my @t = qw( regex regex. regex.1 regex.12 regex.1a regex.a regex.abc regex.a1 oregex.2 );
say s/^regex(?!\.[^\d])(\.\d*)?/|$&|/r for (@t);

##</code><code>##

|regex|
|regex.|
|regex.1|
|regex.12|
|regex.1|a
regex.a
regex.abc
regex.a1
oregex.2