http://www.perlmonks.org?node_id=747329


in reply to Parse::RecDescent matching same line twice

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' ] ];