Hi Melvin,
Did you set the Regex tester to the correct language? Because each programming language has different Regex syntax unfortunately. If it's a microflow, it uses Java syntax. If it's a nanoflow, it uses JavaScript syntax. Make sure to search for a Regex validator for your programming language. I always use Regex101 where you can set the syntax on the left side of the screen.
What are you trying to check? Should the input contain, exactly match, start with those inputs?
Those slashes are a notation from javascript (regular expression literal) for regexes and not needed in Mendix.
It should return true on either one of the following four strings:
/11
12
13
RES/
And false on everything else. Maybe you don’t want the slashes in there?