CODEWITHSUNDEEP
phpregex
Qiao
Qiao

Reputation: 17049

Back reference in negative lookback

Starts with aa or bb. Ends with it (back reference). And has no it in the middle.

It is ok without "or" and back reference -- aa.*?(?!aa).*?aa
But it does not work with them: (aa|bb).*?(?!\1).*?\1

echo preg_match("@(aa|bb).*?(?!\\1).*?\\1@", 'aaxxaaxaa'); // 1 ??
echo preg_match("@(aa|bb).*?(?!\\1).*?\\1@", 'aaxxxaa'); // 1

What is wrong?

Upvotes: 1

Views: 146

Answers (1)

Mark Byers
Mark Byers

Reputation: 838376

You need to check your negative condition for every character.

(aa|bb)((?!\\1).)*\\1

You also need to add anchors.

^(aa|bb)((?!\\1).)*\\1$

See it working online: ideone

Upvotes: 2

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