Question

I’m having trouble with this regular expression:

Construct a regular expression defining the following language over alphabet
Σ = { a,b }

L6 = {All words that begin and end in different letters}

Here are some examples of regular expressions I was able to solve:

1. L1 = {all words of even length ending in ab} 
   (aa + ab + ba + bb)*(ab) 

2. L2 = {all words that DO NOT have the substring ab}
   b*a*  

Answer 1

Would this work:

(a.*b)|(b.*a)

Or said in Kleene way:

a(a+b)*b+b(a+b)*a