使用 Parse::RecDescent

Question

我有以下输入

@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"
}

，我必须为 RecDescent 解析器生成器编写语法。 输出中的数据应针对 xml 结构进行修改，并且应如下所示：

<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>

附加字段和重复字段应报告为错误（带有行号的正确消息，无需进一步解析）。我尝试从这样的事情开始：

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!");

但我什至不知道这是否是正确的方法。请帮忙。

还有一个小问题。输入时的标签可能不按该特定顺序，但每个标签只能出现一次。我是否必须为（作者、标题、年份、出版商）的所有排列编写子规则？因为我想出了：

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

但它显然并不能阻止重复标签。

Answer 1

首先，你有一个拼写错误：tree而不是two。

我运行了您的程序，但添加了以下几行：

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

并得到了此调试输出：

 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.

这表明它卡在子规则 one 处，并且 press, 被放回输入溪流。这是因为您使用 return 而不是 $return = 作为 Parse::RecDescent 手册 说你应该。

此外，一旦分配给 $return 变量，您就无法再返回列表，并且必须手动将字符串连接在一起。

这是最终结果：

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!");