#!perl

for $invalue (qw/ 1 -10 abc /) {

    eval{ local $^W = 1; $invalue + 0 };   ### Is Number?
    if ( $@ ) {
        print "$invalue is *not* a Number; Details: $@\n";
    } else {
        print = "$invalue appears to be a number.\n";
    }

}
[download]

1 appears to be a number.
-10 appears to be a number.
Argument "abc" isn't numeric in add at num.pl line 3.
abc appears to be a number.
[download]

To fix this, you need to force the warning into an error. (And add an empty list assignment to silence the void context warning.):

#!perl

for $invalue (qw/ 1 -10 abc /) {

    eval{ local $SIG{__WARN__} = sub { die @_ };
          local $^W = 1; () = $invalue + 0 };   ### Is Number?
    if ( $@ ) {
        print "$invalue is *not* a Number; Details: $@\n";
    } else {
        print "$invalue appears to be a number.\n";
    }

}
[download]

1 appears to be a number.
-10 appears to be a number.
abc is *not* a Number; Details: Argument "abc" isn't numeric in add at
+ num.pl line 4.
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Note, the new 'use warnings' pragma allows for various fine tuning of default and optional warnings -- including promoting any warning category into FATALs for a given lexical scope. So chipmunk's example can also be written as (assuming 5.6+):

for $invalue (qw/ 1 -10 abc /) {

    eval{
        use warnings FATAL=>qw/numeric/;
        $invalue + 0 
    };   ### Is Number?
    
    if ( $@ ) {
        print "$invalue is *not* a Number; Details: $@\n";
    } else {
        print  "$invalue appears to be a number.\n";
    }

}
[download]

For details see the perllexwarn manpage, and the docs for the warning.pm module.

   MeowChow                                   
               s aamecha.s a..a\u$&owag.print

   Beware that using eval() neither silences perl from
   printing warnings to STDERR, nor does it stuff the text
   of warning messages into $@. To do either of those, you
   have to use the $SIG{__WARN__} facility.
[download]

my $invalue = param( 'INVALUE' );
if ( defined $invalue ) {
    if ( $invalue =~ /\D/ ) {
        $result = "$invalue is *not* a Number@"
    } else {
        $result = "$invalue is a number.";
    }
} else {
    $result = "Enter a value to test."
}
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$less->{'characters'}->{'typed'} == `"time in the pub" | more`; # :-D
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Update: _doh_!! just re-read your post and realised I would have to quickly whip up a regex to allow "-"s and a possible "." so here it is...

/^(?:-)?\d+(?:\.\d+)?$/
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/^(?:-)?(?:\d+(?:\.\d+)?|\.\d+)$/
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I know it's in the FAQ, but I will construct here on my own, for the good of all.

/^(?:-)?(?:\d+(?:\.\d+)?|\.\d+)$/

# becomes

/^-?(?:\d+(?:\.\d+)?|\.\d+)$/

# UPDATE!

/^-?(?=\d|\.\d)\d*\.?\d+$/
[download]

The look-ahead is used to remove the redundancy in your regex. It basically ensures there's either a digit, or a dot and THEN a digit. Then, it matches digits and/or a dot and digits, which we know will be valid.

japhy -- Perl and Regex Hacker

Of course, if you're going to use a regex(p), you might as well make it easy on yourself:

use Regexp::Common;

if ($thing =~ /^$RE{num}{dec}$/) {
    ...
}

(this particular example, of course, requires DWIM.pm... :-)

dha

I can only refer you to the article that MeowChow mentioned. Taking a look at Tom Christiansen's list of benefits that someone who rolls their own RE usually misses, you get zero out of 3 of them.

RE's are not always the right answer...

UPDATE
Changed the link because the page moved. (Thanks footpad.)

cheers for that - don't know what I was thinking with the (?:-)? bit - maybe the smiley just cheered us all up!

correct me if i'm wrong but won't your regex pick up a number which finishes a sentence _and_ the full stop (period to americans)? this would lead to perl treating it as a string in some cases (eg. ++ or --).

4.34
-2.33333
5
20
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4.34
-2.33333
0
5
0
5
12
0
0
0
20
4.3e+015
[download]

use Benchmark;

sub regexstuff() 
{ 
   map 
        {$foo =  $_ if /^-?(?=\d|\.\d)\d*\.?\d+$/} 
        (qw/4.34 -2.33333 abc 5 maurice 5_5_4_1 
          12peter peter12 shoe fred 20 4.3E15/); 
}

sub packstuffD()
{
   map 
        { $foo =  unpack ("d", pack ("d", $_)), "\n" }
        (qw/4.34 -2.33333 abc 5 maurice 5_5_4_1 12peter
          peter12 shoe fred 20 4.3E15/);
}

sub packstuffF()
{
   map 
        { $foo =  unpack ("f", pack ("f", $_)), "\n" }
        (qw/4.34 -2.33333 abc 5 maurice 5_5_4_1 12peter
          peter12 shoe fred 20 4.3E15/);
}

sub evalstuff()
{
   map 
        { eval{ local $^W = 1; $_ + 0 } } 
        (qw/4.34 -2.33333 abc 5 maurice 5_5_4_1 12peter
          peter12 shoe fred 20 4.3E15/);

        if ( $@ ) 
           { 
           $result = "$invalue is *not* a Number; 
                Details: $@" 
        }
        else
           { 
           $result = "$invalue appears to be a number."; 
        }
}

timethese ( 100000, 
   { packstuffD => "packstuffD", 
     regex => "regexstuff", 
     eval => "evalstuff", 
     packstuffF => "packstuffF" 
   } 
);
      eval: 13 wallclock secs (11.97 usr +  0.02 sys = 11.99 CPU) @ 83
+42.37/s (n=100000)
packstuffD: 11 wallclock secs (10.06 usr +  0.00 sys = 10.06 CPU) @ 99
+45.30/s (n=100000)
packstuffF: 10 wallclock secs (10.63 usr +  0.02 sys = 10.65 CPU) @ 93
+93.20/s (n=100000)
     regex: 11 wallclock secs (11.01 usr +  0.01 sys = 11.02 CPU) @ 90
+77.71/s (n=100000)
[download]

-mark