JavaScript regular expression to negate an exact string match

Question

Is there a way in HTML (JavaScript) to write a regular expression to negate an exact string match?

I would like to make sure an input is not equal to "foo". Only "foo" must fail validation, but "fooo" must be allowed.

In other words, I'm looking for a negation of this regex:

<input pattern="^foo$" ...>

Possible duplicate of [HTML5 pattern exclude words](https://stackoverflow.com/questions/13875922/html5-pattern-exclude-words) — Nir Alfasi, Dec 22 '17 at 06:02
It doesn't seem to work: https://codepen.io/smatric/pen/mprBMd — Mateusz, Dec 22 '17 at 06:02

raina77ow · Accepted Answer · 2017-12-22T06:13:11.530

5

One possible way is combining start of string anchor (^) with negative lookahead that includes both the target string and end of string anchor ($):

/^(?!foo$)/

Demo.

But pattern pattern is funny in that account - it has to match something in the string, that's why the original approach doesn't work. This does work, however:

<input pattern="(?!foo$).*">

edited Dec 22 '17 at 06:13

answered Dec 22 '17 at 05:42

raina77ow

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This does not allow "fooo" – oha Dec 22 '17 at 05:44
This does allow `fooo`, check the demo (or just run `/^(?!foo$)/.test('fooo')` in console). Empty string is matched, yes, as `^` is _anchor_. – raina77ow Dec 22 '17 at 05:45
1

Sorry, you are right - didn't see the empty match. Too early in the morning ;-) – oha Dec 22 '17 at 05:47
No problem, has woken up a bit early too. ) – raina77ow Dec 22 '17 at 05:48
I can see your regex works in the demo you linked to, but for some reason, it doesn't work in HTML: https://codepen.io/smatric/pen/mprBMd – Mateusz Dec 22 '17 at 05:55
1

Interesting, didn't know about that difference. I'm not even sure it's not a bug of either spec or implementation. ) Updated the answer. – raina77ow Dec 22 '17 at 06:14

Sweeper · Answer 2 · 2017-12-22T05:54:05.840

1

If Javascript is allowed, why not just negate the result of the match?

if (!yourString.match(/^foo$/)) {
    ...
}

edited Dec 22 '17 at 05:54

answered Dec 22 '17 at 05:48

Sweeper

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That won't work since it won't allow `fooo` as well. But it can easily be fixed: `/^foo$/` – Nir Alfasi Dec 22 '17 at 05:53

score 0 · Answer 3 · edited Dec 22 '17 at 05:55

0

You can use this regex:

\bfooo\b

edited Dec 22 '17 at 05:55

eLRuLL

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answered Dec 22 '17 at 05:43

karthik selvaraj

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JavaScript regular expression to negate an exact string match

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