Look Ahead Behind and Around

The look-ahead is used to only match the string if something lies ahead or does not lie ahead of the current match. The trick is that the look-ahead does not consume parts of the string. A second match would also consider parts of string, where the previous look-ahead matched.

positive look-ahead

(truck|car)s(?=\s)

Do trucks like cars?

This will only match trucks because cars is not followed by a whitespace.

negative look-ahead

(truck|car)s(?!\s)

Do trucks like cars?

This will only match cars because it is not followed by a whitespace.

positive look-behind

(?<=\s)(truck|car)

Are cars scary?

This will only match the first car and not the car in s-car-y because a whitespace needs to be behind the match.

negative look-behind

(?<!\s)(truck|car)

Are cars scary?

This will match the second car in scary because an s is not a whitespace.

look-around

(?<=[\ss])(truck|car)(?=[sy])

Are cars scary?

This will match both ,,car’’ . whitespace or s on the left and s or y on the right.

(?<=[\ss])(truck|car)(?![sy])

Do cars like carpets ?

Now the car in carpets will match.