--
grammar:

# grammar:
order:    quantity color item
quantity: number
color:    id | qstring
item:     id

# tokens:
number:  /\d+/
id:      /[a-zA-Z][-_a-zA-Z0-9]*/
qstring: /"(?:\\["\\]|[^"\\])*"/
            { $return = unquote($item[1]); }
       | /'(?:\\['\\]|[^'\\])*'/
            { $return = unquote($item[1]); }

--
helper (non-grammar) code:

sub unquote {
   local $_ = $_[0];
   s/^['"]//;
   s/['"]$//;
   s/\\(.)/\1/g;
   return $_;
}

--

##</code><code>##

--
helper (non-grammar) code:

my @COLORS = (
   'red'
   'green'
   'blue'
   'cyan'
   'magenta'
   'burnt umber'
);

my %COLORS;
{
  my $idx = 0;
  %COLORS = map { $_ => $idx++ } @COLORS;
}

sub unquote {
   local $_ = $_[0];
   s/^['"]//;
   s/['"]$//;
   s/\\(.)/\1/g;
   return $_;
}

my $error = 0;

... parse ...

die("Validation errors were found.\n") if ($error);

--
grammar:

# grammar:
order:    quantity color item
quantity: number
color:    id | qstring
             { 
                $return = $COLORS{$item[1]};
# Uncomment the following if you want
# to have multiple errors listed.
#                unless (defined($return))
#                {
#                   warn("Line $thisline: Invalid color.\n");
#                   $error = 1;
#                   $return = undef;
#                }
#                1;
             }
item:     id
             { 
                ($return) = $item[1] =~ /^(pen|pencil)$/;
# Uncomment the following if you want
# to have multiple errors listed.
#                unless (defined($return))
#                {
#                   warn("Line $thisline: Invalid item.\n");
#                   $error = 1;
#                   $return = undef;
#                }
#                1;
             }

# tokens:
number:  /\d+/
id:      /[a-zA-Z][-_a-zA-Z0-9]*/
qstring: /"(?:\\["\\]|[^"\\])*"/
            { $return = unquote($item[1]); }
       | /'(?:\\['\\]|[^'\\])*'/
            { $return = unquote($item[1]); }

--