Question

I know that there is a library that do that

use Scalar::Util qw(looks_like_number);

yet I want to do it using perl regular expression. And I want it to work for double numbers not for only integers.

so I want something better than this

$var =~ /^[+-]?\d+$/

thanks.

Answer 1

Constructing a single regular expression to validate a number is really difficult. There simply are too many criteria to consider. Perlfaq4 contains a section “How do I determine whether a scalar is a number/whole/integer/float?

The code from that documentation shows the following tests:

if (/\D/)                          {print "has nondigits\n"      }
if (/^\d+$/)                       {print "is a whole number\n"  }
if (/^-?\d+$/)                     {print "is an integer\n"      }
if (/^[+-]?\d+$/)                  {print "is a +/- integer\n"   }
if (/^-?\d+\.?\d*$/)               {print "is a real number\n"   }
if (/^-?(?:\d+(?:\.\d*)?|\.\d+)$/) {print "is a decimal number\n"}
if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/) {
    print "is a C float\n"
}

• The first test disqualifies an unsigned integer.
• The second test qualifies a whole number.
• The third test qualifies an integer.
• The fourth test qualifies a positive/negatively signed integer.
• The fifth test qualifies a real number.
• The sixth test qualifies a decimal number.
• The seventh test qualifies a number in c-style scientific notation.

So if you were using those tests (excluding the first one) you would have to verify that one or more of the tests passes. Then you’ve got a number.

Another method, since you don’t want to use the module Scalar::Util, you can learn from the code IN Scalar::Util. The looks_like_number() function is set up like this:

sub looks_like_number {
  local $_ = shift;

  # checks from perlfaq4
  return $] < 5.009002 unless defined;
  return 1 if (/^[+-]?\d+$/); # is a +/- integer
  return 1 if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/); # a C float
  return 1 if ($] >= 5.008 and /^(Inf(inity)?|NaN)$/i) 
           or ($] >= 5.006001 and /^Inf$/i);

  0;
}

You should be able to use the portions of that function that are applicable to your situation.

I would like to point out, however, that Scalar::Util is a core Perl module; it ships with Perl, just like strict does. The best practice of all is probably to just use it.