正则表达式，如果它不包含特定字符，则获取完整字符串？

I need help with a regular expression that will find matches in the strings below:

myDOG_test

myCAT_test

Basically, I want to return 'DOG' or 'CAT' from these paths.

Then I have similar strings (all start with 'my') that don't contain the underscore AFTER the value I want, and in that case I just want to return the FULL string -- in a match group.

myCentralReports

myDEMO3

This is the REGEXP that I have so far:

.*?my(.*?)\_.*

This correctly puts CAT & DOG in the matching group, but I'm having problems matching the other 2 strings. Obviously I left the hardcoded underscore in there just to show you what I started with -- but I need to modify this for the other case. Any help is appreciated! Thanks.