Registration fiel validation: "Invalid regular expression"

Hi everyone,
on one of my sites I have many registrations from spam bots. All the registrations have in common, that the last name ends in two capital letters like mugtustintyFY or HerbertImistUM.
Now I try to validate the last name field in my registration form using regex, but Community Bilder complains about my expression not being valid.
Here is the regex string, I try to insert: Could anyone please tell me, what's wrong with my expression?
Best Regards
Markus

You need to supply a delimiter. All REGEX you supply must be complete and valid REGEX. Changing it to the below would give you valid REGEX.

Hello Kyle,
I'm sorry, I forgot to post them along with my expression. I in fact did have the delimiters in the regex field. Just to be sure, I pasted your expression into my field too, but Communiy Builder keeps complaining.

Your REGEXP has to be valid in PHP and JS. It's valid in PHP, but it's not valid in JS. JS does not support possessive quantifiers, which you're using in your REGEXP.