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I'm attempting this challenge:
https://regex.alf.nu/4
I want to match all strings that don't contain an ABBA pattern.
Match:
aesthophysiology
amphimictical
baruria
calomorphic
Don't Match
anallagmatic
bassarisk
chorioallantois
coccomyces
abba
Firstly, I have a regex to determine the ABBA pattern.
(\w)(\w)\2\1
Next I want to match strings that don't contain that pattern:
^((?!(\w)(\w)\2\1).)*$
However this matches everything.
If I simplify this by specifying a literal for the negative lookahead:
^((?!agm).)*$
The the regex does not match the string "anallagmatic", which is the desired behaviour.
So it looks like the issue is with me using capturing groups and back-references within the negative lookahead.
217
In this type of lookahead the regex engine searches for a particular element which may be a character or characters or a group after the item matched. If that particular element is not present then the regex declares the match as a match otherwise it simply rejects that match.
Capturing groups are a way to treat multiple characters as a single unit. They are created by placing the characters to be grouped inside a set of parentheses. For example, the regular expression (dog) creates a single group containing the letters "d" "o" and "g" .
You probably cant perform standard negative lookaheads using grep, but usually you should be able to get equivalent behaviour using the "inverse" switch '-v'. Using that you can construct a regex for the complement of what you want to match and then pipe it through 2 greps.
Lookbehind, which is used to match a phrase that is preceded by a user specified text. Positive lookbehind is syntaxed like (? <=a)something which can be used along with any regex parameter. The above phrase matches any "something" word that is preceded by an "a" word.
^(?!.*(.)(.)\2\1).+$
^^
You can use a lookahead here.See demo.The lookahead you created was correct but you need add .* so that it cannot appear anywhere in the string.
https://regex101.com/r/vV1wW6/39
Your approach will also work if you make the first group non capturing.
^(?:(?!(\w)(\w)\2\1).)*$
^^
See demo.It was not working because \2 \1 were different than what you intended.In your regex they should have been \3 and \2.
https://regex101.com/r/vV1wW6/40
91
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