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Re: Random Tips on Parse::RecDescent

by davistar (Novice)
on Feb 02, 2006 at 19:39 UTC ( #527410=note: print w/replies, xml ) Need Help??

in reply to Random Tips on Parse::RecDescent

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]; }
I translated your xcomment production into the following production:
comment: <skip: qr/[ \t]*/> newline(0..) '/*' { ($text,$return) = main::parse_delimited($text,'/*','*/'); $return = ['comment',$return]; }
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 */
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/ line 2783, <DATA> line 1. Use of uninitialized value in substitution (s///) at (eval 15)[C:/Perl +/site/lib/Parse/] line 22, <DATA> line 1. Use of uninitialized value in concatenation (.) or string at (eval 15) +[C:/Perl/site/lib/Parse/] line 23, <DATA> line 1. Use of uninitialized value in substitution (s///) at (eval 15)[C:/Perl +/site/lib/Parse/] line 23, <DATA> line 1. Use of uninitialized value in substitution (s///) at (eval 15)[C:/Perl +/site/lib/Parse/] line 23, <DATA> line 1.

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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; }

    and the grammar rules :

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

    Successfully matches the following example :

    /* A multiple line /* with nested */ comment */

    Hope this may help someone


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...


    "Never try to teach a pig to wastes your time and it annoys the pig."
      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.

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