Given the following code: 
use strict;
use warnings;
use Parse::RecDescent;

my $grammar = 'startrule: ( "aa" | "a" ) "a"';
my $parser  = Parse::RecDescent->new($grammar);
my $text    = 'aa';

print defined($parser->startrule($text)) ? "Good!\n" : "Bad!\n";

[download]
the output is "Bad!". The reason is that the first branch of the alternation matched, then the next subrule failed.

Now, I understand from the P::RD manpage that bottom-up parsers try all possible matches and select the longest one, so I guess the yacc equivalent would have worked. However, I still found the behaviour of P::RD a little surprising.

My question is: Is there any way to persuade a top-down parser like P::RD to accept the above text as valid? I know that, in this simple example, I could easily rewrite the grammar (either by saying ("a" | "aa" ) "a" or "aa" a" | "a" "a"). What I mean is: Is there any additional feature I have missed which would allow the grammar as is to parse the text successfully?

To put the question another way, can I get P::RD to behave more like a regex engine? After all, even an NFA engine would backtrack to try all possible alternatives before failing :-) (Perhaps parsers just do not backtrack past individual subrules under any circumstances.)

In reply to Backtracking in Parse::RecDescent by ruscoekm

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