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Ex. 5:
Describe as simply as possible in English the language corresponding to the regular expression (( a + b )3)*( #+ a + b ) .

Solution:
(( a + b )3) represents the strings of length 3. Hence (( a + b )3)* represents the strings of length a multiple of 3. Since (( a + b )3)*( a + b ) represents the strings of length 3n + 1, where n is a natural number, the given regular expression represents the strings of length 3n and 3n + 1, where n is a natural number.