You don't say what should happen with negative numbers, but it's easy enough to exclude them in the same way four-or-more digit patterns are excluded.
Update: A link to Regexp::Common might be helpful.
>perl -wMstrict -le
"use Regexp::Common;
;;
for my $t (qw(
12.12 123.123 1 12 123 1. 12. 123. .1 .12 .123 0.1 1.0 0.0
1234.0 1234 0.1234 .1234 -123 -.12 -0.1
)) {
my $ok = $t =~ m{ \A (?! .* \d{4}) $RE{num}{real} \z }xms;
printf qq{%s '$t' \n}, $ok ? 'accept' : 'REJECT';
}
"
accept '12.12'
accept '123.123'
accept '1'
accept '12'
accept '123'
accept '1.'
accept '12.'
accept '123.'
accept '.1'
accept '.12'
accept '.123'
accept '0.1'
accept '1.0'
accept '0.0'
REJECT '1234.0'
REJECT '1234'
REJECT '0.1234'
REJECT '.1234'
accept '-123'
accept '-.12'
accept '-0.1'
