Beefy Boxes and Bandwidth Generously Provided by pair Networks
laziness, impatience, and hubris

comment on

( [id://3333]=superdoc: print w/replies, xml ) Need Help??

When the score is settled, each character of the text can only be matched by two rules if one rule is a production of another.

text: struct { int foo ; int bar ; } ------ - --- --- - --- --- - - IDENT "{" IDENT IDENT ";" IDENT IDENT ";" "}" --- --- --- --- type var type var --------- --------- decl decl ------------------------- decl_list --------------------------------------------- struct --------------------------------------------- parse

But in reaching that state, a rule can match, then be unmatched by a backtrack. For example, given the grammar

parse : foo1 foo2 | bar1 bar2

foo1 could matched, but PRD will backtrack if it can't follow with a foo2 match. It will then try bar1.

I'm guessing one of your productions has side-effects, so you falsely believed it has matched even though a backtrack unmatched it. I could very well be wrong because I have very little data to go on.

Update: In the following example, you'll see foo1 on the screen even though it wasn't matched.

use strict; use warnings; use Parse::RecDescent qw( ); my $grammar = <<'__EOI__'; { use strict; use warnings; } parse : foo1 foo2 /\Z/ { [ @item[0,1,2] ] } | bar1 bar2 /\Z/ { [ @item[0,1,2] ] } foo1 : "X" { print("$item[0]\n"); [ @item[0,1] ] } foo2 : "Y" { print("$item[0]\n"); [ @item[0,1] ] } bar1 : "X" { print("$item[0]\n"); [ @item[0,1] ] } bar2 : "Z" { print("$item[0]\n"); [ @item[0,1] ] } __EOI__ Parse::RecDescent->Precompile($grammar, 'Grammar') or die("Bad grammar\n");
use strict; use warnings; use Data::Dumper qw( Dumper ); use Grammar qw( ); my $parser = Grammar->new(); my $matches = $parser->parse('XZ') or die("Bad input\n"); print("\n"); print(Dumper($matches));
foo1 bar1 bar2 $VAR1 = [ 'parse', [ 'bar1', 'X' ], [ 'bar2', 'Z' ] ];

In reply to Re: Parse::RecDescent matching same line twice by ikegami
in thread Parse::RecDescent matching same line twice by kudra

Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post; it's "PerlMonks-approved HTML":

  • Are you posting in the right place? Check out Where do I post X? to know for sure.
  • Posts may use any of the Perl Monks Approved HTML tags. Currently these include the following:
    <code> <a> <b> <big> <blockquote> <br /> <dd> <dl> <dt> <em> <font> <h1> <h2> <h3> <h4> <h5> <h6> <hr /> <i> <li> <nbsp> <ol> <p> <small> <strike> <strong> <sub> <sup> <table> <td> <th> <tr> <tt> <u> <ul>
  • Snippets of code should be wrapped in <code> tags not <pre> tags. In fact, <pre> tags should generally be avoided. If they must be used, extreme care should be taken to ensure that their contents do not have long lines (<70 chars), in order to prevent horizontal scrolling (and possible janitor intervention).
  • Want more info? How to link or How to display code and escape characters are good places to start.
Log In?

What's my password?
Create A New User
Domain Nodelet?
and the web crawler heard nothing...

How do I use this?Last hourOther CB clients
Other Users?
Others perusing the Monastery: (7)
As of 2024-05-20 17:48 GMT
Find Nodes?
    Voting Booth?

    No recent polls found