More examples

Ex. 4:
Describe as simply as possible in English the language corresponding to the regular expression a*b(a*ba*b)*a* .

Solution:
A string in the language can start and end with a or b, it has at least one b, and after the first b all the b's in the string appear in pairs. Any numbe of a's can appear any place in the string. Thus simply put, it is the set of strings over the alphabet { a, b } that contain an odd number of b's.
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