Systems Analysis and Synthesis

3.3. Regular Expressions

We can model an atom in two distinct ways: we can describe its behaviour, or we can describe how it changes its state. In a behavioural description we focus on what sequences of events are possible. In a state-based description, we take the view that the current state of the system is determined by prior events and that the current state of the system then determines what future events can occur. These descriptions are equivalent; we can derive behaviour from states or derive states from behaviour.

We can describe the possible sentences accepted by an automaton using a formula, called a regular expression , which uses three familiar constructs: sequence, choice, and iteration. We may have one thing followed by another, ...