Perl 'eval' for data validation

© January 2006 Anthony Lawrence

I recently did a little web based Service Schedule. This is something that gets its data from another program: in other words, the details of what will be serviced and when are supplied by that program; this web based app validates things, assigns technicians and does some other things that the first program doesn't.

One of the things entered is the requested date for service, which might look like "Sat 2/6/06". If you happen to have a 2006 calendar handy ("cal 2 2006" will give you one on any Unix/Linux box), you'll notice a small problem: 2/6/2006 is not a Saturday. Because the data entry to the first application isn't validated, my program can receive just about anything and it has to check it.

Checking the day is not too hard in Perl. Here's a little test script that demonstrates.

#!/usr/bin/perl
use Time::Local;
@days=qw(SUN MON TUE WED THU FRI SAT);
checkdate($ARGV[0],$ARGV[1]);

sub checkdate {
    $date=shift;
    $day=shift;
    @d=split /\//,$date;
    $newdate="";
    $newdate=timelocal(0,0,0,$d[1],$d[0]-1,$d[2]+100);
    @newd=localtime($newdate);
    my $line="$date ";
    if ($day !~ /$days[$newd[6]]/i) {
          $warn="!! $days[$newd[6]] !!";
    }
    $line .=  "$day $warn";
    print "$line\n";
}

However, there's a big problem. What happens if you pass that script absolute garbage?

$ ./checkdates.pl 2/6/06 sat
2/6/06 sat !! MON !!
$ ./checkdates.pl 2/6/06 mon
2/6/06 mon 
$ ./checkdates.pl 2/29/06 mon
Day '29' out of range 1..28 at ./checkdates.pl line 11
 

Oops. That's not going to work.

Fortunately, it's not hard to fix, and that's where "eval" comes in. It's not much of a change:

#!/usr/bin/perl
use Time::Local;
@days=qw(SUN MON TUE WED THU FRI SAT);
checkdate($ARGV[0],$ARGV[1]);

sub checkdate {
    $date=shift;
    $day=shift;
    @d=split /\//,$date;
    $newdate="";
    eval {
    $newdate=timelocal(0,0,0,$d[1],$d[0]-1,$d[2]+100);
    @newd=localtime($newdate);
    };
    my $line="$date ";
    if ($day !~ /$days[$newd[6]]/i) {
          $warn="!! $days[$newd[6]] !!";
    }
    $line .=  "$day $warn";
    print "$line\n";
}
 

The script doesn't crash now:

$ ./checkdates.pl 2/29/06 mon
2/29/06 mon !! SUN !!
 

In this form it's not particularly helpful in identifying that the input was bad data, but that's easy enough to fix if you want: just check to see if $newdate is still blank. If it is, the date conversion failed outright. You can also test the return value of the "eval".

This "eval" protects a block of code from crashing your script. Be sure to notice the ";" following the close of the block; that's necessary here.

The "eval" can also be used to compile and evaluate strings; examples are in your Perl manual.

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