negated lookahead confusion

Question

I'm trying to match strings that do not fit the pattern name (CA) or barbaz (UK). I seem to have solved the problem using the answer here, but this is what I started with, and I'm wondering why it doesn't work:

var r1 = /^.+(?!\([A-Z]{2}\))$/;

r1.test('foo'); //true

r1.test('foo (US)'); //whoops -also true

From reading a negated lookahead doc I was hoping that any string that is not followed by e.g. (JP) would result in a match, while anything that is followed by e.g. (DE) would fail. (Only the former is being met).

I thought perhaps the .+ was kind of "consuming" everything, nullifying the lookahead, so I tried

r2 = /^[^()]+(?!\([A-Z]+\))$/;

r2.test('name (US)'); //false

r2.test('foo('); //whoops -also false

However I need things like foo( to match.

Why did my first attempt fail?

score 1 · Accepted Answer · answered Jun 18 '16 at 20:45

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In your first expression, the .+ consumes the whole string and then the look-ahead is tested - it doesn't match, as there are no characters left. To achieve your goal you can use /^(?!.+$[A-Z]{2}$$).+$/ - first checking the non-presence of the unwanted construct, then matching.

See https://regex101.com/r/aG7xZ0/1 with your samples

answered Jun 18 '16 at 20:45

Sebastian Proske

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Thanks for the link. Could you elaborate how you used $ ? – KnewB Jun 18 '16 at 21:22
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It is simply used to ensure, that `(XX)` is only matched at the end of the string, and not somewhere in the middle, so `Hello (XX) World` would be valid. – Sebastian Proske Jun 18 '16 at 21:25
Cool I think that covers the one inside the lookahead, but then there's `.+$` at the end? – KnewB Jun 18 '16 at 21:35
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Simply matching everything up to the end - though you could drop the `$`, as `+` matches lazy anyways. – Sebastian Proske Jun 18 '16 at 21:37
given that anything placed after `(XX)` should result in `true`, I'm confused by the final `.+` -sorry I'm rubbish! – KnewB Jun 18 '16 at 21:49
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The look-ahead doesn't match anything itself, so you need the `.+` for actual matching. – Sebastian Proske Jun 18 '16 at 21:53
For when you use `exec` rather than `test` and want to see matched text? – KnewB Jun 18 '16 at 22:04
According to your question you wanted to match strings - if you only want to test you could also `! /$[A-Z]{2}$$/.test('whatever')` – Sebastian Proske Jun 18 '16 at 22:08
sorry yes see what you mean. Thanks – KnewB Jun 18 '16 at 22:10
do need to match :) – KnewB Jun 18 '16 at 22:15

negated lookahead confusion

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