#!/usr/bin/perl -wT
use strict;
use CGI;
use HTML::Entities;

my $query = CGI->new;
my $tainted_number = $query->param( "bob" );
my ( $clean_number ) = ( $tainted_number =~ /^\s*(\d+)\s*$/ );

print $query->header,
      $query->start_html( -title => "Input test" );

if ( defined $clean_number ) {
    print $query->p( "You entered '$clean_number'." );
} else {
    print $query->p( "You're only supposed to enter numbers, dummy." )
+,
          $query->p( "You entered '" . encode_entities( $tainted_numbe
+r ) . "'" );
}

print $query->end_html;
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HTML::Entities is used to ensure that the user-supplied data does not interpret as HTML (for example, it converts <> to &lt;&gt;).

See perlsec if you're unsure about why I used the -T switch.

Cheers,
Ovid

Join the Perlmonks Setiathome Group or just click on the the link and check out our stats.

Think "What's a representation of an integer?" One answer is "a string of only digits", but that won't catch the negative integers. There *might* be a "-" at the front of such a string. So you want to see that the string matches that pattern. For pattern matches, use a regular expression.

Here's some info that ought to help you figure it out for yourself:
^ says "match at the beginning of the string"
? says "match 0 or 1 of the previous pattern"
\d says "match digits"
$ says "match at the end of the string

So, your pattern will start with ^ and end with $.

That should be enough to get you started.

HTH.

$foo = $query->param('should_be_int');
warn "Not an integer!" if ($foo =~ /[^0-9\-]/)
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Trinary

That allows the following input:

-54
54-
5-4
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I try to specify exactly what is valid:

/^-?\d+$/
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Update: Yup, Kanji is right (note the lowercase z). I never had that problem because I chomp my input first.

--isotope
http://www.skylab.org/~isotope/

I try to specify exactly what is valid:

Then wouldn't you want /\A-?\d+\z/?

If it's there, $ will match a single newline at the end of your string, and you'd end up matching something bogus like "7\n".

    --k.

Jeez. Of course you're right.

Something about quarter past 4 on a warm friday afternoon seems to have made thinking difficult.

Trinary

-mark