use strict;
use warnings;

my $precision = 4;
my $scale = 3;
my $regex = qr{\A(\d*?)\.?(\d*?)\z};

foreach my $float ("1", ".45", "3.452", "3.4526", "12.3", ".34567", "NaN") {
        if (my ($predec,$postdec) = ($float =~ m/$regex/)) {
                if (length "$postdec" > $scale) {
                        print "Invalid scale in $float ($predec\t$postdec)\n";
                } elsif (length "$predec" > $precision - $scale) {
                        print "Invalid precision in $float  ($predec\t$postdec)\n";
                }
                else {
                        print "Valid number: $float ($predec\t$postdec)\n";
                }
        } else {
                print "Invalid floating point number: $float\n";
        }
}