Java Regex Exclude a Pattern ?! Usage

Question

I understand that ?! is to exclude a pattern, so for example a(?!b) means it will match "a" if "a" is not followed by "b". My question is, suppose I have a file with the following content:

a cat is a cat, a dog is a dog,

a bird is a bird.

How many times will the pattern a(?!.*b) match? Is it 0 times? since all "a"s are before the "b" in the last word "bird"?

I don't know much regex but this site is godly: regex101.com — Anubian Noob
– Anubian Noob, Commented Apr 17, 2015 at 1:25
see regex101.com/r/sI4wE1/1 . You could play with the regex in this site. — Avinash Raj
– Avinash Raj, Commented Apr 17, 2015 at 1:27

Community · Accepted Answer · 2020-06-20 09:12:55Z

1

It will match for every a. The pattern a(?!b) means it will match any a not immediately followed by b. cab wouldn't match, but a bird will.

EDIT

With your new pattern a(?!.*b), no match if all of your text is on the same line. The .* does not match linebreaks, so:

A cat is a cat, a dog is a dog

A bird is a bird

This would have matches for every a of the first line.

edited Jun 20, 2020 at 9:12

CommunityBot

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answered Apr 17, 2015 at 1:27

Docteur

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6 Comments

Docteur Over a year ago

No problem, I will edit my answer, but your assumption is right; no match except linebreaks. See above

StackJay Over a year ago

.* matches any character (except newline). I didn't know * couldn't match linebreaks. Hence the confusion. :D Thank you for your explanation!

Docteur Over a year ago

* can, . can't ;) If you want to check everything, including newlines/linebreaks, I believe you would use [.\s]*. My pleasure! Have fun with regexes :)

Alan Moore Over a year ago

@Docteur, [.\s] matches a literal dot or a whitespace character. If you want to match any character including newlines, use the dot in Singleline/DOTALL mode. One way to do that is with an inline modifier: (?s)a(?!.*b)

Alan Moore Over a year ago

Yeah, that works, too. That idiom comes from JavaScript and other ECMAScript implementations, which don't have a dot-matches-everything mode, but it works in all regex flavors.

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