how do I look for n or less characters?

You want /\d{0,3}/ (three digits or less)?

$x = '1234';
print $x =~ /^\d{0,3}$/ ? "'$x' matches\n" : "'$x' no match\n";
$x = '123';
print $x =~ /^\d{0,3}$/ ? "'$x' matches\n" : "'$x' no match\n";
$x = '12';
print $x =~ /^\d{0,3}$/ ? "'$x' matches\n" : "'$x' no match\n";
$x = '1';
print $x =~ /^\d{0,3}$/ ? "'$x' matches\n" : "'$x' no match\n";
$x = '';
print $x =~ /^\d{0,3}$/ ? "'$x' matches\n" : "'$x' no match\n";
__END__
'1234' no match
'123' matches
'12' matches
'1' matches
'' matches
[download]

I wonder if Regexp::Common::number would help you.