Java Regex negative lookahead wrong match

Question

I'm looking for strings where the two first digits are present (in any order) in the digits that follow the space character.First I tried

(\d)(\d)\s\d*(\1|\2)\d*[\1\2&&[^\3]][\d]*

but it seems that I can't use brackets with backreferences.I tried using the lookahead feature instead with

(\d)(\d)\s\d*(\1|\2)\d*(?!\3(\1|\2))\d*

but I isn't right.The idea was "look for two digits, followed by a space, followed by zero or more digits, followed by either of the captured digits, followed by zero or more digits, followed by one of the captured digits which ISN'T the one I got before, followed by zero or more digits".21 20329 is a match.Why?How do I look for the strings I need?

zx81 · Accepted Answer · 2014-05-17 22:49:04Z

1

This is simpler.

^(\d)(\d) (?=.*?\1)(?=.*?\2)\d+

See demo

The first lookahead ensures that the digit captured by Group 1 is present somewhere later in the string.
The second lookahead ensures that the digit captured by Group 2 is present somewhere later in the string.
If these conditions are met, the \d+ eats up all the digits after the space.

answered May 17, 2014 at 22:49

zx81

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4 Comments

Giovani Granzotto Over a year ago

Thanks, that's useful.That website is really helpful too.I wish I could upvote your answer but I have no reputation, sorry.

Jongware Over a year ago

@user3648292: but you can accept this answer (and you'll get some points for that as well). Please read About SO, as your status suggests you have not done this yet.

Giovani Granzotto Over a year ago

hey, I looked at it again and got confused.It looks like it'll only match when \1 comes before \2, is that right?If so, that will be wrong in some cases, the order shouldn't matter at all

zx81 Over a year ago

@user3648292 Thanks for your feedback, glad it works. No, the answer doesn't matter at all. See [demo](user3648292) That's because when the engines evaluate a lookahead, it does so planted at a certain position in the string. The position after the lookahead hasn't changed, so the two lookaheads could be in any order. See Mastering Lookahead and Lookbehind

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