Which of the following strings match [BD][ae][ae]?d ?

Answer:

String	Bed	Bead	Dead	aed	Ba?d	Ded	Bea
Match or Reject?	Match	Match	Match	Reject	Reject	Match	Reject

The RE [BD][ae]\?d matches the strings Ba?d , Be?d , De?d , and Da?d . The question mark has been "escaped" with the backslash, and so does not function in its special role but merely stands for itself.

Automaton for Zero or One

An automaton for a?bc requires an optional state for the transition that is made when an optional 'a' is consumed.

The state q₁ is reached when the optional 'a' is consumed. There then must be a transition out of that state for the following 'b'.

QUESTION 15:

Could we make the transition for 'a' out of state q₀ loop back to q₀? Would this avoid the need for state q₁?