In Perl, how can I tell if a string is a number?

Question

I am using Perl to convert some XML to JSON. If the XML attribute is a number, I don\'t want to put quotes around it so that JSON will treat it as a number and not a string. How

Answer 1

You could just force it into a number then compare that to the original string.

if( $value eq $value+0 ){
  print "$value is a number\n";
}

( Note: it will only work for simple numbers, like 123 or 12.3 )

Answer 2

I think this question from perlfaq solves your problem.

Generally the problem is defining what exactly you want to read as number.

• is "-1.312" valid number?
• is "inf"?
• 5.34123412E-03 ?

Answer 3

It might be easier for you to just read the XML into a data structure in Perl and let Perl's JSON library figure it out for you. It already has checking for that, so unless your number is actually a string in the XML (e.g. it's got a space after it, etc) JSON->encode() will encode it as a JSON number.

Answer 4

The JSON specification provides fairly clear rules on the format of a number, so the following regex should work:

/^-?(0|([1-9][0-9]*))(\.[0-9]+)?([eE][-+]?[0-9]+)?$/

Answer 5

Try Scalar::Util::looks_like_number:

E.g.:

use Scalar::Util qw(looks_like_number);

if (looks_like_number($thingy)) {
    print "looks like $thingy is a number...\n"
}

Answer 6

Assuming you don't need to support unusual stuff (like sci-notation) this almost works (and is very simple):

#!/usr/bin/perl

my $foo = '1234.5';

if( $foo =~ /\d+/ ){
    print "$foo is a number\n";
}

The reason it doesn't fully work is because you can have hyphens and dots anywhere (as many as you please) as long as you have at least one digit present). '--1--2' evaluates as zero, and '1.2.3.4.5' evals as 1.2 (the second dot and everything after are ignored). This may or may not be an issue for you.