I thought all regular languages could be expressed with regular expressions (if a language is regular, it can be expressed with regex), but I have been told that you need all three of the regular operations (concatenation, union, and star) for that to hold.
For example, I have been told that if I can only use the union and concatenation regex operations (2 out of the 3), there would be a regular language I can't describe with just those two.
Same with just Kleene star and union. What are some examples of this?