效绵鼎 RE's:Introduction Regular expressions describe languages by an algebra. They describe exactly the regular languages. If E is a regular expression,then L(E)is the language it defines. We'll describe RE's and their languages recursively. 22 RE’s: Introduction ◼ Regular expressions describe languages by an algebra. ◼ They describe exactly the regular languages. ◼ If E is a regular expression, then L(E) is the language it defines. ◼ We’ll describe RE’s and their languages recursively