Good catch. Only the Action is removing the filename extension.
(Using Convert Tag-Tag does not remove the extension with above regular expression as you can see in the preview).
As an argument, I could say that the examples you gave has no file extension. 
Just a joke...
For an Action "Replace with regular expression" that keeps the file extension, you could use:
Regular Expression:
(^.*\(19\d\d\)).*\.(.*)
Replace matches with:
$1.$2
