I'm playing Regex Golf (http://regex.alf.nu/) and I'm doing the Abba hole. I have the following regex that matches the wrong side entirely (which is what I was trying to do):
(([\w])([\w])\3\2)
However, I'm trying to negate it now so it matches the other side. I can't seem to figure that part out. I tried:
(?!([\w])([\w])\3\2)
But that didn't work. Any tips from the regex masters?
You can make it much shorter (and get more points) by simply using . and removing unnecessary parens:
^(?!.*(.)(.)\2\1)
It just makes sure that there's no "abba" ("abba" here means 4 letters in that particular order we don't want to match) in any part of the string without having to match the whole word.
Using the explanation here: https://stackoverflow.com/a/406408/584663
I came up with: ^((?!((\w)(\w)\4\3)).)*$
The key here turns out to be the leading caret, ^, and the .*
(?! ...) is a look-ahead construct, and so does not advance the regex processing engine.
/(?! ...)/ on its own will correctly return a negative result for items matching the expression within; but for items which do not match (...) the regex engine continues processing. However if your regex only contains the (?! ) there is nothing left to process, and the regex processing position never advances. (See this great answer).
Apparently since the remaining regex is empty, it matches any zero-width segment of a string, i.e. it matches any string.
[begin SWAG]
With the caret ^ present, the regex engine is able to recognize that you are looking for a real answer and that you do not want it to tell you the string contains zero-width components.
[end SWAG]
Thus it is able to correctly fail to match when the (?! ) succeeds.
来源：https://stackoverflow.com/questions/20774437/regex-negation