How to tell apart numeric scalars and string scalars in Perl?

Question

Perl usually converts numeric to string values and vice versa transparently. Yet there must be something which allows e.g. Data::Dumper to discriminate between

Answer 1

When a variable is used as a number, that causes the variable to be presumed numeric in subsequent contexts. However, the reverse isn't exactly true, as this example shows:

use Data::Dumper;

my $foo = '1';
print Dumper $foo;  #character
my $bar = $foo + 0;
print Dumper $foo;  #numeric
$bar = $foo . ' ';
print Dumper $foo;  #still numeric!
$foo = $foo . '';
print Dumper $foo;  #character

One might expect the third operation to put $foo back in a string context (reversing $foo + 0), but it does not.

If you want to check whether something is a number, the standard way is to use a regex. What you check for varies based on what kind of number you want:

if ($foo =~ /^\d+$/)      { print "positive integer" }
if ($foo =~ /^-?\d+$/)    { print "integer"          }
if ($foo =~ /^\d+\.\d+$/) { print "Decimal"          }

And so on.

It is not generally useful to check how something is stored internally--you typically don't need to worry about this. However, if you want to duplicate what Dumper is doing here, that's no problem:

if ((Dumper $foo) =~ /'/) {print "character";}

If the output of Dumper contains a single quote, that means it is showing a variable that is represented in string form.

Answer 2

You might want to try Params::Util::_NUMBER:

use Params::Util qw<_NUMBER>;

unless ( _NUMBER( $scalar ) or $scalar =~ /^'.*'$/ ) { 
   $scalar =~ s/'/''/g;
   $scalar = "'$scalar'";
}

Answer 3

One simple solution that wasn't mentioned was Scalar::Util's looks_like_number. Scalar::Util is a core module since 5.7.3 and looks_like_number uses the perlapi to determine if the scalar is numeric.