http://www.perlmonks.org?node_id=527410


in reply to Random Tips on Parse::RecDescent

hsm,
Thanks for the excellent tips on PRD! I'm trying to use tip #16 for C-like multiline comments '/*' and '*/'.
I don't quite understand your syntax for the following production: 
xcomment:
        <skip: qr/[ \t]*/> newline(0..) '<!--'
        {
            ($text,$return) = main::parse_delimited($text,'<!--','-
++->');
             $return = ['xcomment',$return];
        }

[download]
I translated your xcomment production into the following production: 
comment:
        <skip: qr/[ \t]*/> newline(0..) '/*'
        {
            ($text,$return) = main::parse_delimited($text,'/*','*/');
             $return = ['comment',$return];
        }

[download]
My question is I don't understand the newline(0..) part I must missing something obvious. I had errors if I used newline(0..) and main::parse_delimited so I removed newline(0..) and changed main:: to $thisparser-> and ran the following test with no compile errors but it fails to match the comment production. What am I missing? Any help debugging would be appreciated! 
#!/usr/bin/perl -w
use strict;
use Parse::RecDescent;
use Text::DelimMatch;

$::RD_ERRORS = 1;
$::RD_WARN = 1;
$::RD_HINT = 1;
$::RD_TRACE = 1;

my $grammar = q{
{
   sub parse_delimited {
        my $text       = shift;
        my $startdelim = shift;
        my $enddelim   = shift;

        my $mc = new Text::DelimMatch( $startdelim, $enddelim );
        my ( $p, $m, $r ) = $mc->match( $text );

        if ($p) {
            $text = $p;
        }
        else {
            $text = "";
        }
        $text .= $r if ($r);
        $m =~ s/^$startdelim//;
        $m =~ s/$enddelim$//;
        return $text, $m;
    }
}
file: line(s) eofile { use Data::Dumper 'Dumper'; print Dumper @item}

line:
    comment
    | <error>

eofile: /^\Z/

comment:
        <skip: qr/[ \t]*/> '/*'
        {
            ($text,$return) = $thisparser->parse_delimited($text, '/*'
+, '*/');
             $return = ['comment',$return];
        }

};

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

while (<DATA>)
{
    chomp;
    print "$_...\n";
    defined($parser->file($_)) or print "Bad text!\n";
}

__DATA__
/*Hello
World
*/

[download]
I also get the following errors in the trace which are disconcerting but I don't think it's causing the problem: 
Argument "/*" isn't numeric in addition (+) at C:/Perl/site/lib/Parse/
+RecDescent.pm line 2783, <DATA> line 1.
Use of uninitialized value in substitution (s///) at (eval 15)[C:/Perl
+/site/lib/Parse/RecDescent.pm:2618] line 22, <DATA> line 1.
Use of uninitialized value in concatenation (.) or string at (eval 15)
+[C:/Perl/site/lib/Parse/RecDescent.pm:2618] line 23, <DATA> line 1.
Use of uninitialized value in substitution (s///) at (eval 15)[C:/Perl
+/site/lib/Parse/RecDescent.pm:2618] line 23, <DATA> line 1.
Use of uninitialized value in substitution (s///) at (eval 15)[C:/Perl
+/site/lib/Parse/RecDescent.pm:2618] line 23, <DATA> line 1.

[download]

Replies are listed 'Best First'.
Re^2: Random Tips on Parse::RecDescent
by Anonymous Monk on Apr 27, 2012 at 10:53 UTC

    The example in random tip #16 does not explicitly spell out that you need to :

    use Text::DelimMatch;

    and in the grammar definition you need a rule :

    newline: "\n"

    With those two things in place the code works fine for parsing multi line HTML comments.

    Parsing multi line C style comments is complicated by the fact that * is a regexp character so it needs escaping. I managed to get the following code to work OK based on technique outlined in the tip. I'm sure it could be done better but I was struggling with the escaping

    # Function to cope with multiline comments
# Must be placed in main section of program
sub parse_multilinecomment
{
        my $text       = shift;

        my $mc = new Text::DelimMatch( '\\/\\*', '\\*\\/' );
        my ( $p, $m, $r ) = $mc->match( '/*' . $text );

        if ($p) {
            $text = $p;
        }
        else {
            $text = "";
        }
        $text .= $r if ($r);
        $m =~ s/^\/\*//;
        $m =~ s/\*\/$//;
        return $text, $m;
 }

    [download]

    and the grammar rules :

        newline: "\n"
        
    multilinecomment:
        <skip: qr/[ \t]*/> newline(0..) '/*'
        {
            ($text,$return) = main::parse_multilinecomment($text);
            print $return . "\n";
             $return = ['xcomment',$return];
        }

    [download]

    Successfully matches the following example :

    /*
    A multiple line
    /*
        with nested
    */

    comment
    
    

*/

    [download]

    Hope this may help someone

    Adrian

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[d/l]
[select]
Re^2: Random Tips on Parse::RecDescent
by hsmyers (Canon) on Feb 03, 2006 at 02:03 UTC
    I'll take a look as soon as I blow the dust off of the section of my brain that once retained this stuff!! Please do not hold your breath while I am doing so, but know that I am in fact checking it out...

    --hsm

    "Never try to teach a pig to sing...it wastes your time and it annoys the pig."
[reply]
      hsm,
      I realize this is an old post and I won't hold my breath ;^)
      Please don't spend too much time on it I just was hoping it might ring some bells. I will also continue to investigate.
      Thanks!
[reply]

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