Menu
I am trying to use negative lookahead – standart regex-posix practice:
"foobarbaz" =~ "^(?!.*bar).*$" :: Bool
and getting error *** Exception: user error (Text.Regex.Posix.String died: (ReturnCode 13,"repetition-operator operand invalid"))
How can I use negative lookahead in Haskell?
684
The negative lookahead construct is the pair of parentheses, with the opening parenthesis followed by a question mark and an exclamation point. Inside the lookahead, we have the trivial regex u. Positive lookahead works just the same.
It indicates that the subpattern is a non-capture subpattern. That means whatever is matched in (?:\w+\s) , even though it's enclosed by () it won't appear in the list of matches, only (\w+) will.
Negative lookahead That's a number \d+ , NOT followed by € . For that, a negative lookahead can be applied. The syntax is: X(?! Y) , it means "search X , but only if not followed by Y ".
Negative lookahead, which is what you're after, requires a more powerful tool than the standard grep . You need a PCRE-enabled grep. If you have GNU grep , the current version supports options -P or --perl-regexp and you can then use the regex you wanted.
Disclaimer: there are too many GREP dialects!
Your original GREP works on InDesign (it uses a slightly amended boost implementation); so does this alternative:
^((?!bar).)*$
i.e., try to match (?!bar). on each character in turn.
87
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With
