Something using zero width negative look ahead (?!) should work:

my @t = qw( regex regex.1 regex.12 regex.1a regex.a regex.abc regex.a1
+ oregex.2 );
for (@t) {
    print $_;
    print ' matches!' if /^regex(?!\.[^\d])(\.\d*)?/;
    print "\n";
}
[download]

regex matches!
regex.1 matches!
regex.12 matches!
regex.1a matches!
regex.a
regex.abc
regex.a1
oregex.2
[download]

Checkout the "Look-Around Assertions" section in the perlre documentation. In the example above, the negative look-ahead says fail if the next characters are a period followed by anything that isn't a digit.