Using Parse::RecDescent

Question

I have the following input

@Book{press,
  author    = "Press, W. and Teutolsky, S. and Vetterling, W. and Flannery B.",
  title     = "Numerical {R}ecipes in {C}: The {A}rt of {S}cientific {C}omputing",
  year      = 2007,
  publisher = "Cambridge University Press"
}

and I have to write a grammar for RecDescent parser generator. Data at output should be modified for the xml structure and should look like this:

<book>
    <keyword>press</keyword>
    <author>Press, W.+Teutolsky, S.+Vetterling, W.+Flannery B.</author>
    <title>Numerical {R}ecipes in {C}: The {A}rt of {S}cientific {C}omputing</title>

    <year>2007</year>
    <publisher>Cambridge University Press</publisher>
</book>

Additional and repeated fields should be reported as errors (proper message with line number and no further parsing). I tried to start with something like this:

use Parse::RecDescent;

open(my $in,  "<",  "parsing.txt")  or die "Can't open parsing.txt: $!";

my $text;
while (<$in>) {
    $text .= $_;
}

print $text;

my $grammar = q {
    beginning: "\@Book\{" keyword fields "\}"           { print "<book>\n",$item[2],$item[3],"</book>"; }
    keyword: /[a-zA-Z]+/ ","                            { return "    <keyword>".$item[1]."</keyword>\n"; }
    fields: one "," two "," tree "," four               { return $item[1].$item[3].$item[5].$item[7]; }
    one: "author" "=" "\"" /[a-zA-Z\s\.\,\{\}\:]+/ "\"" {   $item[4] =~ s/\sand\s/\+/g;
                                                            return "    <author>",$item[4],"</author>\n"; }
    two: "title" "=" "\"" /[a-zA-Z\s\.\,\{\}\:]+/ "\""  {   $item[4] =~ s/\sand\s/\+/g;
                                                            return "    <title>",$item[4],"</title>\n"; }
    three: "year" "=" /[0-2][0-9][0-9][0-9]/            {   return "    <year>",$item[3],"</year>\n"; }
    four: "publisher" "=" "\"" /[a-zA-Z\s\.\,\{\}\:]+/ "\"" 
                                                        {   $item[4] =~ s/\sand\s/\+/g;
                                                            return "    <publisher>",$item[4],"</publisher>\n"; }
};

my $parser = new Parse::RecDescent($grammar) or die ("Bad grammar!");
defined $parser->beginning($text) or die ("Bad text!");

But I don't even know if it's the correct way to do it. Please help.

There's a one more tiny problem. Tags at input might not be in that particular order, but each tag can appear only once. Do I have to write subrules for all permutations of (author,title,year,publisher)? Because I came up with:

fields: field "," field "," field "," field
field: one | two | three | four

but it obviously doesn't prevent from repeating tags.

Answer 1

Firstly, you have a typo: tree instead of three.

I ran your program but added the lines:

use strict;
use warnings; # you should always have strict and warnings on
$::RD_HINT = 1; # Parse::RecDescent hints
$::RD_TRACE = 1; # Parse::RecDescent trace

and got this debug output:

