Which of these things can you do with regular expressions?

A: Compactly describe nondeterministic finite$-$state automata for matching strings

B: Define a context$-$free grammar to describe the syntax of a programming language

C: Compactly describe any set of strings.

D: Write a regular expression for matching any number of the letter $\text{'}$A$\text{'},$ followed by the same number of the letter $\text{'}$B$\text{'}.$