Hm. You've described what you don't want to match rather than what you do want to match, and what you've described doesn't correspond to the examples you've given.

Why should the last example not match given it doesn't contain a separator at all?

Updated with addition rule:

#! perl -slw
use strict;

my @good = qw[ aa1.2 aa1_bb2 ];
my @bad  = qw[ aa1_2 aa1_2bb 1aa ];

my $re = qr[^[0-9]|^.*[_-][0-9]];

print $_, $_ !~ $re ? ' matches.' : ' doesnt match' for @good, @bad;

__END__
C:\test>junk
aa1.2 matches.
aa1_bb2 matches.
aa1_2 doesnt match
aa1_2bb doesnt match
1aa doesnt match
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