#!/usr/bin/perl -w
use strict;
sub arabic;# Changes a Roman Numeral to an Arabic Number
           # input Roman Numeral < 4000,
           # output Arabic Number as string.
sub isroman; # Check if string is Roman Numeral and return true if so.
my $roman_numeral = "MMCXLII";
my $result = "";
my $digit = "";
my $last_digit = "";
my $roman_digit = "";
# Hash table follows for converting A Roman Numeral Digit to an
# Arabic Number.
my %roman2arabic = qw(I 1 V 5 X 10 L 50 C 100  D 500 M 1000);
sub isroman {  # This will return True as long the Roman Numeral
               # contains only valid letters and is less than 
               # 4000, i.e. I  TO MMMCMXCIX
               # First we check if it begins with 0 to 3  M's.   
               # then D or 0 to 3 C's, or contains CD or CM,
               # then L or 0 to 3 X's, or contains XL or XC,
               # then V or 0 to 3 I's, or contains IV or IX.
               # We do this ignoring case and making the expression 
               # more readable using white space as dentoed
               # by the "ix" at the end.
## Perl 5 From O'Reilly book, check for Roman Numeral
# $str =~ m/ ^ m* (?:d?c{0,3}|c[dm]) (?:l?x{0,3}|x[lc]) (?:v?i{0,3}|i[vx]) $ /ix;
## End O'Reilly Perl Cookbook Quote
   my($arg) = shift;
   $arg ne '' and 
     $arg =~ /^(?: M{0,3})
               (?: D?C{0,3} | C[DM])
               (?: L?X{0,3} | X[LC])
               (?: V?I{0,3} | I[VX])$/ix;
}
sub arabic { # Input Roman Numeral , Return Arabic Number
   my($arg) = shift; # Get Input of Roman Numeral
   isroman $arg or return undef; # Check it or return undefined, leaving                                 # module
   my $last_digit = 1000; # The last digit we will write is the
                          # thousands place if it is there.
                          # M would map to 1000 as the last_digit.
   my $arabic_number = 0; # initialize to meaningless value
# note there is no zero in Roman Numerals so
# we will never get zero as our Arabic Number.
   foreach (split (//, uc $arg)){ # Loop through the Roman Numeral
     #starting with the left most Roman Numeral Digit, i.e. M.
     # We use the Hash Table to look up the corresponding Arabic Number
     # for the Roman Numeral. At first glance it looks like we could
     # just use these two lines:
     # $last_digit = $digit;
     # $arabic_number = $arabic_number + $last_digit;
     # This will loop through each Roman Numeral and sum it to the
     # previous total. However, pay attention what happens when we
     # encounter XL for 40. We would interpret this as 60, we fix this
     # by noting here the last_digit X is less than L . Therefore we
     # put in the correction of subtracting out twice the last digit.
     # This gives 60 - 20 (2 * X) yielding 40 the proper amount.
     # This was the trickiest part of the algorithm.
     # Note, I have defaulted to the more readable perl code instead
     # of combining everything in a line with the += and -= operators.
     
     my $digit = $roman2arabic{$_};
     if ($last_digit < $digit)
          {$arabic_number = $arabic_number - (2 * $last_digit);}
      $last_digit = $digit;
      $arabic_number = $arabic_number + $last_digit;
}  
   $arabic_number ;
}
  
# End Subroutine Section
#
# Main Section
  $result = arabic($roman_numeral);
  print "This is arabic number: $result \n"; 
  print "for roman numeral: $roman_numeral \n";
# End Main Section
exit;