Regex no two consecutive characters are the same

Question

How do I write a regular expression where x is a string whose characters are either a, b, c but no two consecutive characters are the same

For example

abcacb

Answer 1

Since the number of chars is limited, you can get away without a back reference in the look ahead:

^(?!.*(aa|bb|cc)[abc]*$

But I like tenub's answer better :)