问题
I'm trying to use Parse::RecDescent make a parser which can parse parenthetical expressions and the unary operator ?
.
What I have so far is failing when I create the parser because the rule expression
is left-recursive:
use strict;
use warnings;
use Parse::RecDescent;
my $test = <<END;
((foo)? bar)
END
my $grammar = q(
parse: expression(s)
expression: string | parend | expression(s)
parend : "(" (string | expression) ")" /\??/
string : /\w+/ /\??/
);
my $parser = Parse::RecDescent->new($grammar);
my $result = $parser->parse($test);
if($result){
print $result;
}else{
print STDERR "Invalid grammar\n";
}
回答1:
First, you go from lowest priority to highest priority.
parse : expr /\Z/
expr : list
list : unary(s?)
unary : unary '?'
| term
term : '(' expr ')'
| STRING
STRING : /\w+/
Of course,
unary : unary '?'
| term
doesn't work because it's left-recursive. Operator Associativity and Eliminating Left-Recursion in Parse::RecDescent can help you get rid of it. We get
unary : term unary_(s?)
unary_ : '?'
But that's not going to construct the right tree for us. So let's start by flattinging out the "(s?)
".
unary : term unary_
unary_ : '?' unary_
|
Then we can use subrule args to create the right tree.
unary : term unary_[ $item[1] ]
unary_ : '?' unary_[ [ 'postfix?' => $arg[0] ] ]
| { $arg[0] }
All together:
use strict;
use warnings;
use Data::Dumper qw( Dumper );
use Parse::RecDescent qw( );
my $grammar = <<'END';
{
use strict;
use warnings;
}
parse : expr /\Z/ { $item[1] }
expr : list
list : unary(s?) { [ $item[0] => @{ $item[1] } ] }
unary : term unary_[ $item[1] ]
unary_ : '?' unary_[ [ 'postfix?' => $arg[0] ] ]
| { $arg[0] }
term : '(' expr ')' { $item[2] }
| STRING { [ string => $item[1] ] }
STRING : /\w+/
END
my $parser = Parse::RecDescent->new($grammar)
or die "Invalid grammar\n";
my $tree = $parser->parse("((foo bar)? baz)\n")
or die "Invalid text\n";
print(Dumper($tree));
来源:https://stackoverflow.com/questions/11350898/parsing-string-with-nested-parentheses-using-parserecdescent