http://stackoverflow.com/questions/28192282/bnf-for-a-hexadecimal-number 28192282

ambiguous

Ambiguous parse: 5 alternatives.
# Alternative 1:
['hex',['hex',['hex',['hex',['digit','0']],['hex',['digit','1']]],['hex',['letter','F']]],['hex',['letter','E']]]
# Alternative 2:
['hex',['hex',['hex',['digit','0']],['hex',['hex',['digit','1']],['hex',['letter','F']]]],['hex',['letter','E']]]
# Alternative 3:
['hex',['hex',['hex',['digit','0']],['hex',['digit','1']]],['hex',['hex',['letter','F']],['hex',['letter','E']]]]
# Alternative 4:
['hex',['hex',['digit','0']],['hex',['hex',['hex',['digit','1']],['hex',['letter','F']]],['hex',['letter','E']]]]
# Alternative 5:
['hex',['hex',['digit','0']],['hex',['hex',['digit','1']],['hex',['hex',['letter','F']],['hex',['letter','E']]]]]

q28192282.pl

use 5.010;
use strict;
use warnings;

use Data::Dumper;
$Data::Dumper::Indent = 0;
$Data::Dumper::Terse = 1;
$Data::Dumper::Deepcopy = 1;

use Carp::Always; # force stack trace

use Marpa::R2;

my $g = Marpa::R2::Scanless::G->new( { source => \(<<'END_OF_SOURCE'),

    :default ::= action => [ name, value]
    lexeme default = action => [ name, value] latm => 1

#this is highly ambiguous:
<hex> ::= <digit> | <letter> | <hex> <hex>
#this is unambiguous: <hex> ::= <digit> | <letter> | <hex> <digit> | <hex> <letter>
<digit> ::= '0' | '1' | '2' | '3' | '4' | '5' | '6' | '7' | '8' | '9'
<letter> ::= 'A' | 'B' | 'C' | 'D' | 'E' | 'F'

    :discard ~ whitespace
    whitespace ~ [\s]+

END_OF_SOURCE
} );

my $input = <<EOI;
01FE
EOI

my $r = Marpa::R2::Scanless::R->new( {
    max_parses => 100,
    grammar => $g,
} );
eval { $r->read(\$input) } || warn "$@\nProgress report is:\n" . $r->show_progress;

my $ast = $r->value;

unless (defined $ast){
    die "No parse";
}

if ( $r->ambiguity_metric() > 1 ){
    # gather parses
    my @asts;
    my $v = $ast;
    do {
        push @asts, ${ $v };
    } until ( not $v = $r->value() );
#    push @asts, ${ $v };
    say "Ambiguous parse: ", $#asts + 1, " alternatives.";
    for my $i (0..$#asts){
        say "# Alternative ", $i+1, ":\n", Dumper $asts[$i];
    }
}
else{
    say Dumper $ast;
}

unambiguous

['hex',['hex',['hex',['hex',['digit','0']],['digit','1']],['letter','F']],['letter','E']]