CODEWITHSUNDEEP
regexregex-negationregex-lookarounds
James Monger
James Monger

Reputation: 10685

Regex to match strings containing two of any character but not three

I want a Regex to match strings containing the same character twice (not necessarily consecutive) but not if that character appears three times or more.

For example, given these two inputs:

abcbde
abcbdb

The first, abcbde would match because it contains b twice. However, abcbdb contains b three times, so that would not match.

I have created this Regex, however it matches both:

(\w).*\1{1}

I've also tried to use the ? modifier, however that still matches abcbdb, which I don't want it to.

Upvotes: 4

Views: 2214

Answers (2)

BlueTrin
BlueTrin

Reputation: 10113

The accepted solution does not work for:

This regex will work (\w)(?<!(?=\1)..)\1(?!\1), see demo https://regex101.com/r/qIisdD/1

Explanation:

  1. (\w) captures any letter
  2. (?<!(?=\1)..) is a negative look ahead to prevent triple repeat
  3. \1 will repeat the captured letter from 1.

Upvotes: 0

jaytea
jaytea

Reputation: 1959

You need two checks: a first check to ensure no character exists 3 times in the input, and a second check to look for one that exists 2 times:

^(?!.*(\w).*\1.*\1).*?(\w).*\2

This is horribly inefficient compared to, say, using your programming language to construct an array of character frequencies, requiring only 1 pass through the entire input. But it works.

Upvotes: 3

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