negative lookaround catches nothing

Asked Jul 29 '16 at 13:40

Active Jul 29 '16 at 13:40

Viewed 20 times

I've come across a number of places on the interwebs showing that if you want to negate an entire regex pattern you can just embed it inside: (?! pattern here )

But instead it catches on nothing for me. I've tried it on a number of sites and apps that use regex, but no dice. Here's an example.

Suppose I have these strings:

strangecowfeet

mycow

yourcow

hiddendonkey

cowfishyummy

This pattern matches all except hiddendonkey:

cow

But then If I do:

(?!cow)

It doesn't match on anything. I've tried a bunch of variants I've seen around (trailing .* and .+ and spaces and anchors, etc), but to no avail.

Looks like I've overlooking a very basic flaw.

edited May 23 '17 at 12:32

Community

asked Jul 29 '16 at 13:40

micahhoover

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It is called a lookaround, a zero-width assertion, because it does not *consume* (=return) any text. – Wiktor Stribiżew Jul 29 '16 at 13:43
http://stackoverflow.com/questions/2973436/regex-lookahead-lookbehind-and-atomic-groups – Wiktor Stribiżew Jul 29 '16 at 13:43
1

And http://stackoverflow.com/questions/406230/regular-expression-to-match-line-that-doesnt-contain-a-word explains how to use the lookahead inside a tempered greedy token, and much more. – Wiktor Stribiżew Jul 29 '16 at 13:45
Just use `^(?!.*cow).*$` and be done with it. – Jul 29 '16 at 15:50

negative lookaround catches nothing

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