Question

Give a regular expression for the language of strings over {a,b} in which each substring of length 2 contains two distinct characters

Answer 1

b(ab)*a? + a(ba)*b?

Explanation:
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if string starts with b, then string can have any number of (ab) strings, then may or may not end with an a.
b(ab)*a?

if string starts with a, then string can have any number of (ba) strings, then may or may not end with a b.
a(ba)*b?