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01.23.06
Perl 'Eval' For Data Validation
 By
A.P. 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:
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#!/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.
*Originally published at APLawrence.com
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