use v5.10;
use warnings;
use List::Util qw< reduce >;
my $calculator = do{
use Regexp::Grammars;
qr{
\A
<Answer>
<rule: Answer>
<[_Operand=Mult]>+ % <[_Op=(\+|\-)]>
(?{ $MATCH = shift @{$MATCH{_Operand}};
for my $term (@{$MATCH{_Operand}}) {
my $op = shift @{$MATCH{_Op}//=[]};
if ($op eq '+') { $MATCH += $term; }
else { $MATCH -= $term; }
}
})
<rule: Mult>
<[_Operand=Pow]>+ % <[_Op=(\*|/|%)]>
(?{ $MATCH = reduce { eval($a . shift(@{$MATCH{_Op}}) . $b) }
@{$MATCH{_Operand}};
})
<rule: Pow>
<[_Operand=Term]>+ % <_Op=(\^)>
(?{ $MATCH = reduce { $b ** $a } reverse @{$MATCH{_Operand}}; })
<rule: Term>
<MATCH=Literal>
| <.OpenParen= (\() >
<MATCH=Answer>
<.CloseParen= (\)) >
<token: Literal>
<MATCH=( [+-]? \d++ (?: \. \d++ )?+ )>
}xms
};
#say $calculator; die;
while (my $input = <>) {
if ($input =~ $calculator) {
say '--> ', $/{Answer};
}
}