正则表达式没有两个连续的字符是相同的

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 is true

acbaac is false

Answer 1

^(?!.*(.)\\1)[abc]+$ works if you follow the original question exactly. However, this does not work/check multiple "words" of characters a/b/c, ie. "abc cba".

The way it works is it asserts that any character is not followed by itself by utilizing a capture group inside a lookahead and that the entire string consists only of characters "a", "b", or "c".

Answer 2

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 :)

Answer 3

Other solutions apart from the two mentioned already:

using negative lookbehind: ^((a|b|c)(?<!(aa|bb|cc)))*$

using negative lookahead: ^((?!(aa|bb|cc))(a|b|c))*$