 1|beginning |>>Matched terminal<< (return value:   |
  |          |[@Book{])                             |
 1|beginning |                                      |"press,\n author = "Press,
  |          |                                      |W. and Teutolsky, S. and
  |          |                                      |Vetterling, W. and Flannery
  |          |                                      |B.",\n title = "Numerical
  |          |                                      |{R}ecipes in {C}: The {A}rt
  |          |                                      |of {S}cientific
  |          |                                      |{C}omputing",\n year =
  |          |                                      |2007,\n publisher =
  |          |                                      |"Cambridge University
  |          |                                      |Press"\n}\n"
 1|beginning |Trying subrule: [keyword]             |
 2| keyword  |Trying rule: [keyword]                |
 2| keyword  |Trying production: [/[a-zA-Z]+/ ',']  |
 2| keyword  |Trying terminal: [/[a-zA-Z]+/]        |
 2| keyword  |>>Matched terminal<< (return value:   |
  |          |[press])                              |
 2| keyword  |                                      |",\n author = "Press, W. and
  |          |                                      |Teutolsky, S. and
  |          |                                      |Vetterling, W. and Flannery
  |          |                                      |B.",\n title = "Numerical
  |          |                                      |{R}ecipes in {C}: The {A}rt
  |          |                                      |of {S}cientific
  |          |                                      |{C}omputing",\n year =
  |          |                                      |2007,\n publisher =
  |          |                                      |"Cambridge University
  |          |                                      |Press"\n}\n"
 2| keyword  |Trying terminal: [',']                |
 2| keyword  |>>Matched terminal<< (return value:   |
  |          |[,])                                  |
 2| keyword  |                                      |"\n author = "Press, W. and
  |          |                                      |Teutolsky, S. and
  |          |                                      |Vetterling, W. and Flannery
  |          |                                      |B.",\n title = "Numerical
  |          |                                      |{R}ecipes in {C}: The {A}rt
  |          |                                      |of {S}cientific
  |          |                                      |{C}omputing",\n year =
  |          |                                      |2007,\n publisher =
  |          |                                      |"Cambridge University
  |          |                                      |Press"\n}\n"
 2| keyword  |Trying action                         |
 1|beginning |>>Matched subrule: [keyword]<< (return|
  |          |value: [    <keyword>press</keyword> ]|
 1|beginning |                                      |"press,\n author = "Press,
  |          |                                      |W. and Teutolsky, S. and
  |          |                                      |Vetterling, W. and Flannery
  |          |                                      |B.",\n title = "Numerical
  |          |                                      |{R}ecipes in {C}: The {A}rt
  |          |                                      |of {S}cientific
  |          |                                      |{C}omputing",\n year =
  |          |                                      |2007,\n publisher =
  |          |                                      |"Cambridge University
  |          |                                      |Press"\n}\n"
 1|beginning |Trying subrule: [fields]              |
 2|  fields  |Trying rule: [fields]                 |
 2|  fields  |Trying production: [one ',' two ','   |
  |          |three ',' four]                       |
 2|  fields  |Trying subrule: [one]                 |
 3|   one    |Trying rule: [one]                    |
 3|   one    |Trying production: ['author' '=' '\"' |
  |          |/[a-zA-Z\s\.\,{}\:]+/ '\"']           |
 3|   one    |Trying terminal: ['author']           |
 3|   one    |<<Didn't match terminal>>             |
 3|   one    |<<Didn't match rule>>                 |
 2|  fields  |<<Didn't match subrule: [one]>>       |
 2|  fields  |<<Didn't match rule>>                 |
 1|beginning |<<Didn't match subrule: [fields]>>    |
 1|beginning |<<Didn't match rule>>                 |
Bad text! at parser.pl line 32, <$in> line 6.

This shows that it's getting stuck at subrule one, and that press, is getting put back onto the input stream. This is because you're using return rather than $return = as the Parse::RecDescent manual says you should.

Furthermore, once you are assigning to the $return variable, you can no longer return a list, and must concatenate the strings together manually.

Here's the final result:

use strict;
use warnings;
use Parse::RecDescent;

open(my $in,  "<",  "parsing.txt")  or die "Can't open parsing.txt: $!";

my $text;
while (<$in>) {
    $text .= $_;
}

print $text;

my $grammar = q {
    beginning: "\@Book\{" keyword fields /\s*\}\s*/           { print "<book>\n",$item[2],$item[3],"</book>"; }
    keyword: /[a-zA-Z]+/ ","                            { $return = "    <keyword>$item[1]</keyword>\n"; }
    fields: one /,\s*/ two /,\s*/ three /,\s*/ four               { $return = $item[1].$item[3].$item[5].$item[7]; }
    one: "author" "=" "\"" /[a-zA-Z\s\.\,\{\}\:]+/ "\"" {   $item[4] =~ s/\sand\s/\+/g;
                                                            $return =  "    <author>$item[4]</author>\n"; }
    two: "title" "=" "\"" /[a-zA-Z\s\.\,\{\}\:]+/ "\""  {   $item[4] =~ s/\sand\s/\+/g;
                                                            $return =  "    <title>$item[4]</title>\n"; }
    three: "year" "=" /[0-2][0-9][0-9][0-9]/            {   $return = "    <year>$item[3]</year>\n"; }
    four: "publisher" "=" "\"" /[a-zA-Z\s\.\,\{\}\:]+/ "\"" 
                                                        {   $item[4] =~ s/\sand\s/\+/g;
                                                            $return = "    <publisher>$item[4]</publisher>\n"; }
};

my $parser = new Parse::RecDescent($grammar) or die ("Bad grammar!");
defined $parser->beginning($text) or die ("Bad text!");