#! perl
use strict;
use warnings;
use feature qw( say );

my $decimal = qr{
                    (
                        (?: \d+ \.? \d* ) |
                        (?: \d* \.? \d+ )
                    )
                }x;

while (<DATA>)
{
    chomp;
    next unless length;
    say "$_ => [", (/$decimal/x ? "$1]" : ']');
}

__DATA__
0.12
.12
12.
12
no numbers here
abc.def42
.7zx