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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?
362
Similarly, the negation variant of the character class is defined as "[^ ]" (with ^ within the square braces), it matches a single character which is not in the specified or set of possible characters. For example the regular expression [^abc] matches a single character except a or, b or, c.
In negative lookbehind the regex engine first finds a match for an item after that it traces back and tries to match a given item which is just before the main match. In case of a successful traceback match the match is a failure, otherwise it is a success.
Negated Character Classes If you don't want a negated character class to match line breaks, you need to include the line break characters in the class. [^0-9\r\n] matches any character that is not a digit or a line break.
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.
98
Using the explanation here: https://stackoverflow.com/a/406408/584663
I came up with: ^((?!((\w)(\w)\4\3)).)*$
44
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