How do I tell if a variable has a numeric value in Perl?

Question

Is there a simple way in Perl that will allow me to determine if a given variable is numeric? Something along the lines of:

if (is_number($x))
{ ... }


        

Answer 1

I don't believe there is anything builtin to do it. For more than you ever wanted to see on the subject, see Perlmonks on Detecting Numeric

Answer 2

if ( defined $x && $x !~ m/\D/ ) {} or $x = 0 if ! $x; if ( $x !~ m/\D/) {}

This is a slight variation on Veekay's answer but let me explain my reasoning for the change.

Performing a regex on an undefined value will cause error spew and will cause the code to exit in many if not most environments. Testing if the value is defined or setting a default case like i did in the alternative example before running the expression will, at a minimum, save your error log.

Answer 3

Check out the CPAN module Regexp::Common. I think it does exactly what you need and handles all the edge cases (e.g. real numbers, scientific notation, etc). e.g.

use Regexp::Common;
if ($var =~ /$RE{num}{real}/) { print q{a number}; }

Answer 4

Use Scalar::Util::looks_like_number() which uses the internal Perl C API's looks_like_number() function, which is probably the most efficient way to do this. Note that the strings "inf" and "infinity" are treated as numbers.

Example:

#!/usr/bin/perl

use warnings;
use strict;

use Scalar::Util qw(looks_like_number);

my @exprs = qw(1 5.25 0.001 1.3e8 foo bar 1dd inf infinity);

foreach my $expr (@exprs) {
    print "$expr is", looks_like_number($expr) ? '' : ' not', " a number\n";
}

Gives this output:

1 is a number
5.25 is a number
0.001 is a number
1.3e8 is a number
foo is not a number
bar is not a number
1dd is not a number
inf is a number
infinity is a number

See also:

• perldoc Scalar::Util
• perldoc perlapi for looks_like_number

Answer 5

Personally I think that the way to go is to rely on Perl's internal context to make the solution bullet-proof. A good regexp could match all the valid numeric values and none of the non-numeric ones (or vice versa), but as there is a way of employing the same logic the interpreter is using it should be safer to rely on that directly.

As I tend to run my scripts with -w, I had to combine the idea of comparing the result of "value plus zero" to the original value with the no warnings based approach of @ysth:

do { 
    no warnings "numeric";
    if ($x + 0 ne $x) { return "not numeric"; } else { return "numeric"; }
}

Answer 6

Not perfect, but you can use a regex:

sub isnumber 
{
    shift =~ /^-?\d+\.?\d*$/;
}