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Question 10.7.5: Find a regular expression corresponding to the set of all st......

Find a regular expression corresponding to the set of all strings com-posed of a and b that contain an even number of as.

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The expression b∗ defines the set of all strings without a; the expression (b∗ab∗ ab∗) defines the set of all words with exactly two as. It remains to take the union of these two sets and then to apply iteration:

((b∗ab∗ab∗) \mid b*)∗

Another possible answer:

( (b∗ ab∗a)∗ b∗)

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