How can I put [rnt]* into a regex look behind?

If I have some text like:

"My name is  
    XXX"

and I want to match the “XXX” but the between “is” and “XXX” is any number of either of either rnt ‘s, how can I do this?

If I try (?<=My name is[rnt]*)[a-zA-Z]{3} the following error message:

A quantifier inside a lookbehind makes it non-fixed width

Any help would be appreciated

I have seen the post for doing the with spaces but I don’t believe this is the same using multiple different characters.