#!/usr/bin/perl -w
use strict;

my @stack = ();
my %ops = ( '+' => \&sum, '-' => \&difference,
            '*' => \&product, '/' => \&quotient,
            'p' => sub { print $_, "\n" foreach @stack; },
            'q' => sub { exit 0; }
          );

while(<>) {
  chomp;
  if (exists $ops{$_}) {
    $ops{$_}->();
  # The badass regex is out of the Perl Cookbook, page 44
  # Makes sure non-ops are valid numbers
  } elsif ($_ =~ /^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/) {
    push @stack, $_;
  } else {
    warn "Bad input: $_\n";
  }
}

sub pop_stack {
  my $rhs = pop @stack;
  my $lhs = pop @stack;
  if ((! defined $lhs) || (! defined $rhs)) {
    warn "Not enough values on stack!\n";
    push @stack, $rhs if defined $rhs;
    return undef;
  }
  return $lhs, $rhs;
}

sub sum {
  my ($lhs, $rhs) = pop_stack;
  return unless defined $lhs;
  my $res = $lhs + $rhs;
  print "$res \n";
  push @stack, $res;
}

sub difference {
  my ($lhs, $rhs) = pop_stack;
  return unless defined $lhs;
  my $res = $lhs - $rhs;
  print "$res \n";
  push @stack, $res;
}

sub product {
  my ($lhs, $rhs) = pop_stack;
  return unless defined $lhs;
  my $res = $lhs * $rhs;
  print "$res \n";
  push @stack, $res;
}

sub quotient {
  my ($lhs, $rhs) = pop_stack;
  return unless defined $lhs;
  my $res = $lhs / $rhs;
  print "$res \n";
  push @stack, $res;
}