Write a regular expression for each of the following languages (for each of them, the alphabet is {a, b}): Strings that begin with an a and end with a b. Strings whose length is a multiple of 3, where the letters at positions 3, 6, 9, etc. are all b's. Strings that don't contain either the substring ab or the substring ba. Strings that contain both the substring ab and the substring ba (they are allowed to overlap - for example, aba is in this language) Explain in English what language is represented by each of the following regular expressions: a ab ba aa sigma sigma sigma sigma* sigma* aa sigma* ab sigma* sigma*ab sigma*aa sigma* sigma*aab sigma*