Matching terminals with Parse::RecDescent

fce2 has asked for the wisdom of the Perl Monks concerning the following question:

When matching terminals, Parse::RecDescent seems to produce a match if a token begins with the terminal, rather than trying for an exact match. This code demonstrates this:

#!/usr/bin/perl

use warnings;
use strict;

use Parse::RecDescent;

my $grammar = q{
    FOO: "foo"
};

my $p = Parse::RecDescent->new($grammar);

my @tests = ("foo", "foo ", "foo bar", "football", "fog", "barfoo", "b
+ar foo");

for my $test (@tests) {
    if (my $ret = $p->FOO($test)) {
        print "match: '$test' -> '$ret'\n";
    }
    else {
        print "no match: '$test'\n";
    }
}
[download]

This isn't what I want. I want it to produce an error/return undef when what was entered doesn't match in full, rather than simply eating the remainder.

I doubt this is a bug, because it seems too trivial, so I suspect that this is actually a lack of understanding of parsing in general on my part. Could someone please set me straight here? Thanks!

Comment on Matching terminals with Parse::RecDescent Download Code

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Re: Matching terminals with Parse::RecDescent by GrandFather (Saint) on Dec 11, 2006 at 21:30 UTC
You need to force a end of line (or end of file) match. For the sample given, changing your grammar to: `FOO: /foo$/` [download] does the trick. In the context of a file being parsed you are more likely to want `FOO: "foo" /\Z/` [download] DWIM is Perl's answer to Gödel	[reply] [d/l] [select]
Re^2: Matching terminals with Parse::RecDescent by fce2 (Sexton) on Dec 12, 2006 at 04:01 UTC
/foo$/ works for the example script. "foo" /\Z/ didn't match anything (though /foo\Z/ did). All fall down if I test with this grammar: `line: FOO BAR FOO: /foo\Z/ BAR: /bar\Z/` [download] (and use $p->line instead of $p->foo). I probably should have been more specific, but I'm looking for a more generalised solution to this, or at least some understanding of where my expectations are wrong.	[reply] [d/l]
Re^3: Matching terminals with Parse::RecDescent by ikegami (Patriarch) on Dec 12, 2006 at 06:36 UTC
or at least some understanding of where my expectations are wrong. Your grammar expects to match 0 or more whitespace characters (see `<skip>`), followed by "foo", followed by the end of the string to parse, 0 or more whitespace characters, followed by "bar", followed by the end of the string to parse.	[reply] [d/l]
Re^3: Matching terminals with Parse::RecDescent by GrandFather (Saint) on Dec 12, 2006 at 06:25 UTC
With the grammar: `line: FOO BAR /$/ FOO: "foo" BAR: "bar"` [download] and the invocation changed line to: `if (defined (my $ret = $p->line($test))) {` [download] I get: `no match: 'foo' no match: 'foo ' match: 'foo bar' -> '' no match: 'football' no match: 'fog' no match: 'barfoo' no match: 'bar foo'` [download] Is that not what you expect? DWIM is Perl's answer to Gödel	[reply] [d/l] [select]