Loops: You read the OP a bit more slowly and carefully, and came closer, I think, to answering codz67's question as a result. But the  (\.\d+)? is redundant for the given inputs and outputs if the negative look-ahead is used. (Had it been needed, it would probably have been better as a  (?:...) non-capturing group.)

c:\@Work\Perl>perl -wMstrict -le
"my @t = qw( regex regex.1 regex.12 regex.1a regex.a regex.abc regex.a
+1 oregex.2 );
 for (@t) {
   printf qq{'$_'};
   printf ' matches!' if /^regex(?!\.[^\d])/;
   print '';
   }
"
'regex' matches!
'regex.1' matches!
'regex.12' matches!
'regex.1a' matches!
'regex.a'
'regex.abc'
'regex.a1'
'oregex.2'
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