rpn.pl

token Op {'/' || '*' || '+' || '-'};
token Value { \d+[\.\d+]? };
token Item { <Value> || <Op> };
token Expr { [<ws> <Item> <ws>]+ };

sub do_op($lhs, $rhs, $op) {
  given $op {
    when '*' { $lhs * $rhs }
    when '+' { $lhs + $rhs }
    when '-' { $lhs - $rhs }
    when '/' { $lhs / $rhs }
  }
}

# Read from the command line.
my $str = @*ARGS[0];

if $str ~~ /^ <Expr> $/ {
  my @stack;
  for $/<Expr><Item> -> $item {
    if $item<Value> {
      @stack.push($item<Value>);
    } else {
      my $v1 = @stack.pop;
      my $v0 = @stack.pop;
      @stack.push(do_op($v0,$v1,$item<Op>));
    }
  }
  say @stack[0];
} else {
  say "This is not an RPN expression.";
}

# from http://daniel.carrera.bz/2009/06/rpn-calculator-in-perl-6/

rpn.rb

Op    = /[\/*+-]/
Value = /\d+[\.\d+]?/
Item  = /#{Value}|#{Op}/
Expr  = /(\s*#{Item}\s*)+/

def do_op(lhs, rhs, op)
  case op
    when "*" then lhs * rhs
    when "+" then lhs + rhs
    when "-" then lhs - rhs
    when "/" then lhs / rhs
  end
end

# Read from the command line.
str = ARGV[0]

if str =~ /^#{Expr}$/
  stack = []
  str.scan(Item) do |item|
    if value = item[Value]
      stack.push(value)
    else
      v1 = stack.pop.to_i
      v0 = stack.pop.to_i
      stack.push(do_op(v0, v1, item[Op]))
    end
  end
  puts stack[0]
else
  puts "This is not an RPN expression."
end