim=str_delim>
       | (To silence this warning, use: \<ldelim=str_delim>
       | 
  warn | Possible invalid subrule call:
       |     <rdelim=(?{; *Regexp::Grammars::LOGFILE
       | (To silence this warning, use: \<rdelim=(?{; *Regexp::Grammar
+s::LOGFILE
       | 
  warn | Possible invalid subrule call:
       |     <______0_88_str_delim>
       | (To silence this warning, use: \<______0_88_str_delim>
       | 
  warn | Possible invalid subrule call:
       |     <str_delim>
       | (To silence this warning, use: \<str_delim>
       | 
Eval-group not allowed at runtime, use re 'eval' in regex m/(?{; *Rege
+xp::Grammars::LOGFILE
                        = Regexp::Grammars::_open_log('>>','-'); })((?
+{; @! = () if !.../ at ./main.pl line 10.

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Re: The usage of <ALIAS=\IDENT> of Regexp::Grammars
by DamianConway (Beadle) on Aug 16, 2012 at 04:12 UTC

    The bug is that Perl does not always pass the complete raw regex into an overload 'qr', so the module cannot always convert it correctly. :-(

    In this case, Perl intercepts the \l of the \ldelim and treats it as a "convert the following to lowercase" escape, before Regexp::Grammars ever gets to see the grammar.

    The workaround for the current release is not to use rulenames that start with 'l', 'u', 'L', 'U', 'Q', or 'E' in a matchref. Which sucks, I know.

    The long term solution (which was just uploaded to CPAN as Regexp::Grammars version 1.020) is to change the entire matchref syntax from <\IDENT> to <\_IDENT>. Yes, that's horribly backwards incompatible, but it's the only solution that doesn't involve some deep and scary hacking on the Perl core. I'm sorry.

    Damian

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Re: The usage of <ALIAS=\IDENT> of Regexp::Grammars
by Anonymous Monk on Aug 10, 2012 at 01:29 UTC

    I think its a bug like Bug #75644 for Regexp-Grammars: debug does not work

    You can workaround by following http://search.cpan.org/src/DCONWAY/Regexp-Grammars-1.016/t/backref.t and http://search.cpan.org/src/DCONWAY/Regexp-Grammars-1.016/t/backref_ARG.t, which DO NOT test this (your) example from the docs

    The workaround does work 

    #!/usr/bin/perl --
use strict;
use warnings;

use Data::Dumper;

my $grammar = do {
  use Regexp::Grammars;
  qr{
<debug:on>
<delimited_string>


<token: delimited_string>
    <delim=(['"`])> <content=(.*?)> <rdel=\delim>

}xms; };

my $string = "'hi'";
if ($string =~ $grammar) {
  print Dumper \%/;
}
__END__
=====> Trying <grammar> from position 0
'hi'   |...Trying <delimited_string>
       |   |...Trying <delim=(['"`])>
hi'    |   |    \_____<delim=(['"`])> matched '''
       |   |...Trying <content=(.*?)>
       |   |    \_____<content=(.*?)> matched ''
       |   |...Trying <rdel=\delim>
       |   |    \FAIL <rdel=\delim>
i'     |   |    \_____<content=(.*?)> matched 'h'
       |   |...Trying <rdel=\delim>
       |   |    \FAIL <rdel=\delim>
'      |   |    \_____<content=(.*?)> matched 'hi'
       |   |...Trying <rdel=\delim>
[eos]  |   |    \_____<rdel=\delim> matched '''
       |    \_____<delimited_string> matched ''hi''
        \_____<grammar> matched ''hi''
$VAR1 = {
          '' => '\'hi\'',
          'delimited_string' => {
                                  '' => '\'hi\'',
                                  'content' => 'hi',
                                  'delim' => '\'',
                                  'rdel' => '\''
                                }
        };

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    Regexp::Grammars::Common::String use this syntax

    You should report this bug upstream , maybe write this example as a test case following t/backref*

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