use strict;
use warnings;

my $string = '1,2,a,b,3';

$string =~ s/( (?:(?<!\d)(?:,|\.)(?!\s*\d))     |
			   (?:(?<=\d)(?:,|\.)(?=\s*)(?!\d)) |
			   (?:(?<!\d)(?:,|\.)(?=\s*\d))     )
			/$1 /gx;

print $string, "\n";